Genetic And Functional Analysis Of Hybrid Incompatibility Genes In Drosophila

Abstract

Reproductive isolating mechanisms maintain species boundaries by preventing gene flow and act either before or after mating. Hybrid incompatibility (HI) is an example of a post-zygotic barrier and includes hybrid lethality and sterility. Mating of D. melanogaster females with D. simulans males, which diverged ~3 million years ago, produce viable but sterile daughters, and sons which die as 3rd instar larvae. Lethality is caused by an epistatic interaction in the hybrid background between D. melanogaster Hybrid male rescue (Hmr) and D. simulans Lethal hybrid rescue (Lhr). There is evidence that additional unknown factors contribute this lethal interaction in hybrids. In collaboration with members of the Barbash lab, I performed a screen using the Bloomington Deficiency Kit to identify putative HI factors. We found that there are no additional major-effect HI genes in the D. melanogaster autosomal genome, however, additional HI factors on the X chromosome are required for lethality. HI genes are typically characeterized by rapid evolution driven by selection. ! ! Likewise, Hmr exhibits extensive coding sequence divergence between D. melanogaster and D. simulans. In order to identify the forces driving selection, it is necessary to first describe the intraspecific function of Hmr. I used immunofluorescence and fluorescent in situ hybridization (FISH) to characterize the nuclear localization of the encoded protein and found that Hmr is heterochromatic and localizes to specific satellite sequences. Using functional genomics techniques, I showed that Hmr regulates the transcript abundance of transposable elements (TEs), which comprise a large portion of heterochromatin. In addition, genome-wide profiling of H3K9me3 enrichment and RNA Polymerase II occupancy in Hmr mutants showed that Hmr likely silences TEs posttranscriptionally. The heterochromatic properties of Hmr, as well as its role in TE regulation, are largely conserved between the orthologs. However, the D. melanogaster and D. simulans genomes have diverged in TE content, density, and activity. TEs are highly dynamic and rapidly evolving, and it is possible that the sequence evolution of TEs and other rapidly evolving repetitive elements has contributed to the divergence of Hmr. !