Ultrastructural observations of eu- and paraspermiogenesis in the cottid fish Hemilepidotus gilberti (Teleostei: Scorpaeniformes: Cottidae)

Lacking of sex-lethal gene lowers the fertility of male reproduction in Spodoptera litura (Lepidoptera)

Sex-lethal (Sxl) encodes an RNA-binding protein that acts as the switch of sex determination in Drosophila and influences the genitalia formation and gonadal development. However, its sex-determination roles are not conserved in all insects and its role in the gonadal development of Lepidoptera is not well documented. In this study, three splicing variants of Sxl mRNA were identified in Spodoptera litura and they highly expressed in gonads, particularly in the testis. The mRNA levels of SlSxl exhibited higher expression in the spermatid than the testis sheaths, and gradually increased with the spermiogenesis. Sex-lethal protein (SlSXL) is mainly distributed in the cytoplasm of spermatocytes and the head of spermatid. Knockout of SlSxl resulted in fewer eupyrene sperm bundles and apyrene sperm bundles in the testes of moth and a large number of undeveloped spermatocysts retained in the moth of mutant testis, and leading to the reduction of oviposition and hatch rate in the offsprings after mating with female. These results suggest that SlSxl is a critical player in the spermiogenesis of S. litura.

Dysfunction of dimorphic sperm impairs male fertility in the silkworm

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.

Characterization of an alternative chromatin remodeling to parasperm in a cottid fish, Hemilepidotus gilberti

The dimorphic sperm of Hemilepidotus gilberti, i.e., haploid eusperm and diploid parasperm, have different morphologies corresponding to their own roles in fertilization. To estimate how these specific sperm morphologies were established, we focused on the nuclear morphologies and examined their changing processes in dimorphic spermiogenesis. Electron microscopic observation revealed that, in euspermatids, chromatin condensation first appeared as a mosaic pattern of moderate electrodense material in the peripheral region of the round nucleus. Those materials spread across the whole area to form a uniformly condensed nucleus. Chromatin condensation began similarly in paraspermatids to that in euspermatids. These became localized to one side of a nucleus and further condensed to form strong electrodense chromatin clusters, which are a specific feature of parasperm. From the remodeled nuclei of eusperm and parasperm, we found five and three kinds of sperm-specific basic proteins (SBPs), respectively, substituted to histones. The N-terminus amino acid sequences of the SBPs suggest that, in parasperm, one major SBP and two minor ones were distinct from each other. In eusperm nuclei, two kinds of specific SBPs were detected in addition to the homologs of parasperm SBPs. The specific SBPs had homologous amino acid sequences with huge arginine clusters, and one of them was most dominant among the five kinds of SBPs. The different combinations of SBPs in the eusperm and parasperm may cause a specific pattern of chromatin condensation in the dimorphic sperm nuclei of H. gilberti.

Partial formation of sperm dimorphism from spermatocytes of the cottoid fish, Hemilepidotus gilberti in cell culture

Polymorphism of sperm is considered to be significant for the reproductive strategy in some animal species. The phenomenon is thought to occur in the species-specific stage of spermatogenesis, but how the identical germ cells are differentiated towards polymorphic sperm remains unknown. We here performed a germ cell culture in the cottoid fish, Hemilepidotus gilberti, whose sperm exhibit dimorphism with fertilizable eusperm and unfertilizable parasperm. In the culture, germ cells, which were obtained with an identical morphology, a spherical shape of 5-7 microm in diameter, differentiated into smaller spherical cells with a single nucleus, a moving flagellum and localized mitochondria. In addition, large retroflex-shaped cells with two elongated nuclei were also observed in the cell culture. Germ cells that had each morphological feature were histologically also observed in some cysts of the spermatogenetic testis, suggesting that the former type of cell corresponded to developing eusperm and the latter corresponded to developing parasperm. When BrdU was incorporated into germ cells in the culture, it was detected in both cells with eusperm-like and those with parasperm-like morphologies. These findings suggest that DNA-duplicating spermatocytes are potent to autonomously progress a part of spermatogenesis to form dimorphic sperm.

Intratesticular signals for progression of germ cell stages in vertebrates

The mechanisms underlying the complexity of spermatogenesis and spermiogenesis have deeply been studied in recent years. Transgenic animals, gene-targeting techniques, and lower vertebrate animal models have led to the discovery of some of the intratesticular signals involved in germ cell progression. This review wish to give the state of the art about it with particular emphasis on the comparative approach.