Copyright 0 1997 by the Genetics Society of America Perspectives Anecdotal, Historical and Critical Commentaries on Genetics Edited by James I;. Crow and William F. Dove Seventy Years Ago: Mutation Becomes Experimental James F. Crow* and Seymour Abrahamsont *Genetics Laboratory and +Departmentof Zoology, University of Wisconsin, Madison, Wisconsin 53706 N 1927 H. J. MULLER(1890-1967) published in Sci- against indiscriminate use of high-energy radiation, a I ence a paper entitled “Artificial Transmutation of crusade bolstered by the later demonstration thatmuta- the Gene.” It reported the first experimental produc- tion was linearly related to dose, down to doses as low tion of mutations and opened a new era in genetics. as could be practically studied. The title is curious. Why transmutation rather than muta- In the summerof 1927 MULLERgave a major paper at tion? The answer emerges from another paper(MULLER the Fifth International Congress of Genetics in Berlin. 1928a), written in 1926. After reviewing the repeated Typically, he scribbled the paper in transit and was still failure of efforts by many workers to modify the muta- preparing slides up to the time of its presentation. The tion rate, MULLERasked the question: “Do the preced- talk is said to have been confusing, but themessage was ing results mean, then, that mutation is unique among clear. This time he gave the full details and the skeptics biological processes in being itself outside the reach of were silenced. Helpful as he was throughout his life, modification or control,-that it occupies a position CURTSTERN got the paper typed, and it was published similar to that till recently characteristic of atomic trans- the next year (MULLER 1928b). mutation in physical science, in being purely spontane- Biologists generally accepted mutation as the ulti- ous, ‘from within,’ and not subject to influences com- mate basis of evolution. Furthermore, mutation prom- monly dealt with? Must it be beyond the range of our ised a way to get at the natureof the gene.Yet mutations scientific tools?” MULLERthought of his radiation ex- were so rare that therewas only anecdotal information. periments as parallel to those of RUTHERFORD, onlya In a few months MULLERfound more mutant genes few yearsearlier, demonstrating experimentaltransmu- than the total from all Drosophila labs up to that time. tation of chemical elements. Like the physicists, who Hisdiscovery was independently confirmed by L. J. were attracting a great deal of public attention at the STADLER,who startedexperiments withbarley and time, MULLERhad tampered with a fundamental natu- other plants atabout the same time (ROMAN1988; ral process and had succeeded in mastering it. He was STADLER1997). The slowerlife cycle in these plants an instant celebrity. meant that his results appeared somewhat later, but he The 1927 paper is also curious in another way, for it clearlydeserves recognition along with MULLER, al- presented no data-no dosage measurements, no num- though he didn’t always receive it. Other geneticists bers, no statistical analysis. MULLERsimply reported immediately jumped on the bandwagon, and the field qualitative results and rough comparisons, e.g., a muta- of radiation genetics was on its way. tion-rate increase of “fifteen thousand percent.” But a Although he was actively engaged in many aspects of paper without data invited skepticism, and the skeptics Drosophila genetics and was an active contributor in included no less than T. H. MORGAN,who was always various ways to the MORGANgroup, MULLER’S interest suspicious of speculations and invariably asked for the centered on mutation. His first experiments started in data. 1918, so his radiation paper represented the culmina- MULLER’Sidea was clearly to establish priority. He tion of a decade of work. MULLER’Stwo full reports, noted thatmany of the mutations were repeats of those one written before the X-ray discovery (1928a) and the found earlier.Most were recessive,but a few weredomi- other immediately after (1928b), report a remarkable nant. Many were lethal or sterilizing, and there were saga. The first paper occupies an entireissue of Genetics, dominant lethals, not easy to detect.In addition to gene 79 pages. In minute detail, he described the various mutations, MULLERreported a number of chromosome experiments, each successive one being an improve- rearrangements, especially translocations. He suggested ment and coming closer to providing convincing quan- mutation as a cause of cancer. And in this, his first titative data. His innovations and their updates are now paper on the subject, he began his lifetime crusade standard Drosophila methodology. Genetics 147: 1491-1496 (December, 1997) 1492 J. F. Crow and S. Abrahamson The problem from the beginning was the rarity of visible mutations were immediately apparent in males mutations. Early on, MULLERdecided that lethals were of the next generation. better material than visible mutations for quantitative From the beginning, MULLERthought that perhaps