Viability of first and second generation hybrids of Drosophila virilis and Drosophila lummei.

F1 hybrids of D. virilis and D. lummei survived significantly worse than the parents. When D. virilis was the mother, larger proportion of hybrids failed to emerge from pupal case, but overall survival rate of reciprocals was about the same. Viability of different heterospecific combinations of sex chromosomes and autosomes was studied in backcrosses. All the sex chromosomes survived well on the background of all-heterozygous autosomes. Hemizygous X was reciprocally subvital with homozygous alien autosomes. The reduced viability was male sex limited. The X-linked factor causing this effect was localized in the area of the large, phylogenetically ancient double inversion In(1)a+b. All four large autosomes were heterotic in backcross hybrids, more strongly in females. Five out of six possible first degree interactions between the autosomes were negative, indicating incompatibility between heterospecific autosomes. This effect was stronger in females. The genetic basis for hybrid subvitality is sex-specific.