The size of the sperm head influences the gynogenetic success in teleost fish

Gynogenesis is a phenomenon in which only the maternal genome contributes to offspring development. Artificial gynogenesis induced using genetically inactivated sperm is a useful technique for sex control and rapid production of inbred lines in aquaculture and breeding. For gynogenesis induction, eggs require fertilization stimulus from the sperm; however, the effects of sperm characteristics on gynogenetic success remain unexplored. In this study, we investigated the induction of gynogenesis in medaka embryos by sperm from distant species and the correlation of sperm head size relative to the micropyle with success in gynogenesis. Results revealed that sperm heads of Nile tilapia and rainbow trout had smaller areas than those of the medaka micropyle, and thus, the sperm successfully activated medaka eggs for development. Furthermore, gynogenetic haploid embryos could be induced using genetically inactivated sperm from these species, which were viable after diploidization using heat shock treatment. In contrast, goldfish and zebrafish sperm, with larger head areas than that of medaka micropyle, could not activate medaka eggs for development, suggesting that sperm entry into medaka micropyle was inhibited. Therefore, we propose that the relative physical features of sperm and eggs influence the success of gynogenesis. The findings provide new insights into the application of gynogenesis in aquacultural fish. We developed an automatic method for measuring sperm head size, which can facilitate the recording of physical characteristics of sperm and predicting gynogenesis success.