4

Radium Genetics

Within a decade after its discovery, radium had “come to life” in several different senses and experimental contexts. The powerful association between radium and life was evident not only in the simple use of radium itself—in radium as a tool for experimentation—but also in the key role it played in interpretation of the results of experiments in which it had served as a mutagen. As a fundamental homology between physical and biological transmutation was perceived, the probabilistic and statistical approaches exemplified in the concept of the half-life came to be common to both the half-living element and the study of the mutants it might induce. Even as MacDougal sought to better assess mutation frequency in his radium work, he drew on methods similar to those developed by physicists of radioactivity to better characterize radioactive decay. And even as Gager carried on MacDougal’s work and sought to move beyond the “extravagant,” “superficially analogous,” and putatively “dangerously attractive” analogies of Burke’s radiobes, Gager’s central concept of a “radiotonus” emerged at the intersection of biological concerns with the world of the radioactive and drew on the same sorts of powerful associations between radium and life. And yet Gager associated radium not with life in general, but with the specific physiological and mitotic effects radium might induce, and especially its effects on heredity and the chromosomes. Such work opened up a new realm of investigation into the nature of “mutation” and the various modes and mechanisms by which it might operate.

Indeed, from de Vries’s inaugural address at Cold Spring Harbor in 1904 through MacDougal’s and Gager’s pathbreaking experiments, radium was central to many of the earliest efforts to induce mutations artificially, and it played a crucial role in the emerging plurality of meanings of induced mutation. The increasing success of radium-based techniques and radium-suffused interpretations in eliding any distinction between the natural and the artificial—in which Burke had failed and de Vries succeeded, at least for a time—meant that half-living radium not only came to be associated with the experimental induction of mutations, but resonated with the very qualities attributed to those atoms of life, the elements of heredity, themselves. This chapter, then, tells the story of how radium came to life in a fourth way: having entered the rich realm of research into the nature of biological transmutation in the early twentieth century, radium had everything to do not only with experimental attempts to get at the question of the origin of species, but also with the manipulable stuff of heredity itself: first chromosomes and later genes.

From Oenothera to Drosophila: T. H. Morgan and Jacques Loeb

Although much of the early work on mutation was done in plants, no sooner had Gager’s work received prominent attention than other investigators sought to extend his findings to the animal world. In the year following the publication of Gager’s monograph, J. Arthur Thomson was already deploring the lack of attention paid to the study of mutation in animals:

And yet by 1911, an editorial in the American Breeders Magazine (soon to become the Journal of Heredity) could remark, “There are signs of a scientific awakening in animal breeding such as occurred in plant breeding a decade ago.”2 Gager was thus far from the only one interested in inducing mutation experimentally, in plants or in animals.

De Vries’s idea that “the rays of Roentgen and Curie” could be put to use in the study and control of evolution—itself a particular scientific manifestation of the broader radium craze—proved compelling to many, including the famed geneticist Thomas Hunt Morgan. De Vries and Morgan had been friends for some time, and Morgan had been actively searching for “Oenothera-like mutations” in animals throughout 1906 and 1907. In particular, he was searching for cases of mutation in Drosophila melanogaster that seemed similar to those de Vries had encountered in Oenothera.3 The fruit fly was the “animal Oenothera.”4

Like MacDougal and many others at the beginning of the twentieth century who were interested in investigating and ultimately controlling the evolutionary process, Morgan was keen to find an experimental mutagen. In contrast to MacDougal’s widely reported success in the plant world, however, Morgan regularly failed for a couple of years to produce a single mutation in Drosophila. He employed a variety of techniques, including the injection of various substances “into pupae in the regions of the reproductive cells,” and he tested his way through “wide ranges of temperature, salts, sugars, acids, alkalis,” and other chemicals, and even different kinds of food. He even investigated the effects of changes in temperature. Nothing seemed to work—it was all “without any resulting mutation,” he reported.5

“Morgan apparently did nothing more with insects until the fall of 1907,” according to Robert Kohler, “when he persuaded Fernandus Payne to try inducing mutations experimentally in Drosophila.” As Payne recalled later in life, “I tried the effects of heat and cold, variations in the food and even X rays. All results were negative with the possible exception of X rays. One variation [with wing modifications] occurred but the strain was weak and after 5 or 6 generations the strain was lost. Of course the variation might not have been a mutation. The Physics department gave me no cooperation, I am sorry to say.”6

It was during Payne’s second year at Columbia, 1908–9, that Morgan began his own attempts with Drosophila. “Cells seemed to succeed in setting up barriers against too violent an intrusion of chemical agents,” the radiation geneticist Curt Stern later recounted of Morgan’s efforts. But another possibility still held hope: “It occurred to T. H. Morgan at the very beginning of the Drosophila work that no cell can refuse the entry of γ-rays from radium.”7 Unsuccessful in his attempts thus far and willing to try almost anything, Morgan began exposing Drosophila larvae to the rays of the new element.8

Morgan’s experiments with radium were a success and proved central to the early history of classical genetics. He discovered his first mutation in May 1910—a mutant with a distinctive wing mutation he labeled “beaded” because of the pattern of the wing margin. It had originated in a culture of flies he had treated with radium.9 He reported the similar appearance of his first white-eyed mutant in a paper read at the Society for Experimental Biology and Medicine on May 18, 1910, and to Charles Davenport at the Cold Spring Harbor Station for Experimental Evolution on June 11, 1910.10 Morgan clearly attributed the appearance of this white-eyed mutant to his use of radium, and he published the story of his successes in Science and in the American Naturalist in 1911.11 Many of his contemporaries viewed these early experiments as foundational.

Gager’s monograph Effects of the Rays of Radium on Plants had been published in 1908, the very year Morgan first began working with his flies. By 1910 Morgan was referring to his own work using the same words (as the study of “the effect of the rays of radium” on Drosophila). For his part, MacDougal saw Morgan’s experiments on the occurrence and transmissibility of mutation as an explicit confirmation and extension of Gager’s earlier work: “By the use of similar excitations Morgan has recently induced the appearance of white eyes and short wings in the fly, Drosophila, which characters seem to be fixed and fully transmissible.”12

Sharing a strong interest in the eventual hope of controlling the means and modes of evolution, but ignorant of Morgan’s radium research, Jacques Loeb—already famous for his discoveries in artificial parthenogenesis—and his collaborator F. W. Bancroft built on their own interests in MacDougal’s mutagenic studies and began to conduct their own experiments with radium. (Loeb and Morgan even had a bit of a priority dispute over whose idea it had first been to use radium to induce mutations; Loeb, for one, seems to have received his supply of radium directly from de Vries, who in turn seems to have brought it from Rutherford’s laboratory.13)

Citing other work by Tower on Leptinotarsa and Gager on Oenothera, Loeb and Bancroft reported that in their own work they undertook a “very large number of experiments with radium . . . because it happened that the first culture which we treated with radium chanced to give us mutants.”14 They experienced some difficulty repeating their experiments, however, and according to one review, “the short-winged mutants have appeared thus far only in cultures treated with radium, but in only two out of several hundred such cultures . . . the authors appear to doubt whether or not the treatment was responsible for the mutations.”15 Nevertheless, Loeb and Bancroft noted (in a jab at Morgan’s reported results) that “as long as the full account of his results is not available, it is not easy to judge to what extent it is possible to produce mutations at desire with his method.”16

