Dysfunction of dimorphic sperm impairs male fertility in the silkworm

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.

Oligopaint DNA FISH as a tool for investigating meiotic chromosome dynamics in the silkworm,Bombyx mori

“…To further validate this finding, we repeated the experiment using chromosome 7 stripe paints in whole mount late 5 th instar larval testes (Figure S8). As we predicted based on our testes squashes and previous studies ( 54 and eupyrene (with nuclei) sperm (65)(66)(67)(68)(69)(70)(71). In whole-mount testes, we were clearly able to identify eupyrene secondary spermatocyte bundles (Figure 5C-3) and mature eupyrene sperm (Figures S10 and S11).…”

Silencing the Triacylglycerol Lipase (TGL) Gene Decreases the Number of Apyrene Sperm and Inhibits Oviposition in Sitotroga Cerealella

“…It promotes the separation of eupyrene sperm bundles and plays an important role in the formation or transport of eupyrene sperm [28,29]. Studies in silkworms have shown that the movement of apyrene sperm is a necessary condition for the transfer of eupyrene sperm from the bursa copulatrix to the spermatheca [30]. Combining the functional roles of apyrene sperm, we speculated that there is no significant difference in the number of eupyrene sperm between the two groups after mating, and the decrease in apyrene sperm may affect the transport from the bursa copulatrix to the spermatheca and fertilization process of eupyrene sperm in females and ultimately affect female egg production.…”

Positional Relationships among Male Reproductive Organs in Insects

“…In lepidopteran insects, there are fertile nucleated eupyrene sperm and infertile non-nucleated apyrene sperm [31][32][33][34][35]. Eupyrene sperm fertilize the eggs, whereas apyrene sperm cannot fertilize the eggs because they lose their nuclei during meiosis [36]. In general, apyrene sperm are produced more or transferred to females more than are eupyrene sperm [37][38][39], although spermatogenesis of eupyrene and apyrene sperm is not markedly different in the diamondback moth Plutella xylostella [40].…”