GENETICS OF HYBRID MALE-STERILITY BETWEEN DROSOPHILA SIBLING SPECIES - A COMPLEX WEB OF EPISTASIS IS REVEALED IN INTERSPECIFIC STUDIES

To study the genetic differences responsible for the sterility of thei r male hybrids, we introgressed small segments of an X chromosome from Drosophila simulans into a pure Drosophila mauritiana genetic backgro und, then assessed the fertility of males carrying heterospecific intr ogressions of varying size. Although this analysis examined less than 20% of the X chromosome (roughly 5% of the euchromatic portion of the D. simulans genome), and the segments were introgressed in only one di rection, a minimum of four factors that contribute to hybrid male ster ility were revealed. At least two of the factors exhibited strong epis tasis: males carrying either factor alone were consistently fertile, w hereas males carrying both factors together were always sterile. Disti nct spermatogenic phenotypes were observed for sterile introgressions of different lengths, and it appeared that an interaction between intr ogressed segments also influenced the stage of spermatogenic defect. M ales with one category of introgression often produced large quantitie s of motile sperm and were observed copulating, but never inseminated females. Evidently these two species have diverged at a large number o f loci which have varied effects on hybrid male fertility. By extrapol ation, we estimate that there are at least 40 such loci on the X chrom osome alone. Because these species exhibit little DNA-sequence diverge nce at arbitrarily chosen loci, it seems unlikely that the extensive f unctional divergence observed could be due mainly to random genetic dr ift. Significant epistasis between conspecific genes appears to be a c ommon component of hybrid sterility between recently diverged species of Drosophila. The linkage relationships of interacting factors could shed light on the role played by epistatic selection in the dynamics o f the allele substitutions responsible for reproductive barriers betwe en species.