In his personal correspondence from these early years, however, Morgan was clear that his mutations resulted from the application of radium. He had written to Jacques Loeb in the spring of 1911, “As I told you last summer all my wing mutations go back to my flies treated with radium, as do also at least two of the eye mutations.”17 Immersed in the radioactively tinged terminology of his day, Morgan even once told Loeb that he had submitted a paper for an upcoming conference entitled “The Disintegration of a Species and Its Reconstruction by Artificial Combinations.”18 And Morgan had reported to the American Naturalist in 1914 that “one of the first mutants that I observed in [Drosophila] ampelophila appeared in the offspring of flies that had been treated with radium.”19

Morgan also seems to have further encouraged some of his students—including Alfred Sturtevant—to study the effects of radium on Drosophila.20 The importance of this fundamental association of radium with life in Morgan’s experimental practice has thus far been generally overlooked by historians. Morgan’s promising mutational results led in time to the development of his large-scale program for studying mutation in Drosophila—the heart of research in early classical genetics, as Robert Kohler has shown. Morgan’s lab was thus dependent not only on a particularly fruitful moral economy surrounding the fruit fly—it was also dependent, initially, on radium.21 Inspired by de Vries’s dedicatory speech, Morgan produced his first mutants by the application of radium, and he characterized these experiments with a phrasing identical to that Gager employed—even describing radium as bringing about the “disintegration” of a species of fruit fly. Morgan thus found the interconnections of radium and life as provocative and as productive as Burke, MacDougal, and Gager had.

Morgan experienced some difficulties in getting his mutant flies to breed, however, and—in a context where radium was readily associated with a stimulating effect—didn’t seem to fathom at first that the very radium causing the apparent mutation might itself be a factor in the flies’ sterility.22 Moreover, he soon became increasingly evasive about whether the radium he had used in his earlier experiments had actually been responsible for the mutant flies he observed. He began to claim instead that there was merely a correspondence between the application of the radium and its purported effects. By 1914 he had qualified and “clarified” his findings to such an extent that the exact nature of his earlier claim—and his very accomplishment—were cast into doubt:

Had he discovered a new and effective experimental mutagen or hadn’t he? By 1923, reflecting on early work with the mutant beaded, Morgan (with Calvin Bridges) noted that “subsequent work with radium gave no indication that the appearance of Beaded was due to the use of radium.”24 As Morgan would later explain:

Despite his initial enthusiasm, Morgan thus rapidly backpedaled on his claims for the mutagenic power of radium, even going so far as to emphasize Loeb and Bancroft’s own wariness in claiming that they had induced mutants with radium in their experiments: “They found a black mutant type after treatment with radium but since the same type appeared in the control they do not believe that its appearance had any connection with the radium.” More significantly, Morgan suggested that the stocks Loeb and Bancroft were working with were “contaminated,” even while claiming further priority in the production of the noted mutations:

Morgan recalled similarly later in life:

In all, neither Loeb and Bancroft nor Morgan believed that the other party had succeeded in truly inducing mutations by means of radium. Curiously, then, radium, which had at first held such promise, rapidly came, after a slew of experimental disappointments, to be questioned as a legitimate mutagen. As mutants became increasingly readily available for genetic study, radium’s fall from grace seemed certain. Morgan even cited other investigators (such as Émile Guyénot) who had tried similarly to induce mutations in Drosophila with high temperatures, radium, and X-rays and claimed that they had all been “without result” (the only exception being that UV rays gave rise to black eggs). Even Morgan’s student Payne came to later dispute the classic story of radium-induced mutation: “In 1910, as you know, a fly with white eyes appeared in one of his cultures,” Payne said. “It was not an induced mutation.”28

Morgan’s reluctance to attribute mutagenicity to radium had more to do with questions of experimental setup and the proper understanding of mutation, however, than with any doubt about radium’s disequilibrating power. Just as MacDougal had come up with the concept of “mutation frequency” to replace the potentially inaccurate concept of “mutation period,” Morgan argued for taking greater care in assessing the causal role of potential mutagens:

As the historian of genetics Elof Axel Carlson has claimed, however, at least two of Morgan’s famed mutations—truncate and beaded—in fact did come from lines Morgan had exposed to radium: “The rest did not.” But, as he concluded, “With so many more spontaneous mutations than allegedly induced ones during this ‘mutating period,’ Morgan played it safe and dismissed the role of radium as an agent inducing mutations.”30 Radium began to lose its central role as a mutagen of choice in no small part because of such shifts in conceptions of induced mutation frequency.

One of the most revealing things about Morgan’s apparent retractions is his conception of “mutation” as an event with any number of possible causes—from the external to the internal, and from hybridization (a theme in the history of speciation at least from Linnaeus to contemporary criticisms of de Vries) to errors in crossing-over (a novel mechanism suggested only in the wake of the Morgan school’s own discoveries). Morgan thus shied away from claiming a causal mutagenic role for radium in part because the mechanism of mutation was not yet known. Nor was there yet a clearly articulated concept of what would later be understood to be a “point” or “gene” mutation. As Morgan concluded, “We know now that the white-eyed mutant is one of the commonest mutant types; it has recurred again and again, as have also its allelomorphs. Finding it was not so important as the use to which it was put.”31

The history of the use of radium in the experimental induction of mutations has been largely forgotten not simply because its main practitioners—Loeb and Morgan—doubted each other’s findings, but for the rather more significant reason that doubting radium’s mutagenicity seemed the most promising way to gain clarity on the real nature of mutation. Accordingly, as Morgan gained greater insights into the relationship between hereditary factors, “genes,” and chromosomes, understanding in ever greater detail what a mutation might be, he reinterpreted his earlier experiments with radium as having produced mutants by chance rather than by direct action.32 And although Morgan was soon able to acquire a relatively large number of mutants simply by culturing flies in a system later metaphorically described a “breeder reactor,” he concluded in 1914 that “our experience with Drosophila has given us the impression that mutations are rare events.”33

Not all that rare, however—Charles Davenport commented in 1922 on the relative abundance of mutations, comparing it directly with the phenomena of radioactivity: “There is certainly much in the phenomena of gene mutation with its prevailing recessive tendency, its measurable rate of occurrence, and its predictability, that shows at least many points of similarity to the gradual changes, by loss, of the salts of the uranium-radium-lead series.”34 And a few years later, Davenport reiterated the connection between mutation and transmutation.35

Mutations were rare enough, though, that it wasn’t until the work of the Morgan fly group that evidence for mutations in animals went from being “scanty”—in the words of geneticist Reginald Ruggles Gates in 1920—to being established. Gates even went so far as to claim that “the Drosophila work has therefore given us a look into the constitution of the germ plasm such as no annual-breeding plant or animal could furnish in a lifetime.”36 Gates was just a few years away from being proved wrong by a simple weed, the study of which also grew in part out of radioactive roots.

