Variegation in Drosophila and the Inert Chromosome Regions.

At the time of their discovery, the variegated races of Drosophila (D. virilis, Demerec ('26); D. melanogaster, Muller ('30)) seemed to be promising material for the study of mutation. They have since developed rather into a puzzle sui generis. Members of the group have been discussed variously as mutable genes (Demerec ('28)), unstable translocations (Muller ('30), Patterson and Painter ('31, '32)) and, more recently, as a rather special type of duplication undergoing frequent somatic crossingover (Stern ('35)). The discussions have not been very satisfactory; partly because the data themselves have been inadequate for the formulation of general rules, excepting only Muller's correlation of the "eversporting" types in D. melanogaster with the occurrence of chromosome rearrangement. This paper summarizes the results of experiments with thirteen different variegations in Drosophila melanogaster. All of these belong tq.Tyuller's "eversporting displacements"-they are associated with chromosome rearrangements. Five involve the region around the white locus in chromosome 1; two the yellow region of the same chromosome; and the six remaining, the brown region of the second chromosome. I have attempted to determine what relation there might be between the nature of the rearrangements and the production of variegation. To this end, I have studied the characteristics of the rearrangements cytologically, in the salivary gland chromosomes; and the characteristics of the variegations, genetically, in relation to the different affected genes. Both series of data show a relation between variegation and the so-called "inert" regions. The first evidence of the sort came from the work of Gowen and Gay ('34), on the suppression of white-variegation by a supernumerary Y chromosome. This may now be extended; extent of variegation, that is, the proportion of "mutant" to wild type tissue, depends upon a quantitative relation between active and "inert" chromosome regions. In addition, however, the "inert" regions are involved in the rearrangements themselves. Thus both the chromosome structure associated with variegation and the extent of variegation are related to the "inert" chromosome regions. The variegated races form abnormal configurations of the salivary gland chromosomes as a direct result of their relation to the "inert" regions. In a nucleus with normal chromosomes, the "inert" regions are aggregated to VOL. 22, 1936 27