Sterility and inviability genetic base of interspecific hybrids of "Drosophila"

Hybrid male sterility or inviability is most frequently the only postzygotic mechanism of reproductive isolation in interspecific hybridizations within the genus "Drosophila". It seems clear that this is only the initial phase of a single evolutionary process that leads to sterility or inviability in both sexes: an initial accumulation of alleles causing sterility (or inviability) in hybrid males, is followed by accumulation of alleles causing sterility (or inviability) in hybrid females. The accumulation of these alleles, but mainly of those which happen to be male-specific, is then an essential part of the process of genetic differentiation between populations that leads to speciation. In so far as the effect on the phenotype is concerned, these alleles may be of two kinds: either major genes, each one able to bring about sterility or inviability of the hybrid by itself, or mirror gene