From Oenothera to Datura: Albert F. Blakeslee

Morgan may have had doubts about which experimental mutagens were responsible for which mutants in Drosophila, but the situation was far more complicated in plants—and especially in the case of de Vries’s own favorite, the much studied and hotly debated Oenothera. By the second decade of the twentieth century, it was becoming increasingly difficult to determine just which newly identified mutant forms in Oenothera truly deserved to be considered “new species”—with all that that might mean—and which should be seen as merely the result of the plant’s newly discovered messy chromosomal dynamics. Various cytological investigators struggled to come to grips with Oenothera’s “normal” karyokinetic idiosyncrasies, and with the sheer complexity of the phenomena it presented even in the absence of radium treatment.37

In the years following the publication of his monograph, Gager did his best to keep on top of advancements in radiation genetics, and he reviewed some of the more significant literature in a 1916 piece.38 George Harrison Shull, meanwhile, had taken up the challenge of studying Oenothera at Cold Spring Harbor, but with his departure in 1916, the future course of research on Oenothera at the Station for Experimental Evolution was unclear. Charles Davenport, director of the station, wrote to de Vries directly with his concerns:

De Vries had also recommended the application of “the rays of Röntgen and of Curie” in the study of mutations and in the attempt to accelerate evolution, and although the new man, Blakeslee, would rapidly turn from Oenothera toward other species, he collaborated with Gager for a decade in further investigations of the effects of radium on plants.40

Having begun his botanical career at Harvard under the mycologist Roland Thaxter in 1904, Albert Francis Blakeslee (“Bert”) first encountered de Vries’s mutation theory while teaching at the Connecticut Agricultural College in Storrs. As he recalled in an autobiographical account, it was in 1909 that he first had “the thrill” of reading de Vries’s theory, “and thought that if I scoured the country I too might be able to find a species in the process of mutation.”41 The mutation theory was at the core of Blakeslee’s interest in genetics, and both its promise and its unanswered questions sparked his imagination on more than one occasion. “I have always felt that the Mutation Theory was a strong factor in turning my interests and research toward genetics,” Blakeslee later remembered. His interest in de Vries’s theory remained strong for the rest of his life.42 Even as late as 1949, Blakeslee continued to say that de Vries was “perhaps the greatest biologist of all time” and that “the mutation theory is one of the corner stones of genetic research.”43

Like many others of the period, Blakeslee was keenly interested in the experimental control of evolution, and he gave several lectures over the years with this theme prominently highlighted. “Methods of controlling genetic processes have always been of interest to us,” he remarked.44 At first Blakeslee thought he had found a suitable choice of model organism in the yellow daisy known as the black-eyed Susan (Rudbeckia hirta). He was soon forced to move on to another model organism, however, when the daisy proved to be self-sterile and too “reduced in vigor” after two or three crosses to withstand inbreeding—not to mention the generations of inbreeding required for proper detailed research. Blakeslee began to search for other possible organisms.45 It was at this time, while at Storrs around 1909, that Blakeslee received from the United States Department of Agriculture “a batch of seeds of Datura stramonium as an example of an economic weed.” The seed “happened to give both purple- and white-flowered seedlings,” Blakeslee recalled, “and for several years this species was used to demonstrate Mendel’s laws of inheritance” in his teaching.46

On leave from the college during the 1912–13 year, Blakeslee worked at Cold Spring Harbor, finally joining the staff as a resident investigator in genetics to replace the departing Shull in 1915. Having devoted considerable attention to genetics in his botany work—Blakeslee had offered what was “probably the first organized course in genetics in the United States in 1914–1915”—it was only natural that he chose to bring his work on the “coarse, weedy plant with its beautiful flowers” with him when he moved to Cold Spring Harbor permanently.47 Once there, and at last giving up the multifaceted teaching load he carried at Storrs, where his courses included (among other things) freehand drawing, Blakeslee was free to begin work as a full-time geneticist with access to superb greenhouse and garden facilities. He was on the hunt, as he put it, for “the best possible ‘Versuchstier’” and for the best possible means to do research with it.48 Over the next twenty-seven years, Blakeslee would make full use of six greenhouses and various agricultural test fields, running experiments on a grand scale. These were resources that Gager, having difficulties even finding eight to ten feet of bench space, simply could not match.49

Blakeslee noted that Datura (jimson weed) “was the best organism I could find in the botanical line.”50 He had been drawn to Datura for a variety of reasons, including its hardiness, the ease with which it could be grown, and the fact that four generations could be grown per year in greenhouse environments, making the results of his evolutionary experiments (radium-based and otherwise) that much quicker to uncover.51 “At first,” Blakeslee recalled, echoing newly emerging concerns about Oenothera, his own choice, Datura, “seemed to have too many chromosomes, but we kept at it as a side problem since it was so easy to work with.”52

The decision paid off. Blakeslee’s assistant, B. T. Avery, found the first novel type in Datura—the so-called Globe mutant—in the summer of 1915.53 As Blakeslee later reported, “The Globe mutant differs from normals apparently in all parts of the plant. It forms a complex of characters readily recognized whether the plants in question have purple or white flowers, many or few nodes, or spiny or smooth capsules.”54 This was no ordinary mutation like those found in Drosophila. Much more than one factor had been affected: the entire plant was different from its ancestor, in a whole suite of traits.

Blakeslee became convinced that he had found a new species, and he labeled the original new plant specimen as such (“N.S.”), including a photograph of the plant in the 1919 paper reporting the discovery (fig. 8). Although the plant proved sterile with “normal” plants, it could be self-pollinated successfully and produced progeny that bred true, resulting in further generations with “depressed globose capsules.” Blakeslee concluded that it “seems to have established itself as a distinct new race.”55 He continued:

In the caption to the photograph included in the paper, Blakeslee put the point more plainly: “Tests have shown that this mutant differs from all others investigated in that it breeds true as a distinct new race. Here we appear to be witnessing the birth of a new species.”57

As Blakeslee, Avery, and his other assistants bred the “Globe” mutants, they rapidly discovered that still “other types appeared as mutants in our cultures, and Datura soon became practically our sole object of investigation.”58 As one observer at the station recalled:

Blakeslee was even able to identify types that while “indistinguishable in gross appearance from each other,” were nevertheless “in respect to a whole series of characteristics strikingly different from the normal Jimson Weed from which they have been made up to order, as it were, with definite plan and purpose.” Blakeslee eventually found three types in particular that he thought “perhaps merit the term of synthesized new ‘species,’ since they satisfy the criterion of breeding true and are more different from the normal type than some of the species which already have been described in the genus Datura.”60 He took these newly encountered mutants to be indicative that his team had encountered a situation in Datura similar to that which de Vries had encountered in Oenothera (this was particularly important because de Vries’s theory faced increased criticism and skepticism by the time Blakeslee reported his discovery in 1919):

A year after his initial discoveries, Blakeslee made further explicit reference to the “increasing rôle in experimental evolution” of the de Vriesian “theory of mutations” that had first been laid out two decades earlier. Understanding the exact nature of mutation in plants—which for Blakeslee meant understanding much more than simply gene mutations—piqued his interest and became his central goal.

