Abstract
By backcrossing hybrids from the cross Drosophila mojavensis female × 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 of these incompatibilities will cause the same type of nonreciprocal F1 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.
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