Abstract. -By backcrossing hybrids from the cross Drosophila mojavensis female x Drosophila arizonensis male to both parental species we show that several interspecific combinations of autosomes with one or the other sex chromosome (X or Y) result in sperm abnormalities. Two ofthese incompatibilities will cause the same type of nonreciprocal F, male sterility that is observed in this pair of species, but the possibility of an additional incompatibility that would have the same result, e.g., an incompatibility between the mojavensis Y and the arizonensis X chromosomes, cannot be excluded. The incompatibility between the arizonensis Y chromosome and the mojavensis fourth chromosome found to occur for all tested populations of mojavensis race B (Vigneault and Zouros, 1986) is shown also to occur for race A of this species. We further show that a dominance relationship exists between heterospecific homologous autosomes in their interactions with the sex chromosomes and that the direction of the dominance depends on whether the sex chromosome is the X or the Y. The present role of these incompatibilities in preventing gene flow between the two species may be minor, but their genetic basis and mode of action may provide useful insights about the genetic events that have played a significant role in earlier stages of speciation.Received June 16, 1987. Accepted March II, 1988 It is a widely accepted view that the first that, if one sex is sterile among the progeny step in the process of speciation is the ap-of an interspecific cross, that sex is the hetpearance of some form of reproductive iso-erogametic one. It is obvious that this phelation between two conspecific populations nomenon, or any other phenomenon ofhy- (Dobzhansky, 1940). For operational pur-brid sterility, cannot be attributed to isolated poses, the appearance and reinforcement of effects of single loci, but rather to interacreproductive isolation can be considered as tions (or incompatibilities) among loci. As the "phenotype" of speciation, and any ge-an example, we may consider sperm imnetic locus that may contribute to this iso-motility, a character used in this study. Since lation as a "speciation" locus (e.g., Coyne sperm is motile in males of both species, and Charlesworth, 1986; Hutter and Ash-sperm immotility in hybrids cannot be atburner, 1987). So defined, the phenotype of tributed to genes that are intrinsically inspeciation and its genetic causes can be best compatible with normal spermatogenesis, studied by hybridization. Hybridization has but, rather, to disruptions of synergistic inbeen extensively used for the genetic study teractions among con specific loci. ofboth prezygotic (e.g., Tan, 1946; Ehrman, Using hybrids of the species D. simulans, 1961; Zouros, 1981a) and postzygotic (e.g., D. mauritiana, and D. sechelia of the melDobzhansky, 1936; Sturtevant and Novit-anogaster subgroup, Coyne (1985) has exski, 1941;Prakash, 1972;Schafer, 1978; amined two hypotheses about Haldane's Coyne, 1984, 1985 Naveira and Fontde-rule, the imbalance bet...
