GENETICS OF REPRODUCTIVE ISOLATION IN THE DROSOPHILA-SIMULANS CLADE -DNA MARKER-ASSISTED MAPPING AND CHARACTERIZATION OF A HYBRID-MALE STERILITY GENE, ODYSSEUS (ODS)

In this study, we address the question of whether there exist major ge nes that cause complete male sterility in the interspecific hybrids of Drosophila and, if they do, how these genes may be characterized at t he molecular level. Our approach is to introgress small segments of th e X chromosome from Drosophila mauritiana (or Drosophila sechellia) in to Drosophila simulans by repeated backcrosses for more than 20 genera tions. The introgressions are monitored by both visible mutations and a series of DNA markers. We compare the extent of introgressions that cause male sterility with those that do not. If a major sterility fact or exists, there should be a sharp boundary between these two classes of introgressions and their breakpoints should demarcate such a gene. Furthermore, if male sterility is the only major fitness effect associ ated with the introgression, recombination analysis should yield a pat tern predicted by the classical three-point cross. Both the genetic an d molecular analyses suggest the presence of a major sterility factor from D. mauritiana, which we named Odysseus (Ods), in the cytological interval of 16D. We thus formalize three criteria for inferring the ex istence of a major gene within an introgression: (1) complete penetran ce of sterility, (2) complementarity in recombination analysis, and (3 ) physical demarcation. Introgressions of Ods from D. sechellia do not cause sterility. Twenty-two introgressions in our collection have bre akpoints in this interval of about 500 kb, making it possible to delin eate Ods more precisely for molecular identification. The recombinatio n analysis also reveals the complexity of the introgressed segments-ev en relatively short ones may contain a second male sterility factor an d partial viability genes and may also interfere with crossovers. The spermatogenic defects associated with Ods and/or a second factor were characterized by phase-contrast microscopy.