The Chromosomal Basis of Sexual Isolation I N Two Sibling Species of Drosophila: D. Arzzonenszs

The chromosomal determination of interspecific differences in mating behavior was studied in the interfertile pair, Drosophila arizonensis and Drosophila mojavensis, by means of chromosomal substitutions. Interspecific crossing over was avoided by crossing hybrid males to parental females, and identification of the origin of each chromosome in backcrossed hybrids was possible by means of allozyme markers. I t was found that male mating b e havior is controlled by factors located in the PGM-marked chromosome (which, in other Drosophila species, is part of the X chromosome) and in the Y chromosome. The other chromosomes influence male sexual behavior through their interactions with each other and with the PGM-marked chromosome, but their overall effect is minor. Female mating behavior is controlled by factors located in the ODH-marked and AMY-marked chromosomes, with the other chromosomes exercising a small additive effect. Hence, the two sex-specific behaviors are under different genetic control. Cytoplasmic origin has no effect on the mating behavior of either sex. There appears to be no correlation between a chromosome’s structural diversity (i.e., amounts of inversion polymorphism within a species or numbers of fixed inversions across species) and its contribution to sexual isolation. These findings are in general agreement with those from similar Drosophila studies and may not be specific to the species studied here. EXUAL (or ethological) isolation is among the most important and most widespread forms of reproductive isolation in the animal kingdom (MAYR 1963). According to the classical view (DOBZHANSKY 1940), its development is the product of selection against reproductive waste resulting from hybridization between populations that have already developed some form of postzygotic isolation. There are, however, many known cases of allopatricali y developed ethological isolation (ANDERSON and EHRMAN 1967; CRADDOCK 1974a; KANESHIRO 1976; OHTA 1978). For a number of such cases, there exists evidence that ethological isolation was not preceded by postzygotic isolation (CRADDOCK 1974b; CARSON 1978). These observations have forced CARSON (1978) to call attention to the importance of “mutual adjustment of the sexes to what may be called the intraspecific sexual environment” that “in the early phases of divergence may far outweigh, in evolutionary importance, other types of adjustment to ambient enGenetics 97 : 703-718 March/April, 1981.