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...