Germline and male somatic determinants of sperm length divergence in Drosophila: implications for postcopulatory sexual selection.

Sperm display remarkable diversity in size and morphology, especially in insects. Sperm length varies by several hundred-fold within Drosophila alone. In the model species Drosophila melanogaster, sperm are approximately 1.7 times longer than those of its close relatives Drosophila simulans and Drosophila mauritiana. However, the developmental mechanisms underlying such rapid divergence in sperm traits remain poorly understood. Somatic hub and cyst cells are essential for the maintenance of germline stem cells in Drosophila spermatogenesis, yet their contribution to cell growth later in spermatogenesis remains unclear. Here, we used primordial germ cell (PGC) xenotransplantation to determine whether sperm length regulation is cell autonomous or influenced by the somatic environment of the testes. When D. simulans PGCs were transplanted into agametic D. melanogaster embryos, the resulting sperm were indistinguishable in length from those of D. simulans donor males and remained significantly shorter than the sperm of D. melanogaster males. Our results suggest that the dramatic diversification in sperm length shaped by postcopulatory sexual selection in the Drosophila lineage has been largely driven by cell-autonomous modifications in the germ line.