Background: Gonadal development is driven by a complex genetic cascade in vertebrates. However, related information remains limited in molluscs owing to the long generation time and the difficulty in maintaining whole life cycle in the lab. The dwarf surfclamMulinia lateralisis considered an ideal bivalve model due to the short generation time and ease to breed in the lab.Results: To gain a comprehensive understanding of gonadal development inM. lateralis, we conducted a combined morphological and molecular analysis on the gonads of 30 to 60 dpf. Morphological analysis showed that gonad formation and sex differentiation occur at 35 and 40–45 dpf, respectively; then the gonads go through gametogenic cycle. Gene co-expression network analysis on 40 transcriptomes of 35–60 dpf gonads identifies seven gonadal development-related modules, including two gonad-forming modules (M6, M7), three sex-specific modules (M14, M12, M11), and two sexually shared modules (M15, M13). The modules participate in different biological processes, such as cell communication, glycan biosynthesis, cell cycle, and ribosome biogenesis. Several hub transcription factors includingSOX2,FOXZ,HSFY, FOXL2andHES1are identified. The expression of top hub genes from sex-specific modules suggests molecular sex differentiation (35 dpf) occurs earlier than morphological sex differentiation (40–45 dpf).Conclusion: This study provides a deep insight into the molecular basis of gonad formation, sex differentiation and gametogenesis inM. lateralis, which will contribute to a comprehensive understanding of the reproductive regulation network in molluscs.

Vasa (Ddx4, DEAD box polypeptide 4), an extremely specific marker of germ cells in vivo, is an ATP-dependent RNA helicase that plays an essential role in germ cell development and gametogenesis. However, the expression and function information about this gene in groupers remains lacking. Here, vasa homolog termed Plvasa was isolated and identified Plvasa as a putative germ cell marker in the leopard coral grouper, (Plectropomus leopardus). Results indicated that Plvasa contained 17 exons in the genomic sequence and 9 conserved motifs of the DEAD-box protein by sequence analysis. The sequence comparison, phylogenetic analyses and synteny analyses showed that Plvasa was homologous with other teleosts. Additionally, the expression of Plvasa was significantly higher in gonads than in other tissues in adult individuals (p < 0.05). Further, the distribution of Plvasa revealed that it was only expressed in the germ cells, such as spermatids, germline stem cells and oocytes at different stages, and could not be detected in the somatic cells of gonads. The current study verified that the Plvasa gene is a valuable molecular marker of germ cells in leopard coral grouper, which potentially plays an important role in investigating the genesis and development of teleost germ cells.

The scallops of the genus Argopecten hold a significant position in the international shellfish aquaculture industry. However, due to their hermaphroditic nature, selfing reproduction has led to an increasingly severe phenomenon of germplasm degradation. The development of interspecific hybridization can effectively address this issue, but the fertility of interspecific hybrid offspring is greatly affected. Backcrossing appears to be a more practical and feasible solution, yet current research in this regard remains extremely scarce. In this study, the F1 generation of bay–Peruvian hybrid scallops (Aip derived from Ai ♀ × Ap ♂) and Peruvian–bay hybrid scallops (Api derived from Ap ♀ × Ai ♂) were successfully backcrossed with Peruvian scallops (Argopecten purpuratus, Ap) and bay scallops (Argopecten irradians, Ai) to produce F2 backcrossed population (Aipi derived from Aip ♀ × Ai ♂; Aipp derived from Aip ♀ × Ap ♂; Apii derived from Api ♀ × Ai ♂ and Apip derived from Api ♀ × Ap ♂). These F2 populations were comprehensively analyzed in terms of heterozygosity, gonadal development and differentiation, fertility, genetic diversity, and offspring identification. The results revealed that the fertilization and hatching rates of the F2 generation were significantly lower than those of the parental generations. However, they exhibited normal development through larval, juvenile, and adult stages, albeit with prolonged larval development periods. Notably, the juvenile and adult stages demonstrated significant hybrid vigor. The F2 generation retained hermaphroditic characteristics but also included individuals with exclusively female gonads. Cloning of Dmrt1 and Foxl2 genes in these scallops showed high similarity with the bay scallop’s corresponding genes, and Dmrt1 is specifically expressed in the testis, while Foxl2 is specifically expressed in the ovary, as is the case in the bay scallop. Eleven pairs of polymorphic loci were selected from developed microsatellite markers for bay scallops, and the analysis revealed that the polymorphism levels in the F2 backcrossed population were higher than those in purebred parental self-crosses. Additionally, the reliability of parental genetic information in the backcrossed population has also been corroborated according to the scallop descent identification experiments. The findings of this study enhance the understanding of Aipi, Aipp, Apii, and Apip, providing a theoretical foundation for the selection and breeding of scallop species with superior traits and addressing inbreeding decline in scallop culture.