Dysfunction of dimorphic sperm impairs male fertility in the silkworm

Sperm, which have a vital role in sexual reproduction in the animal kingdom, can display heteromorphism in some species. The regulation of sperm dichotomy remains a longstanding puzzle even though the phenomenon has been widely documented for over a century. Here we useBombyx morias a model to study a form of sperm dimorphism (eupyrene and apyrene sperm), which is nearly universal among Lepidoptera. We demonstrate thatB. mori Sex-lethal(BmSxl) is crucial for apyrene sperm development, and thatB. mori poly(A)-specific ribonuclease-like domain-containing 1(BmPnldc1) is required for eupyrene sperm development. BmSXL is distributed in the nuclei and cytoplasm of somatic cyst cells in a mesh-like pattern and in the cytoplasm of germ cells enclosed in spermatocysts and sperm bundles. Cytological analyses of dimorphic sperm inBmSxlmutants ( increment BmSxl) showed deficient apyrene sperm with abnormal nuclei, as well as loss of motility associated with malformed mitochondrial derivatives. We define the crucial function of apyrene sperm in the process of fertilization as assisting the migration of eupyrene spermatozoa from bursa copulatrix to spermatheca. By contrast,BmPnldc1deficiency ( increment BmPnldc1) caused eupyrene sperm abnormalities and impaired the release of eupyrene sperm bundles during spermiation. Although apyrene or eupyrene sperm defects impaired fertility of the mutated males, double copulation of a wild-type female with increment BmSxland increment BmPnldc1males could rescue the sterility phenotypes induced by single copulation with either gene-deficient male. Our findings demonstrate the crucial functions ofBmSxlandBmPnldc1in the development of sperm dimorphism and the indispensable roles of nonfertile apyrene sperm in fertilization.