DESCRIPTION: Dr. H. Allen Orr proposes three related projects concerning the genetics of speciation in this amended application for a FIRST award. There is considerable debate as to whether the initial stages of reproductive isolation result from a few major genes or a large number of genes of smaller effect. While studies have typically found a modest number of factors involved in isolation, essentially all of these studies have relied on taxa separated for some time, making it difficult to distinguish between initial events and those occurring much later during the evolution of isolation. Dr. Orr proposes to examine an evolutionary "young" taxa, the "Bogota" subspecies of Drosophila pseudoobscura. Dr. Orr has already studied the genetics of isolation for this subspecies at the whole chromosome level, finding evidence for a very strong epistatic interaction between normal pseudoobscura second and third chromosomes with the X chromosome from the Bogota population. The proposed work will use at least 32 mapped markers to much more finely examine how many separate chromosomal regions are involved in isolation. The second project proposed is to examine specific predictions on the nature of a hybrid rescue mutation, which restores the viability of normally inviable species hybrids. Dr. Orr proposes two specific tests of the Hmr (Hybrid male rescues) gene, namely that deletion of Hmr+ will also rescue hybrids, while addition of Hmr+ kills normally-viable hybrids. These predictions follow from the model proposed by Hutter, Roote, and Ashburner that Hmr is a loss-of-function allele at a locus otherwise involved in hybrid inviability. The final project is a screen for mutants that rescue hybrid fertility in crosses of D. melanogaster and D. simulans. While mutants what rescue hybrid viability are know (such as Hmr), there are no mutants that rescue fertility and apparently no one has screened for such mutants. The investigator proposes three screens based on using individuals from natural populations, from EMS mutagenized-flies, and from a P-element mutagenesis screen.