RELATIVE PAUCITY OF GENES CAUSING INVIABILITY IN HYBRIDS BETWEEN DROSOPHILA-MELANOGASTER AND DROSOPHILA-SIMULANS

Using deficiencies from Drosophila melanogaster, we looked for genomic regions in the sister species D. simulans that could cause lethality when hemizygous on a hybrid genetic background. Such genotypes allow h emizygous genes from one species to interact with heterozygous genes f rom other species and may correspond to the kinds of genotypes causing Haldane's rule, the observation that if only one gender is sterile or inviable in species hybrids, it is nearly always the heterogametic se x. A survey of roughly 50% of the D. simulans genome (114 chromosome r egions) revealed only four regions causing hybrid lethality and five c ausing severe reductions in hybrid viability. However, the viability o f all of these genotypes was at least partially restored by rearing hy brids at lower temperature or using different genetic backgrounds from D. simulans. We therefore detected no D. simulans chromosome regions causing unconditional hybrid lethality, although several regions were shown to be deleterious under most tested temperatures and genetic bac kgrounds. The relative paucity of ''inviability genes'' supports the i dea, suggested by work on other species, that hybrid inviability betwe en closely related species might be caused by interactions among relat ively few genes, while hybrid sterility may involve many more loci.