Dysfunction of dimorphic sperm impairs male fertility in the silkworm

Shuqing Chen,Yongping Huang,Jun Xu,Xu Yang,Xingyu Luo,Zulian Liu,Yujia Liu

doi:10.1038/s41421-020-00194-6

Abstract

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 use Bombyx mori as a model to study a form of sperm dimorphism (eupyrene and apyrene sperm), which is nearly universal among Lepidoptera. We demonstrate that B. mori Sex-lethal (BmSxl) is crucial for apyrene sperm development, and that B. 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 in BmSxl mutants (∆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, BmPnldc1 deficiency (∆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 ∆BmSxl and ∆BmPnldc1 males could rescue the sterility phenotypes induced by single copulation with either gene-deficient male. Our findings demonstrate the crucial functions of BmSxl and BmPnldc1 in the development of sperm dimorphism and the indispensable roles of nonfertile apyrene sperm in fertilization.

Highlights

Remarkable diversity of sperm exist for sexual reproduction in Animalia
Using the cDNA of testes from fifth instar larvae as template, we performed PCR amplification of the B. mori Sex-lethal (BmSxl) open reading frames (ORF) and obtained four fragments, 804 base pairs, 873 bp, 942 bp, and 1011 bp in length, which we confirmed by DNA sequencing as four BmSxl variants (Supplementary Fig. S1a)
We found that BmSxl was highly expressed in sperm, whereas it was expressed at significantly lower levels in the testis carcass at all time points (Supplementary Fig. S1d)

Summary

Introduction

Males of some species exhibit sperm polymorphism, regularly producing multiple distinct classes of sperm by a single male[1]. Dichotomous spermatogenesis is found widely in phyla from invertebrates to vertebrates, including Mollusca[1,2], Annelida[3,4], Rotifera[5], Arthropoda[6,7,8], Echinodermata[9], and Chordata[10]. The regulatory mechanism for dichotomous spermatogenesis is still unclear at the molecular level. Sperm dichotomy is a characteristic of lepidopteran species, regularly generating two kinds of co-. Several species of the genus Drosophila bear a form of sperm dimorphism termed dimegaly, producing two size classes of nucleated sperm, which is different from the extreme sperm dichotomy seen in Lepidoptera[15,16,17,18]

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Results

Conclusion