Male-biased retrogenes in Drosophila melanogaster: integration of novel genes into pathways governing sperm development

Sexual selection and co-evolution with the female reproductive tract drives the rapid evolution of the Drosophila sperm proteome. It is important to reveal the genetics underlying the rapid evolution, and understand that the sperm proteome often evolves via turnover in composition, including the incorporation of novel proteins. Retrotransposition is one mechanism that gives rise to novel genes; this process involves the mRNA from an ancestral gene being converted back to DNA and incorporated back into the genome. Genes originating through the processes of retrotransposition often acquire testis-specific expression, and a novel role in spermatogenesis. Further, in Drosophila, retrogenes with testis-specific expression are enriched for putative roles in mitochondrial energetic pathways. The mitochondria in sperm cells fuse to form the nebenkern which associates with microtubules to provide crucial support during sperm tail elongation. Evidence suggests that tail length affects competitive ability within Drosophila melanogaster, and that individuals that produce longer sperm have an advantage in positioning sperm relative to the female seminal receptacle. Through the use of the UAS/GAL4 system for RNA interference we targeted twelve testis-specific, mitochondrial enriched retrogenes and assessed knockdown phenotypes via sperm tail measurements, seminal vesicle imaging, and fertility assays. Two RNAi lines demonstrated significant deviation from control sperm length measurements, indicating a role for these genes during sperm elongation. Three additional RNAi lines showed a reduced ratio of hatched to laid eggs, indicating that retrogenes participate in mitochondrial roles other than supporting tail elongation. Our work helps deepen the understanding of retrogene function, and the mechanisms that drive rapid sperm evolution