Genetics of Sexual Isolation between DROSOPHILA PSEUDOOBSCURA and DROSOPHILA PERSIMILIS.

OTWITHSTANDING their morphological similarity, Drosophila pseudoN obscura and Drosophila persimilis are distinct species. No hybrids between them have been recorded in natural populations, although the two species frequently occur together in the same habitats. Although in laboratory experiments species hybrids may be obtained without much difficulty, intraspecific matings succeed more easily than interspecific ones. The principal if not the only cause of this is the pronounced sexual isolation between the species. If females of the two species are placed together with males of one of them, a greater proportion of conspecific than of alien females are inseminated (DOBZHANSKY and EPLING 1944). Although the degree of sexual isolation between D. pseudoobscura and D. persimilis depends upon the environment in which they live (MAYR and DOBZHANSKY 194s), there can be no doubt that the principal cause of the isolation is genetic. The genetic basis of sexual isolation has never been studied. The experiments reported in the present article represent an attempt to explore this little known field. Since female hybrids between D. persimilis and D. pseudoobscura are fertile when backcrossed to either parental species, it is, theoretically, possible to obtain individuals with any combination of chromosomes of the parents. The sexual preferences of such individuals of mixed parentage may, then, permit inferences regarding the genetic mechanisms underlying the isolation between the species. It may be noted that differences in sexual preferences sometimes occur between mutants and the wild type of the same species (SPETT 1931; NIKORO, GUSSEV, PAVLOV, and GRIASNOV 193s; RENDEL 194s), as well as between geographic strains of a single species (DOBZHANSKY and KOLLER 1938; STALKER 1942; DOBZHANSKY and MAYR 1944; DOBZHANSKY and STREISINGER 1944). However, these differences in sexual preferences seldom assume the character of a true sexual isolation-that is, of a regular preference for endogamic, as opposed to exogamic, mating. This of couse, is what would be expected on theoretical grounds if, as is usually supposed, species arise from geographic races by a process of gradual divergence. The genetic elements from which an isolation may be built are available within a species, but these elements are combined to form a functional isolating mechanism only during the process of speciation-that is, during the transformation of the diverging races into distinct species.