[Report] Using transcriptomes to search for positive selection and adaptation in gene expression of Drosophila species

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Summary Findings

EVE model results. The EVE model implements a formal test for branch-specific shifts (selection) in gene expression levels while taking into account phylogenetic distances among species. I found several candidate genes in each species, among them GTPase, chromatin binding proteins and odor receptors. In almost all cases it was difficult to link the findings with potential cases of directional selection leading to adaptive traits for one particular species, which was the ultimate goal of the study. Importantly, I found 7 genes with signatures of positive selection in D. mojavensis. Out of these 7 genes two are involved in cuticle metabolism and one in chitin catabolism. D. Mojavensis is a fly species that is endemic to the deserts south to the United States and also north of Mexico. It has been proposed that in these locations it evolved and adapted to at least two things: 1) To live and feed exclusively from cactus; it’s a cactophilic species. 2) To live and resist the very high temperatures of the desert. To be able to resist high temperature it’s been shown and documented that it has a very distinct exoskeleton composition. The exoskeleton in Drosophila species consists mainly in a cuticle. In the specific case of D. mojavensis the cuticle has a very high proportion of long-chained lipids compared to other species. This peculiarity allows D. mojavensis to lose less water in the arid conditions of the dessert. Quitin is also a long-chained carbon polymer and a main component of the exoskeleton. To summarize, I found three genes with signals of selection at gene expression levels. This makes an interesting case of potential selection at gene expression levels leading to the evolution of an adaptive trait in D. mojavensis. Further investigation is required to test and confirm this hypothesis.

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