Chromosomes Regnant

Unlike the drosophilists, who fairly readily shared their stocks and data across the fly room and with other centers of fly research, Blakeslee kept full control of his Datura data. But Blakeslee was nothing if not collaborative: having collected the seeds of ten different species of Datura from around the world, he engaged in a series of ongoing collaborative ventures over the years, working with the geneticist Edmund W. Sinnott, an expert in the internal anatomy of the Daturas who could recognize most mutants from tissue samples alone (and who also happened to come from Blakeslee’s old stamping ground in Storrs), and John T. Buchholz, an expert on “the growth of pollen-tubes and the abortion of ovules as problems in developmental selection,” among others.62

One of Blakeslee’s earliest ongoing collaborations was with the cytologist John Belling, who had joined Blakeslee’s group in 1920 and helped him in his “study of the nuclear condition of our mutants.”63 Blakeslee, Belling, and greenhouse manager M. E. Farnham published a “preliminary report” of their findings in Science in 1920.64 And it was Belling’s cytological work—on the appearance and behavior of chromosomes—that was later held to have given “the greatest possible assistance in the interpretation of the originally baffling phenomenon of mutation in Datura.”65 Indeed, it was largely as a result of this “fruitful association” with Belling—as well as the invention of the acetocarmine staining method that permitted chromosomes to be readily enumerated in “smear preparations”—that Blakeslee was rapidly able to establish that “each mutant was the result not of a gene difference but of a third chromosome added to a particular pair of the twelve in this plant.” Such mutants were termed “trisomics” or “2n + 1” types. More generally, this discovery enabled Blakeslee at last to interpret his results: he had found mutant plants that differed by a whole “complex of characters” that were “transmitted collectively” and which segregated “in a very unusual fashion”—on a chromosomal, rather than a genic, basis that would presumably otherwise have required the simultaneous mutation of a number of different genes.66

While acknowledging that it was “sudden germinal changes, large or small in amount” that were the basis of “perhaps the most fundamental work in modern genetics,” Blakeslee noted that “mutations could not be confined to cells associated with sexual reproduction.” In an apparent reference to the remarkably productive and groundbreaking work of the drosophilists and other more gene-oriented investigators, Blakeslee’s remarks emphasized that botany had already applied the mutation concept in ways that extended far beyond the genes that many animal geneticists were most concerned with. Somatic mutations, for instance, were those mutations that took place in cells in which sexual processes were not involved. While fairly “less common phenomena in animals,” such somatic mutations—or “bud sports,” as they were also frequently called—were common in plants, and many were even quite well known (recall MacDougal’s interests). After examining some cases in the “nonsexually propagated races of Mucor genevensis,” Blakeslee had concluded that mutation also took place “in lowly organized plants and animals in which nonsexual reproduction is the rule or in which sexual reproduction is not known to occur.”67 In other words, mutation need not be restricted to the gene or the soma; it could also take place in the context of reproduction, even if that reproduction was not itself sexual. Such instances of mutation were real, and yet they were clearly beyond the ken and the techniques of the drosophilists—no matter how powerful and innovative these investigators were in identifying and mapping mutant genes. Blakeslee argued that all these categories, including those whose “inheritance could not be established by breeding experiments,” had been and should continue to be called “mutations.”68

Blakeslee held that the effects of chromosomal duplication or other alterations in producing phenotypic change were also valid additional instances of mutation:

Blakeslee fully acknowledged that classical Mendelian research up to this time had “dealt almost exclusively with disomic inheritance.”70 But he noted that “distinct variations, provisionally termed mutations . . . [have] regularly recurred whenever a sufficiently large number of plants have been subjected to observation,” and that these, “so far as investigated . . . have been found to be connected with a duplication of one or more of the normal chromosomes.”71 Blakeslee’s mutant plants thus revealed that phenotypically distinct mutations could result from genically identical types, simply with different arrangements or numbers of chromosomes.72 Mutation could thus take place at a level that was neither organismal nor genic, but chromosomal.

Charles Davenport, director of the Cold Spring Harbor Station for Experimental Evolution, was completely convinced. With Blakeslee, he held that it was through the study of the nature and structure of chromosomes and their alterations, and not just genes, that the phenomena of heredity would be properly understood. In 1921 Davenport had already prominently noted the work of his researchers in “demonstrating the close relationship between variations in chromosome number and specific variations in the form and other qualities of the body,” and there is little doubt that it was Blakeslee’s work in particular that drove this new understanding: “The work on Datura stramonium is offering remarkable explanations of the complexities of de Vriesian mutation, a form of mutation of possibly not less general significance than Mendelian mutation.”73 By 1922, Davenport noted:

In the hands of Blakeslee, Davenport, and many others, de Vriesian mutation—like atomic physics before it—was “inward bound,” from the organismal level to the chromosomal level, while still remaining distinct from the Mendelian, “factorial,” or genic mutations that were of such interest to the drosophilists. Here at the station, one of the very centers of early experimental genetics, a distinction was being drawn not only between organismal mutants and chromosomal mutations—a distinction unknown to de Vries’s original mutation theory—but also between what Davenport called more generally “extrachromosomal changes,” or “changes in numbers of chromosomes” (such as the phenomena that Blakeslee had discovered), and “intrachromosomal changes,” or “changes in the genes.”75 Davenport even explicitly referred to Blakeslee’s mutative variants as cases of “interchromosomal mutation.” A mutation did not need to be genic in order to be genetic.

In short order, Blakeslee and his collaborators, colleagues, and competitors identified many other varieties of “chromosomal mutants,” including reciprocal translocation among trisomics, the existence of haploids in higher plants (theretofore unknown), and even mutants with chromosomes arranged in sets and rings (precisely that phenomenon determined to be responsible for the seemingly endless bedeviling of an earlier generation of investigators of Oenothera). While the drosophilists acknowledged the phenomenon of nondisjunction at the microscopic level, it was Blakeslee who connected the dots to its effects at the phenotypic level and brought nondisjunction and other related chromosomal phenomena into the realm of “mutation” proper. Davenport agreed: “It has remained for Datura to reveal in the hands of Blakeslee and his associates, Belling, Farnham, and others, an extensive system of inter-chromosomal mutation and corresponding somatic change the like of which had been entirely unknown.”76

With Datura, Blakeslee had found a “genetically simpler mutating plant material” entirely relevant to unraveling the more complicated mysteries of the chromosomal dynamics of Oenothera. As Davenport had once reported to de Vries, “Here we are, as you know, submerged in Datura and feel, as you feel yourself, that it throws light also upon Oenothera.”77 And as he reported elsewhere:

Although undoubtedly invested in the success of the research program conducted at his own station, Davenport could still hardly praise the significance of the Datura work enough: “The Datura work is of such great theoretical importance that it deserves all the cooperation that can be secured for it.” Time and again he trumpeted the significance of the “variations of the chromosomal complexes and their corresponding somatic mutation.”79 The genic mutations of the drosophilists were important, Davenport argued, but were properly understood as complementary to the work coming out of Cold Spring Harbor and its focus on the chromosome as a primary agent in evolution:

The chromosome was central. Oenothera may have once upon a time struck investigators as peculiarly problematic, but with a better understanding of chromosomal dynamics and the effects this had on the production of actual mutant organisms, Datura was saved from a similar fate. As Blakeslee noted sotto voce: “We do not believe . . . that the jimson weed is peculiar among plants in giving rise to chromosomal mutants.”81

Elemental Heredity: Atomizing the Chromosome

Even as the precise mechanisms of heredity remained unknown and the understanding of mutation continued to evolve in these early years, important areas of overlap and resonance between the phenomena of mutation and the phenomena of radioactivity continued. Sir George Darwin’s presidential address to the BAAS was one prominent example, but there are countless others, even decades after the initial radium craze. To H. G. Wells and Julian Huxley in 1931, for example, “it seemed that mutations were like the transformation of radio-active elements—something truly spontaneous, in the sense of being determined from within, not to be influenced in their rate of occurrence by any treatment which could be devised.”82 J. Arthur Thomson proclaimed in the same year that “in the domain of things the processes that come nearest those of organic evolution are to be found in radio-active changes,” and he described the transmutation of uranium as “in some ways like the transformation of species; but, nowadays, the known chemical-physical clocks are all running down, whereas the vital clocks are able to wind themselves up.”83 And Morgan, even years after his preliminary experimentation with radium, continued to stake out a position regarding radium’s relationship to life, on one occasion even taking Henry Fairfield Osborn to task for quasi-vitalist claims relating radium to life processes. (Morgan characterized Osborn’s claims as to “the atomic constitution of the chromatin” and its possible constitution by as yet “undetected chemical elements” as nothing more than “a sort of poetic outburst.”84)