study. For one thing, they were far more numerous. the best way to getat the question of whether themuta- For another, the results were unambiguous; with rare tion rate was immutable was to study the effect of tem- exceptions there were no survivors (or only a few piti- perature. If mutation behaved like ordinary laboratory fully weak ones). The “personal equation” was elimi- chemical reactions, the rate should double or triple nated; lethals could be identified as well by the average with each rise of 10”. Therefore, throughout most of technician as by a sharp-eyed CALVIN BRIDGES.Soon the several years of studyin the early 1920s,temperature MULLERbegan devising ways to identify all the lethal was varied, with increasingly precise results in the later mutations on a chromosome, thereby multiplying the experiments. Controlling temperature was no easy task per-locus mutation rate by the numberof lethal-produc- in those days of poor equipment andpoorly supported ing loci on the chromosome. labs. MULLERdid experiments in the MORGANlab in Early in his work MULLERrealized the value of “C New York,at Woods Hole in Massachusetts, at Rice Insti- factors,” crossover suppressors, later proven to beinver- tute in Houston, and at the University of Texas in Aus- sions. He exploited these to construct balanced lethal tin. It was especially difficult to arrange cool tempera- systems in which lethal mutations, in addition to those tures in the hot Texas summers. He covered the cul- necessary to keep the system balanced, could be accu- tures with a wet cloth on which an electric fan blew. mulated overmany generations. Then, by elaborate Despite the crudity of the experimental conditions, he mating schemes, he could render the new mutations was able to compare two sets of experiments that,while homozygous and locate their approximate position by each varied considerably, differed on the average by linkage with known markers. This permitted further about 8”. enrichment of the number of mutations by summing By showing that the mutation rate could be influ- the mutations that had accumulated over manygenera- enced, MULLERmade mutation a researchable subject. tions. In his words (1928a): “Perhaps the most hopeful fea- In these accumulation experiments, MULLERfore- ture of the present datais that they show that mutation shadowed the work of MUKAI(1964), who employed is indeed capable of being influenced ‘artificially” this idea to measure the spontaneous rate of mutations that it does not stand as an unreachable god playing with small effects on viability. MULLERhad argued that its pranks upon us from some impregnable citadel in mutations producing small, statistically detected effects the germ plasm.” Later thetemperature effect was on viability and fertility were the most numerous class, given a theoretical interpretation (see SCHR~DINGER and MUKAIshowed it. 1944). MULLER’Smost famous Drosophila stock involved an It is a shortintellectual step from realization that the X chromosome that he called CZB. C stands for a cross- mutation process has a high temperature coefficient to over suppressor, Z is a recessive lethal, and B is the Bar thinking of radiation as a source of activating energy. “gene,” later proven to be a duplication. Using the Although MULLERjustly receives great credit forradia- crosses that arenow familiar to every student of elemen- tion mutagenesis, the greater contribution is his devel- tary genetics, MULLERwas able to measure the rate of opment of techniques whereby mutation could be stud- lethal mutation on the Xchromosome with almost no ied experimentally and measured reproducibly. In ambiguity. MULLERwas constantly on the lookout for three monumental papers-actually only two,since the time-saving methods, and this was one of his best. Since Science paper gave no details-MULLER provided essen- an F2 culture descended from a new recessive lethal tially all the basic techniques used in the burst of radia- contained no males, it was not necessary to examine tion experiments done in the next several decades. the flies in detail, but only enough toascertain whether In those days, many geneticists thought that best way males were present. This could be done by examining to get at the natureof the mysterious gene was through the flies in the vial with the naked eye or, a bit better, mutation. Perhaps the way the gene mutates could tell a hand lens sufficed. Figure 1 shows MULLERdemon- them what it is. Of course, genetic history has been strating his favorite low-tech instrument. The CZB quite different. The nature of the gene was discovered, method provides an easy way to map the location of not by the kinetics of mutation, but by the identification new lethals, and MULLERquickly exploited this. He of DNA as the genetic material, by advances in the showed that lethal mutations are distributed over the chemistry of large molecules, and by some clever model entireX chromosome, roughly uniformly, although building by WATSON and CRICK.Now the tables are concentrated at the “left” end.