Sterility of male and female hybrids of Drosophila virilis and Drosophila lummei

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Abstract
The sterility of interspecific hybrids between the sibling species Drosophila virilis and D. lummei was tested in reciprocal F1s and different second generation combinations of cytoplasm, sex chromosomes and autosomes. Males with motile sperm and females with at least one mature egg were scored as fertile. When D. virilis was the mother, about 5 per cent of the F1 male progeny was sterile, the reason being that the X of D. virilis was weakly incompatible with the heterozygous second, fourth and fifth chromosomes. The F1 males carrying the X chromosome of D. lummei were all fertile. The X chromosome of D. lummei, and in particular its species-specific double inversion In(1)a + b, nevertheless caused sterility in 70 per cent of males in the presence of homozygous autosomes of D. virilis. Sterility was rare among the females, although some weakly disadvantageous X versus autosome and autosome versus autosome interactions were detected. The male and female hybrid sterilities were based on different genetic systems. The results emphasize the central role of the X chromosome in hybrid male sterility.