Poetic it may have been, but such “outbursts” specifically relating atoms to the phenomena of the cell were commonplace. “There is something about the . . . declarations of Professor Edmund B. Wilson regarding the structure of the cell that reminds one of Sir Ernest Rutherford’s description and Bohr’s graphs of the atoms,” wrote F. M. Getzendaner in the American Naturalist in 1924. He suggested that the journal’s readership explore what he called the “periodic differences in species,” even going so far as to identify eight “super phyla” of animal life and explicitly noting that this was the same number of groups as in the periodic table.85 Indeed, the relationship of the physicochemical to the realm of heritable variation had always been provocative. According to one early historian of genetics, Bateson had earlier “derived the discontinuity of substantive variations . . . from chemical differences which were determined by a chemical stability,” as opposed to meristic variations, which were determined by a more “purely mechanical” stability.86 Inverting the anxiety of influence, meanwhile, MacDougal claimed that it was de Vries’s “speculative insights” and his theory of heredity that had led to the “present conception of the ions of the physical chemist”!87

Either way, mutations were strongly associated with changes in chemical elements, and the implications of a biology analogous to chemistry or physics were widely commented on. As R. C. Punnett noted as early as 1909:

With only the most nascent of cytogenetics to depend on, it was not beyond the pale in 1913 to consider mutation as having to do primarily with change in the number of chromosomes in an organism, given that chromosomes were understood to be the primary vehicles of heredity. In fact, de Vries’s Oenothera mutants were being explained in just this way. Chromosomes were, for a time, the undisputed atoms of heredity in many quarters. As such, one commentator in 1913 noted:

Gates also explicitly linked the phenomena of heredity with those of the atom:

Gates even called this view of heredity the “elementalist” or “particulate” view.91

Such thoughts conjured visions of a new kind of periodic table of the chromosomes—presaging the kinds of charts of “chromosomal types” that Blakeslee was to produce in the 1920s. Even Loeb, referring to Morgan’s mapping work, noted that “biology has thus reached in the chromosome theory of heredity an atomistic conception, according to which independent material determiners for hereditary characters exist in a linear arrangement in the chromosomes.”92 Morgan was mapping genes, but it was references to chromosomes being the units of hereditary mutation—just as atoms were the units of radioactive transmutation—that were legion. Lancelot Hogben made the connection clear in the late 1930s:

The Meaning of Mutation

The wider community of geneticists and other students of heredity were already well aware that it appeared possible to make a distinction between genic and chromosomal mutation. Another important and complicated shift in the meaning of mutation was taking place at the same time, however: a distinction was also emerging between the process and the object of mutation (arguably, “mutant” vs. “mutation”). This latter distinction led to a carefully reasoned exchange between Shull, then the editor of the Journal of Genetics, and Blakeslee following Shull’s editing of the title of one of Blakeslee’s submissions to read “mutation” in place of “mutant.”94 Blakeslee wrote to Shull to complain about the change, and the two engaged in an exchange that captures an important moment of transition in the terminology of the period. Blakeslee first complained to Shull on April 15, 1921:

Shull replied:

Not everyone was in agreement with Shull, nor would they necessarily be in the years to come. James Neel remarked later on a further distinction: “Mutant vs. mutation. I have polled the geneticists here, and they seem to agree that it is unfortunate but true that the term mutation covers both the changed condition in the genome and the process of change.” A note attempting to standardize genetic nomenclature that had been published in the 1921 American Naturalist, Neel remarked, “does not help much” as it did not touch on the issue.97

Shull was also fully aware of the first axis in the meaning of mutation, and whether chromosomal variations were “mutations” became a matter of debate in the field. Although Shull initially seemed to agree with the designation of chromosomal aberrations as mutations—as his initial reply to Blakeslee’s complaint shows—a week later he had coined a new word for such chromosomal mutations and tried to get Blakeslee to use it. The word was “anomozeuxis”:

Shull’s two responses to Blakeslee’s work are illustrative of how the differing levels of analysis employed by competing groups of biologists ensured competing definitions of “mutation.” For example, by traditional observable botanical and morphological criteria, and by the simple fact that they bred true, Blakeslee’s plants were clearly mutants, and any botanist (as de Vries himself had often remarked) would have classified such new organisms discovered in the field as mutants belonging to a new species. By the standards of the drosophilists who used genetic mapping techniques and some other geneticists, however, these were clearly not new mutants (or mutations), but merely chromosomal aberrants.

Within a decade this unresolved issue—were mutations genic or chromosomal?—would start to resolve itself in ways that had a long-standing negative effect on the assessment of the significance, scope, and legacy of Blakeslee’s work. As “mutation” increasingly became genic, as Morgan’s recantation of the mutagenic power of radium settled in, and as Blakeslee’s work increasingly came to be overlooked or devalued as irrelevant to a proper understanding of mutation, the important ways in which radium had been instrumental and even central in the early study of heredity would come to be forgotten. Even the important and pathbreaking work with radium that Blakeslee was to undertake in the 1920s in collaboration with Gager would, as a consequence, be almost entirely forgotten by the mid-1930s in the wake of exciting new findings about inducible genic mutation.

: : :

In the early 1920s Blakeslee was fully aware of the polyvalent meaning of “mutation” and of the declining influence of de Vries’s theory among biologists of all stripes. In an article entitled “Types of Mutations and Their Possible Significance in Evolution,” he compared the influence of Mendel with that of De Vries:

Having laid out the relevant details—from the drosophilist H. J. Muller’s work on balanced lethals in the 1910s to the importance of the study of the behavior, association, and mechanism of chromosomes and chromosomal duplication and polyploidy—Blakeslee asked:

In all, Blakeslee’s approach represented a distinct modification and reworking of de Vries’s theory.101 Although Blakeslee acknowledged that “strictly speaking I should not call chromosomal aberrations mutations when the changes are purely quantitative,” the table accompanying his 1921 article labeled those forms in precisely that way.102 The situation was further complicated by the fact that not only could phenotypically different species be genically identical while differing at the chromosomal level (as Blakeslee had shown), but phenotypically and genically identical species could still be chromosomally distinct through simple and well-known processes such as translocation.

Blakeslee was fully aware of the drosophilists’ genocentric focus, and he gave their understanding of mutation a certain priority in his 1921 article in the American Naturalist: “We have seen that chromosomal duplications and related phenomena may simulate gene mutations in their effects upon the individual.” And yet his focus always remained on understanding the nature of chromosomal mutations: “What is their possible significance in evolution?” he asked, since this is where the fundamental question of speciation ultimately resided. (He also noted that “sudden genetic changes are not necessarily associated with sexual processes,” meaning genic changes, while chromosomal changes often were.)103 Blakeslee sidestepped any firm answer on the nature of mutation in 1921:

Chromosomal mutations, or “chromosomations,” thus served as a halfway point between the classic de Vriesian organismal mutants and the drosophilists’ clear identification of gene mutations. (By 1933 Hurst had proposed an alternative coinage with radioactive roots: changes “due to chromosome transmutations and not to gene mutations . . . may be distinguished as transmutants.”105) Only when the effects of a mutagenic treatment that produced phenotypic candidates for new species could be shown not to be the result of chromosomal mutation, and to have resulted in the formation of a new mendelizing character, was Blakeslee willing to attribute the visible aberrant effects to gene mutations. In a new collaboration with Gager, Blakeslee was soon to use radium to explore the natural history of the chromosome in greater depth. The secret of life, like the secret of radioactivity before it, was inward bound.

