Induction of Mutations by High Temperature in Drosophila.

Exposure of the germ cells of living organisms to X-rays or to radium has so far proved to be the only reliable method for artificially inducing mutations, as shown by MULLER and others. In nature, organisms or groups of organisms carrying “favorable” mutations are supposed to be chosen for survival by the sorting action of natural selection, and MULLER and MOTT-SMITH (1930) have concluded that natural radiation is inadequate as a cause of spontaneous or natural mutations. The search continues therefore for some agent in the natural environment of living things which produces mutations under controlled conditions, and is of sufficiently general occurrence to account for mutations in nature. That this factor may be temperature change is of course an old idea. Temperature changes are ubiquitous and palaeontologists have often suggested that periods of rapid evolutionary change coincided with, or followed, periods of marked climatic fluctuation. Most of the observations of the pre-Mendelian period fail to satisfy the genetic criteria, and until recently, controlled genetic tests have proved negative. More than fifteen years ago the senior author looked for mutations in Drosophila strains which had been exposed to high and low temperatures with negative results (PLOUGH 1917). MULLER and ALTENBERG (1919) and later MULLER (1928) reported a slight but significant increase in the number of lethals from Drosophila cultures bred at 27°C. But the work of GOLDSCHMIDT (1929) was the first which seemed to give clear-cut evidence in support of this view. He reported that a large number of mutations of all classes appeared among the offspring of flies which had been exposed as larvae to sub-lethal temperature (35”-37”) for periods of about 24 hours. This work was confirmed and extended in a series of reports by JOLLOS (1930-1933). The latter has carried out a long series of experiments not yet completely summarized and found a marked increase in the number of mutations as well as of non-inherited variations in successive generations following heat, as compared with untreated controls. Some of the non-inherited variations reappeared among the offspring of females showing them, and many of the mutations found resembled in their expression certain of these non-inherited variations. In certain cases, too, subsequent heating of flies bearing mutations already induced gave rise to more extreme alleles in