Nuclear introgression in Drosophila subobscura and D. madeirensis despite distinct mitochondrial genomes. Danielle Herrig, Ana Llopart. 143 Biology Building University of Iowa, Iowa City, IA.

   Speciation typically occurs when a single species splits into two populations in which gene flow is severely reduced. Over time, the two populations accumulate genetic differences that eventually produce two independent species. While hybridization between two species, and thus the potential for gene flow, has been traditionally viewed as a reproductive mistake, recent studies suggest that it is not as rare as once believed. In this study, we examine if gene flow has occurred in the sister species Drosophila madeirensis and D. subobscura. D. madeirensis is endemic to the ecologically unique island of Madeira while D. subobscura is distributed throughout the palearctic region. Both species are now found on Madeira Island where they hybridize and produce partially fertile hybrids. We obtained the sequence of 26 loci randomly distributed throughout the genome in 33 lines (16 from each sister species and one from the outgroup D. guanche). Our data includes genes on the Y chromosome and the mitochondrial genome. To estimate gene flow after the split, we used maximum likelihood methods applied to multilocus data as well as individual genes. Our results indicate that there has been effective gene exchange between the two species. In particular we detect D. subobscura genes in a largely D. madeirensis genetic background. Interestingly, this signature of introgression does not include the mitochondrial genome and is limited to a small fraction of the nuclear genes.