Abstract
Sperm exhibit dramatic evolutionarily divergent morphologies in almost all taxa. Some sexually reproductive species show polymorphisms in the sperm produced by single males. Here, we focused on Sex-lethal (Sxl), which is the master sex-determination gene in Drosophila melanogaster, and investigated its function in the lepidopteran insect Bombyx mori. Our genetic analyses revealed that Sxl is essential for the formation of anucleate nonfertile parasperm. It is not expected that Sxl would be involved in sperm polymorphisms. Yet, whereas many morphological observations and ecological surveys have been conducted on sperm polymorphisms, this paper identifies the gene involved in sperm polymorphisms. Moreover, we clearly demonstrate that parasperm of B. mori is necessary for sperm migration in female organs.
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 Sex-lethal (Sxl), which is the master sex-determination gene in Drosophila melanogaster and is necessary for female germline development. It has been widely shown that the sex-determination function of Sxl in Drosophilidae species is not conserved in other insects of different orders. We investigated the function of Sxl in the lepidopteran insect Bombyx mori. 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 Sxl mutants revealed that the gene is indispensable for proper morphogenesis of apyrene sperm. Similarly, our analyses using Sxl 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 B. mori. Although Sxl is essential for oogenesis in D. melanogaster, it also plays important roles in spermatogenesis in B. mori. Therefore, the ancestral function of Sxl might be related to germline development.
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