Germ line dynamics in bivalve molluscs

Studies about germ line specification in Mollusca have been neglected for years. Available data based on few species indicate that molluscs exhibit the same cleavage mechanism of other spiralians, in which much of the mesoderm comes form the 4d blastomere, including the germinal tissue, whose specification has been identified by localizing the products of vasa orthologs (VASA is highly conserved in the animal kingdom). Despite the relevance of bivalve molluscs in marine ecosystems and aquaculture, their mechanism of sex determination is still unknown, as well as the details of their seasonal gonad reconstitution (i.e. the gonad is reabsorbed after spawning and reconstructed at the beginning of the subsequent reproductive season). In order to determine general features of bivalve germ line development, we employed two antibodies produced against the VASA ortholog of Ruditapes philippinarum (Subclass Heterodonta, Family Veneridae) to investigate three additional species. We chose two species of the Subclass Pteriomorphia, Scapharca inaequivalvis (Family Arcidae) and Crassostrea gigas (Family Ostreidae), and another species of the Subclass Heterodonta, Mya arenaria (Family Myidae). The immunoreactivity of anti-VASA was confirmed by Western Blot in which single specific bands were obtained in each species, although of different molecular weight. On the other hand, the presence of two different bands in males and females of the gonochoric R. philippinarum has been previously related to sex-biased isoforms, already found in other animal species outside bivalves. The detection of a single band in the two Pteriomorphia may be tentatively related to their state of proterandric hermaphrodites. M. arenaria is reported to be gonochoric, as R. philippinarum, but the analyzed specimens were all females, so we cannot exclude the existence of different sex-related isoforms, although no sign of a second band was present. The immunohistological data obtained support a conserved mechanism of proliferation of primodial germ cells (showing the VASA labeling at one side of the cell cytoplasm) among the simple columnar epithelium of the gut. Moreover, their seasonal migration to the reconstituting gonad appears to be a common feature of bivalves. The study of bivalve reproductive biology can clarify important aspects of their development, but can also be useful for conservation management and breeding programs.