A novel role for Eip74EF in male reproduction in promoting sperm elongation at the cost of fecundity

Spermatozoa are the most morphologically variable cell type, yet little is known about genes controlling natural variation in sperm shape. Drosophila fruit flies have evolved the longest sperm known, which are evolving under postcopulatory sexual selection, driven by sperm competition and cryptic female choice. Long sperm outcompete short sperm but primarily when females have long seminal receptacles (SRs), the primary sperm storage organ. Thus, selection on sperm length is mediated by SR length, and the two traits are coevolving across the Drosophila lineage, driven by a genetic correlation and fitness advantage of long sperm and long SR genotypes in both males and females. Ecdysone induced protein 74EF (Eip74EF) is rapidly evolving under positive selection in Drosophila, and it is expressed during post-meiotic stages of spermatogenesis, when spermatid elongation occurs. Partial knockdown of Eip74EF leads to shorter sperm but does not affect SR length, suggesting that Eip74EF is involv...