Dimorphic sperm formation by <i>Sex-lethal</i>.

Abstract

Sex is determined by diverse mechanisms and master sex-determination genes are highly divergent, even among closely related species. Therefore, it is possible that homologs of master sex-determination genes might have alternative functions in different species. Herein, we focused on <i>Sex-lethal</i> (<i>Sxl</i>), which is the master sex-determination gene in <i>Drosophila melanogaster</i> and is necessary for female germline development. It has been widely shown that the sex-determination function of <i>Sxl</i> in Drosophilidae species is not conserved in other insects of different orders. We investigated the function of <i>Sxl</i> in the lepidopteran insect <i>Bombyx mori</i> In lepidopteran insects (moths and butterflies), spermatogenesis results in two different types of sperm: nucleated fertile eupyrene sperm and anucleate nonfertile parasperm, also known as apyrene sperm. Genetic analyses using <i>Sxl</i> mutants revealed that the gene is indispensable for proper morphogenesis of apyrene sperm. Similarly, our analyses using <i>Sxl</i> mutants clearly demonstrate that apyrene sperm are necessary for eupyrene sperm migration from the bursa copulatrix to the spermatheca. Therefore, apyrene sperm is necessary for successful fertilization of eupyrene sperm in <i>B. mori</i> Although <i>Sxl</i> is essential for oogenesis in <i>D. melanogaster</i>, it also plays important roles in spermatogenesis in <i>B. mori</i> Therefore, the ancestral function of <i>Sxl</i> might be related to germline development.