EXPRESSION OF MITOCHONDRIAL AND NUCLEAR ELEMENTS DURING PRIMORDIAL GERM CELL PROLIFERATION AND DIFFERENTIATION IN THE BIVALVE SPECIES RUDITAPES PHILIPPINARUM (BIVALVIA VENERIDAE)

The first representatives of germ line to appear in the embryo are primordial germ cells (PGCs), and some conserved proteins play a role in their determination and specificity (e.g.: VASA). Also mitochondria have an active role in germ line development and to further understand this role, it is fundamental to deal with their inheritance dynamics. Metazoa generally experience strictly maternal inheritance (SMI) of mitochondria. The only known exception is represented by some bivalve molluscs that show doubly uniparental inheritance (DUI). Recently, a novel protein (RPHM21) was shown to be encoded by the male-transmitted mtDNA of the DUI species Ruditapes philippinarum. In silico analyses suggested its viral origin, and we hypothesized that the endogenization of a viral element provided sperm mitochondria with the ability to invade male germ line, thus being transmitted to the progeny. We investigated the dynamics of germ line development in relation to RPHM21 and expression patterns. We used specifically produced antibodies to detect germ cell proliferation and to compare the localization of VASPH (R. philippinarum VASA homolog) and RPHM21. Bivalves show a mechanism of seasonal gonad production, in which the gonad is re-adsorbed after spawning, and rebuilt de novo at the beginning of the subsequent reproductive season. Based on VASPH staining, we determined that in R. philippinarum the proliferation of PGCs begins among the simple columnar epithelium (batiprismatic cells) of the gut. Then PGCs appear to migrate in the connective tissue. Germ cells at initial stage of gametogenesis were also VASPH-stained. Since adult specimens showed an intense proliferation of PGCs in the gut, we deduced that this reservoir of PGCs migrate from the intestinal epithelium and reach the presumptive gonad localization where they rebuild the gonad at every reproductive season. Both VASPH and RPHM21 were localized in male PGCs, at one side of the cell cytoplasm, but while VASPH was detected in all PGCs, RPHM21 appeared to be expressed only in a subpopulation of them. Since RPHM21 was detected in all spermatozoa, we propose that the germ cells expressing it could gain advantage over the others during spermatogenesis. Specifically, RPHM21 might have a role in activation and proliferation of male PGCs, or might be involved in a process of active elimination of germ cells not expressing it, something that would resemble a meiotic drive.