Making a Go of It

Although Blakeslee initially planned to stay at the Station for Experimental Evolution only two years before returning to full-time teaching at Storrs, fate intervened, and he became assistant director in 1923. He took over the reins as acting director after Davenport’s retirement in 1934 and finally became director in 1935. By the time he retired in 1941, Blakeslee had spent twenty-seven years at Cold Spring Harbor, during which time he uncovered so many new and important data from his work on Datura that other geneticists began to refer to “the Datura Klondike”—a gold mine that revolved in no small measure around his experiments with radium.106

Blakeslee laid out the problem: If plant mutants were due to alterations in chromosomes and not just in genes, then “it should be possible by breeding tests to connect up mutants with as many chromosome sets as there are known Mendelian factors, or factor groups.” This, however, was not always the case, as there were unusual situations (such as various forms of chromosome duplication) in which varied effects also needed to be taken into account. The discovery of what were termed “balanced” and “unbalanced” types—that is, mutant types with all paired chromosomes and types in which an additional chromosome was left unpaired—provided a new means of exploring the influence of mutation (see figs. 9, 10, and 11 for later visualizations of this phenomenon). In effect, Blakeslee argued, it meant that there was now a means to avoid depending on the random appearance of mutations in a population:

“Knowing the mechanism to be affected,” Blakeslee concluded—that is, the behavior, mechanism, and association of the chromosomes—“we may be able ultimately to induce chromosomal mutations by the application of appropriate stimuli.”108 Radium was one of the first of those stimuli to which Blakeslee turned.

Although Blakeslee was unaware of de Vries’s inaugural remarks at the station, his use of radium echoed de Vries’s hope that it might be used to induce artificial mutation in the chromatin. As noted in chapter 3, Gager had been the last major figure to investigate the effects of radium rays on plants. After finishing his first round of experiments at the New York Botanical Garden, Gager had gone back to his benefactor, Hugo Lieber, in 1909 to request a further 10 mg of radium for a further series of experiments (doubling the 5 mg he had previously used). As he explained to Lieber, “I hope to give a little more finish to some of my first work, and then to specialize more along the lines of the next to the last chapter in the Memoir, i.e., experimental heredity by means of radium rays.”109

While other researchers had investigated the effects of radium in inducing mutations after Gager’s initial work (such as Emmy Stein, working with the snapdragon Antirrhinum in August 1921, “exposing the vegetative tip of shoots . . . and by exposing seeds to radium rays”), little of significance was found that had not already been reported in Gager’s work. “The net results of the experiments reported . . . leave it wholly an open question as to whether mutation can be caused by exposure to radium rays,” Gager and Blakeslee would later report.110

In fact, it wasn’t until the winter of 1921 that “the three essentials” needed to properly investigate and finally establish the effects of radium on plants came together once again: “a supply of radium preparations, carefully pedigreed plant materials, and sufficient time and cooperation to make the exposures and to follow the behavior of the plants developed from seeds produced from ovules that had been exposed to radium rays either during gametogenesis, fertilization, or the development of the fertilised egg.”111 Although it seems unlikely that Gager got the additional radium from Lieber this time around, his interests in experimental heredity drew him to Cold Spring Harbor, where he brought his experience in irradiation to a full-scale collaboration with Blakeslee and his Datura. Gager was soon hard at work at the station, “investigating the possible effect of radium emanations upon gene and chromosomal mutations.”112

Theirs was a close professional friendship: Gager and Blakeslee often shared train cabins and hotel rooms at botanical conferences, and they visited each other’s homes with their wives. Having begun their collaboration in 1921, Gager and Blakeslee were already well aware of reports that Oenothera’s “mutating period” might be due to complicated chromosomal dynamics. And yet they were committed to seeing with a sort of double vision, not only recognizing phenotypic mutants but also retaining a conceptual space for “mutations” that were neither (now disparaged) cases of ring chromosome nondisjunction, as in Oenothera, nor strictly genocentric, as the drosophilists were wont to hold. As Blakeslee himself reported to the Botanical Society of America on December 28, 1921, the year before he and Gager presented the first fruits of their collaborative work, “The fact has recently been emphasized that two distinct types of mutation may occur in plants—those which are due to the change of a single factor or gene and those which are due to the addition of one or more entire chromosomes.”113 Blakeslee claimed to have thus far discovered three “factor” mutations and twelve “chromosome” mutations in the jimson weed, all of which were “identified by various external characters.” It was in this presentation that Blakeslee first publicly outlined the goals of his collaboration with Gager:

Blakeslee’s approach to mutation studies was thus intended to complement other studies in the field and to better highlight the different ways mutations could be produced—by both chromosomal and genic factors.

With most radium in the hands of physicists and hospitals at this time, Blakeslee found a benefactor in a certain Halsey J. Bagg, of the Memorial Hospital of New York City. (As Blakeslee explained, “Anything that you can do for us in getting hold of the rays, will be greatly appreciated. The Jimsons are becoming more interesting and ought I feel to be attacked from every standpoint possible.”115) Blakeslee then wrote a note to Gager on a copy of the letter: “Hope we can make a go of the radium work. I can bring in the plants any time you are ready for them.”

Blakeslee expected that the application of radium to the plants would have one of two major categories of effects. As he once asked Gager, “Just what do you anticipate the results will most likely be—induction of the mutations or effect upon somatic growth?”116 These two possible outcomes were not entirely distinguished in Gager’s earlier 1908 work—in many cases, effects on somatic growth were mutations in the early days of the century.117

In mid-April Blakeslee brought potted Datura plants from Cold Spring Harbor to the Brooklyn Botanic Garden, where Gager was now director. Gager then either took the plants to the hospital for irradiation at some point in the spring of 1921 or, more likely, borrowed radium needles to expose them himself.118 (As the glass of the radium needles would absorb most of the α-and β-rays emitted by the element, it was primarily the γ-rays that could be held responsible for the effects observed.) The plants were then transferred back to Cold Spring Harbor for the summer growing season, where both Gager and Blakeslee tended them.

