Seasonal composition of immature germ cells in the Yesso scallop identified by vasa-like gene (my-vlg) and protein expression, with evidence of irregular germ cell differentiation accompanied with a high mortality event

The objective of this study was to understand germ cell renewal and differentiation in the Yesso scallop Mizuhopecten yessoensis throughout its reproductive cycle. First, we identified a Yesso scallop vasa-like gene (my-vlg) as a useful marker for immature germ cells. Both in situ hybridization and immunohistochemistry confirmed the germ-cell-specific expression of my-vlg and protein, especially in spermatogonia, oogonia, and primary oocytes. An immunohistochemistry with anti my-Vasa antibody further revealed that immature germ cells (i.e., gonial cells) are located on the basal membrane of germinal acini and are present during all maturational stages in both the ovary and testis. Quantitative analysis revealed a shift in germ cell number and change in germ cell composition associated with the gonadal maturation process, indicating a persistence of my-Vasa-positive immature germ cells throughout the reproductive cycle. In the testis, a gradual increase in my-Vasa-positive immature germ cells was seen from November to March. In the ovary, an increase in my-Vasa-positive immature germ cells was observed from September until November. We also used gonad index (GI) to summarize and evaluate the maturational changes of scallops in the 2018 generation. This generation had a background of relatively high mortality and shell deformity in the previous year, and we noted a small irregular peak of GI in the summer (July–August). We confirmed that the germ cells proliferating in the summer were my-Vasa-positive, implying that germ cell developmental progression occurred in the 2018 generation during the non-reproductive period. In conclusion, this study establishes an immuno-identification method for my-Vasa-positive immature germ cells and reveals germ cell composition in a bivalve species throughout its reproductive cycle. These results will support monitoring techniques for gonad development and the reproductive status of brood stock in shellfish seed production hatcheries.