EVIDENCE FOR COMPLEX GENIC INTERACTIONS BETWEEN CONSPECIFIC CHROMOSOMES UNDERLYING HYBRID FEMALE STERILITY IN THE DROSOPHILA-SIMULANS CLADE

F-1 hybrid females between the sibling species Drosophila simulans, Dr osophila mauritiana and Drosophila sechellia are completely fertile. H owever, we have found that female sterility can be observed in F-2 bac kcross females who are homozygous for D. simulans X chromosomes and ho mozygous for autosomal regions from either D. mauritiana or D. sechell ia. Our results indicate that neither D. mauritiana autosome (2 or 3) can cause complete female sterility in a D. simulans background. The s imultaneous presence of homozygous regions from both the second and th ird chromosomes of D. mauritiana, however, causes nearly complete fema le sterility which cannot be accounted for by their individual effects . The two autosomes of D. sechellia may show a similar pattern. From t he same crosses, we also obtained evidence against a role for cytoplas mic or maternal effects in causing hybrid male sterility between these species. Taken with the results presented elsewhere, these observatio ns suggest that epistatic interactions between conspecific genes in a hybrid background may be the prevalent mode of hybrid sterility betwee n recently diverged species.