The experimentation was far from easy, and Gager himself was not particularly sanguine that the results would be all that revealing: “My forecast is that we shall probably not get any results that can be attributed to the radium unless possibly a dwarfing.”119 Gager seemed continually beset by obstacles, and he found by the end of June 1921 that his schedule had been so overtaken with other work that he was unable to carry out further experiments. He had hoped to be able to continue with the experiments the next spring, but was again unable to do so, and he found in subsequent summers that his busy schedule, hay fever, and other matters interfered with further experiments.120 Gager called the disappointing results of that first summer useful only for studying the “the best method of procedure,” and he considered that the work done thus far served only “generally to indicate errors in method to be avoided.”121

Blakeslee was considerably more enthusiastic about the prospects of their work, writing in 1922 to Gager, “This spring the offspring of the mutants which came from your rayed capsules showed albino seedlings in the proportion of one albino to three normally green plants.” If only he could find capsules heterozygous for that albino character, Blakeslee concluded, “we will have proven the presence of a new mendelian character for the Jimson Weed and can feel the probability that the cause of its appearance was the radium treatment.”122 Taking Gager’s own initial investigations and carrying them further, Blakeslee continued to come up with other experimental possibilities to try out, drawing on his increasing knowledge of chromosomal (and not just genic) mutations. In one case he proposed assessing the “differential mortality or stimulation of the various mutants caused by a single extra chromosome” when exposed to radium in order to better study the “stimulation and retardation” of the rays of radium “upon the various physiological processes of mutants.”123 Gager, for his part, was more concerned to know whether any of the mutations were totally new or whether they were simply more of the same kinds that had already been identified (MacDougal had made a point of measuring the frequency only of new mutations).124

Gager’s earlier 1908 work had found effects that “seemed confined to purely somatic characters of the offspring and did not appear to affect their genetic constitution.”125 He had concluded, therefore, “that most of the variants were not true mutations, and that further evidence is needed before we may say . . . that mutation may be induced by exposure to radium rays.”126 His collaboration with Blakeslee extended this earlier work in a new direction, and with provocative new results. Together they hoped “that it may be possible, soon, to take up again the study of the effects of radium treatment upon the genetic constitution of the offspring and to determine more precisely at what stage or stages the stimulus is effective.”127 In other words, even in the wake of Gager’s earlier claims to have failed to produce “true mutations,” Blakeslee’s awareness that mutational changes could be somatic or chromosomal while still not being genic led to a new understanding crediting radium with mutagenic power.

Blakeslee only occasionally expressed minor impatience with the speed of their progress. As he wrote to Gager late in 1923, “Wish we had some more radium work to report. You must find a cure for the Ragweed and come down and play with them again next summer.”128 Nevertheless, by this time, the two had already presented their preliminary results at the 1922 meeting of the Botanical Society of America in Boston. And already by 1921 they had encountered a peculiar mutant, “Nubbin,” that had clearly arisen from a “radium-treated parent” and was probably the result of ray-induced “breaking up and the reattachment of parts of non-homologous chromosomes.”129 (As Blakeslee later reported in the CIW Year Book, some of the “three chromosomes were fragments, and the fragments of one were attached each to a fragment of the other two.”130) Blakeslee thought that “Nubbin,” with its interchanged chromosomes, was thus “probably the first induced chromosomal mutation.”131 He held that an albino character might also have been due to radium treatment.132 In short, Blakeslee believed that the radium treatment increased the proportion of mutants, but he remained open-minded as to whether it could cause new gene mutations—such as the albino mutant—and waited for evidence that such traits acted as Mendelian characters.133

By the following year, the two had begun to draft a paper, eventually to be published in the Proceedings of the National Academy of Sciences. Gager took the time in this paper to explain in detail the “exact nature of the stimulus,” by which he meant “the different kinds of rays given off by radium,” expecting his audience to be botanists not well acquainted with the properties of the various radioactive elements. (Gager wondered aloud to Blakeslee as he sent the draft “whether I have said more than is desirable” on this front.) Gager understood well, however, that his previously published monograph

In fact, neither Blakeslee nor Cold Spring Harbor had copies of Gager’s work on the shelf.135

Production of their paper became bogged down for years, both because of the inherent difficulties of the project and because Gager’s other commitments kept him away from the radium work. By the dawn of 1927, Gager wrote to Blakeslee, “I have just glanced the paper through. Apparently, it will need very considerable revision, if not re-writing. Among other things, it might be desirable to mention the results of Mavor on the production of nondisjunction and crossing over . . . by X-rays, though reference to those papers should, I think, be very brief.”136 James Mavor’s results, published in Science in 1922 under the title “The Production of Non-Disjunction by X-Rays,” had indicated that the phenomena of nondisjunction, first identified in Drosophila by Calvin Bridges as the cause of various heritable traits, could be induced artificially.137

Fully aware that some of MacDougal’s earlier successes in inducing mutations had come into question, Blakeslee and Gager were concerned that their own work not fall prey to the same criticisms. Though certain that they had discovered two radium-induced mutations, Blakeslee nonetheless advocated caution: “It seems to me that in view of the trouble which McDougall [sic] got into with his induction of mtations [sic] it behooves us to be extremely cautious, perhaps unnecessarily so, in claiming much for our preliminary experiment.”138

The idea that new mutations (Mavor’s cases of nondisjunction) could be induced with X-rays had found little favor at this time. As Blakeslee noted:

Blakeslee was not alone in seeing difficulties in attributing the results in fruit flies to the effects of irradiation. In one of his early unpublished manuscripts, A. H. Sturtevant recorded his own inconclusive experiments on the effects of radiation on Drosophila funebris, in which he exposed some 902 flies to radium and compared them with 2,348 control flies. As Sturtevant’s notes reveal:

Encouraging a bit of devil’s advocacy, Blakeslee therefore recommended to Gager that

Blakeslee was concerned about a number of issues, from the lack of an ideal control (which he took to be their own fault) to the effects of cold temperature in producing mutants (as was “known from other experiments”). And he raised another potential objection: “we shouldhave [sic] been able to control the production of mutants and we get 2 cases where the radium had no effect to only one inwhich [sic] it seemed to have an effect.” All in all, he concluded, “I am wondering if we ought not to do a little more work with the radium and get more than an isolated capsule effected [sic] before we get out a formal paper.”141 A little over a month later, he wrote to Gager again:

Radium-Induced Chromosome and Gene Mutations

Blakeslee and Gager rapidly “made a go” of their radium work. Their collaboration also ultimately led, among other things, to the discovery of various abnormalities besides visible changes in the chromosomes, such as “definite proportions of aborted pollen . . . abnormalities in pollen-tube growth . . . including non-germination of half the pollen grains, bursting of half the pollen tubes and bimodal curves of pollen-tube growth.”143 More significantly, after years of delay, their joint paper “Chromosome and Gene Mutations in Datura Following Exposure to Radium Rays” finally appeared in the Proceedings of the National Academy of Sciences in February 1927.144 While they acknowledged that when they first presented their results in 1922 they had not yet “a sufficient body of data in regard to the mutability of untreated parents to permit us properly to evaluate the significance of the results,” they now claimed to have accumulated “considerable” data regarding both “gene and chromosomal mutations in closely comparable normal material which can be handled as control to the treated material.”145 Finding great surprise in their success, they reported that they had discovered a variety of what they called “chromosomal mutants,” mostly of the 2n + 1 form—having a complete diploid set of chromosomes with an additional chromosome.

