Genetics of hybrid inviability and sterility in Drosophila: Dissection of introgression of D. simulans genes in D. melanogaster genome

Interspecific crosses between Drosophila melanogaster and Drosophila simulans usually produce sterile unisexual hybrids. The barrier preventing genetic analysis of hybrid inviability and sterility has been taken away by the discovery of a D. simulans strain which produces fertile female hybrids. D. simulans genes in the cytological locations of 21A1 to 22C1-23B1 and 30F3-31C5 to 36A2-7 have been introgressed into the D. melanogaster genetic background by consecutive backcrosses. Flies heterozygous for the introgression are fertile, while homozygotes are sterile both in females and males. The genes responsible for the sterility have been mapped in the introgression. The male sterility is caused by the synergistic effect of multiple genes, while the female sterility genes have been localized to a 170 kb region (32D2 to 32E4) containing 20 open reading frames. Thus, the female sterility might be attributed to a single gene with a large effect. We have also found that the Lethal hybrid rescue mutation which prevents the inviability of male hybrids from the cross of D. melanogaster females and D. simulans males cannot rescue those carrying the introgression, suggesting that D. simulans genes maybe non-functional in this hybrid genotype. The genes responsible for the inviability have not been separated from the female sterility genes by recombination.