Adaptation to various and extreme environments is key to long-term species persistence. Reducing water loss is important for organisms adapting to different terrestrial environments. In Drosophila fruit flies and other terrestrial insects, their small body size and large surface areas to volume ratios make them vulnerable to desiccation stress. Their ability to prevent water loss is crucial for their survival. Previous studies have suggested that cuticular water loss accounts for the majority of water loss in insects and hypothesized that differences in cuticular hydrocarbon (CHC) content accounted for differences in desiccation resistance between mesic and desert species. However, the specific association between different CHC components and desiccation has not been established, and the genetic mechanisms underlying the evolution of these CHC components that confer high desiccation resistance have not been elucidated. This dissertation investigated how the evolution of CHCs in insects affected desiccation resistance and elucidates the genetic mechanisms underlying their evolution. With a comprehensive association study of desiccation resistance and CHCs in 46 Drosophila species and 4 species in closely-related genera, the analyses showed that mbCHC chain lengths were important predictors of desiccation resistance and longer mbCHCs contributed to higher desiccation resistance. This dissertation further investigated the genetic and molecular mechanisms underlying longer chain mbCHCs and higher desiccation resistance in a desert Drosophila species, Drosophila mojavensis. A fatty acyl-CoA elongase gene, mElo (methyl-branched CHC Elongase), was identified in Drosophila species for the elongation of mbCHCs. Overexpression experiments in D. melanogaster demonstrated that coding changes in mElo from D. mojavensis lead to longer mbCHCs and higher desiccation resistance. Further experiments using CRISPR-Cas9 to knock out mElo from D. mojavensis showed that knockout of this gene decreased the production of the longest mbCHCs and significantly reduced desiccation resistance at their ecological-relevant temperature. Results from this dissertation elucidate the molecular and evolutionary mechanisms that enable species to reduce water loss and maintain water balance as our planet gets warmer and more arid in the next few decades.
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