Although these types of chromosomal mutants had first been mentioned in the Anatomical Record as early as 1923, what was significant in Blakeslee and Gager’s new publication was the sheer rate of production of these mutants.146 While overall they had discovered some 73 “2n + 1” forms among 15,417 progeny in the controls (a rate of 0.47 percent), in one case they found “[a] percentage of 17.7 chromosomal mutants in over 100 offspring from a single capsule” of one of the treated plants—a rate they described as “enormously greater than we have ever obtained before or since.” They concluded, “In view of the above figures, we believe the radium treatment was responsible for the increased proportion of chromosomal mutations, as also for the appearance of the compound chromosomal type Nubbin.”147

While Mavor had noted in 1925 that it was still unclear just how X-rays and other “modifying agents” affected the germ cell, saying that “it is quite possible that . . . there may have occurred only a loss or an abnormal distribution of chromosomes,”148 by 1927 Blakeslee and Gager had given such cases of chromosomal mutation clear and proper standing, and they cited Mavor’s own production of nondisjunction as an explanation of their own 17.7 percent rate of chromosomal mutants attained from a single capsule. They indicated that the number of these chromosomal mutants, which were chiefly nondisjunctional forms, represented “a much higher percentage than ever obtained from untreated capsules.”149 Radium, they made clear, could induce chromosomal mutations and, as such, was an important first tool in the experimental control of heredity. After all, as Blakeslee had noted time and again, gaining control of chromosomal mutation was one key way not to have to wait for unpredictable mutations in genes. (Control was a key desideratum of the station’s efforts at “experimental evolution”—what in a few years Blakeslee would begin to characterize as the work of a “genetics engineer” after he turned to using colchicine as a mutagen.)150

Even years before the publication of his 1927 PNAS paper with Gager, Blakeslee was convinced that he had found both chromosomal and gene mutants as a result of the radium experiments, as when he had discovered the swollen mutant (which came from an earlier generation of Gager’s irradiated capsules).151 As Blakeslee had written to Gager in 1923, “You may be interested to know that a new mutant which we had called swollen seems to be a gene mutant rather than a chromosomal mutant as we had at first believed it to be.” The evidence at hand “strongly indicates” that this was the case, Blakeslee remarked, which meant that “we will have had two gene mutations following radium treatment and these are the only gene mutants we have ever identified in all our cultures.”152

By 1927, in a new round of radium treatment, Blakeslee and Gager had discovered two more induced gene mutations among the offspring of eighteen irradiated individuals.153 While the discovery of two new genes might be a small number in absolute terms, they argued, it “is very large if the proportion of gene mutations can be considered significant with so few individuals tested.” (As they did not save seeds of the normal offspring from the treated capsule to test for heterozygosity in other new genes, however—they were looking only for chromosomal mutations—they were unable to work up these results on genic mutation further.)154

The end result of their collaboration was clear. There was no longer any doubt that radium could transmute species, and that it did so in at least two different ways: “It is our belief that for most, if not for all, of these three types of results”—the compound chromosomal type Nubbin, the chromosomal mutants, and the gene mutants—“the radium treatment may be held largely responsible,” they concluded.155 Blakeslee summarized the significant findings of their collaboration thus:

Blakeslee identified three main “chromosomal types” that they encountered in their radiation research on Datura: “prime types,” which were the result of segmental interchange and other related forms of translocation; “compensating types” such as “Nubbin,” with interchanged chromosomes; and the third and perhaps most interesting, “synthesized pure breeding types, which correspond to synthesized new ‘species’” resulting from radiation treatment. Blakeslee was firmly convinced that these synthesized pure breeding types—the result of chromosomal and not merely genic mutation—were indeed new species in an evolutionary sense: they bred true, generation after generation, and they were recognized as new types by botanists (the traditional criteria for demarcating new species).157 Blakeslee was also aware of other effects that were clearly the result of gene mutations, though their direct relevance to evolutionary processes—the emergence and maintenance of new species—was not as readily apparent. Though they included altered pollen tube growth, the non-germination of pollen, and the early or late abortion of pollen grains, “visible gene effects of radium treatment” were, at any rate “not yet . . . common in Datura.”158 Nevertheless, Blakeslee’s research program proved tremendously successful: over the years, he found 541 gene mutations, 81 of which he was able to map to specific chromosomes.159 Intriguingly, outside the world of drosophilists, it was not at all clear that gene mutations were in any way more fundamental to the nature of evolution and the origin of species than the chromosomal mutations Gager and Blakeslee were uncovering.

Blakeslee’s emphasis on the significance of chromosomal mutation was long-standing. He had written to MacDougal as early as 1923, “I feel very strongly that a study of the chromosomal distribution is likely to explain irregularities in behavior in other plants than the Datura and that chromosomal changes in number have been responsible for evolution.”160 Blakeslee was also aware, however, and most especially at the Boston meeting in 1922, “that I have been obliged to caution people with whom I have talked about the Datura work from being over-enthusiastic and thinking the chromosome irregularities would explain phenomena which appeared to be explainable on ordinary factorial basis.”161

Blakeslee’s work was warmly and widely received, and many of his contemporaries were impressed with the scope and significance of his many discoveries. Lewis J. Stadler, who was working on maize, and who would later share some portion of credit for the subsequent discovery of X-ray-induced mutation, reviewed the “several investigations of the genetic effects of penetrating radiation in plants in progress” that had already attained “some positive results” and found the work of Gager and Blakeslee to be “especially noteworthy.” He described their findings at length, noting in particular that a “single treated capsule of Datura included variants resulting from three diverse types of germinal variation, namely, change in the distribution of chromosomes, internal reorganization of chromosomes, and changes in individual genes.”162 As he wrote to a colleague in 1931, “there are undoubtedly diverse types of mutation.”163

After Blakeslee’s death, Edmund Sinnott, who had coauthored a paper with Blakeslee in 1922 entitled “Structural Changes Associated with Factor Mutations and with Chromosome Mutations in Datura,” wrote that “botany has lost one of its notable leaders.” Milislav Demerec likewise eulogized Blakeslee’s decades of work that had “brought forth spectacular results,” especially in “our understanding of polyploidy, polysomic types, segmental interchange . . . and chromosomal differences between geographically separated strains or different species of Datura.” Another remarked that Blakeslee’s “investigations on extra-chromosomal types and the role played by each chromosome in inheritance are genetical classics.”164

Both chromosomal and gene mutations were important to Gager and Blakeslee, as they were to Stadler and many others, especially those interested in botanical cytogenetics.165 Gager and Blakeslee’s efforts successfully demonstrated the multifaceted nature of radiation-induced hereditary changes (mutations in several different levels and senses). But they were well aware of MacDougal’s earlier reception, and they double-checked their results and qualified most of their claims in the few articles they published, including their PNAS paper, which was finally published in February 1927.

They paid for their caution with their future fame. By July 22, 1927, Science had published other results on the induction of mutations in Drosophila under the provocative title “Artificial Transmutation of the Gene.”166 The author was none other than Hermann J. Muller, who was one of the century’s most remarkable and brilliant geneticists and would soon become one of its most famous. On July 31, Blakeslee wrote to Gager and mentioned only in passing what was later to be taken as the most momentous news in the history of mutation research:

After all of Blakeslee’s and Gager’s agonizing over whether what they had were examples of radium-induced mutation, thinking of all possible counterarguments, acknowledging that they lacked proper quality controls in some cases, and worrying that the argument was not as strong as it could be, the publication of Muller’s work suddenly cast their work in a new light—at least initially. Blakeslee, who had been reluctant to publish before additional research could be done—research that Gager could never quite seem to get around to doing—was now glad that they had published when they did. Blakeslee saw Muller’s new research as a call to the renewal of his own—if not with Gager, then with Buchholz.168

Muller, like Blakeslee, saw powerful associations between radium and life in his studies of the fruit fly Drosophila melanogaster. But with Muller, the story of radium’s association with life was bound further inward, toward the gene (rather than the chromosome) as containing within itself the secret of life, the mutation as the quantum of evolution, and ultimately the X-ray as the ideal means to explore the relationship between the two. While Blakeslee recalled that the gene had once been “considered an imaginary concept like the equator,”169 Muller and his Nobel Prize–winning experiments on gene mutations would prove to be a further profound transmutation of the powerful associations between radium and life.