Evolutionary change in fatty acid synthase expression underlies ecological divergence and reproductive isolation in a pair of Australian Drosophila species. Henry Chung¹, David Loehlin¹, Kathy Vacarro¹, Heloise Dufour¹, Jocelyn Millar², Sean Carroll¹. 1) HHMI and Laboratory of Molecular Biology, University of Wisconsin, Madison, WI; 2) 2Department of Entomology, University of California, Riverside, CA.

   Evolutionary changes in traits during ecological adaptation may contribute to reproductive isolation and speciation if they also play a role in mating. However, the genes underlying the production of such dual traits and the functional evolutionary changes within them have largely not been identified. Methyl-branched cuticular hydrocarbons (CHCs) of insects are potentially one such trait. These compounds can protect animals from desiccation but also have roles in sexual signaling, as in Drosophila serrata, a fruit fly widely distributed in Australia. Its rainforest-restricted sibling species, D. birchii, in contrast, produces low amounts of methyl-branched CHCs, and is extremely sensitive to desiccation. Here, we identify a fatty acid synthase gene, mFAS (CG3524), that is responsible for the production of methyl branched CHCs in Drosophila oenocytes, and show that mFAS expression is undetectable in D. birchii oenocytes. We demonstrate that transgenic RNAi-mediated knockdown of mFAS in D. serrata dramatically reduces desiccation resistance, as well as mating success. We suggest that ecologically-influenced changes in the expression of mFAS in the evolving D. birchii lineage have contributed to the reproductive isolation between the two species.