| Summary: | The life cycle of many organism features an early moment in which cells split between two lineages, the somatic line and the germ line. These represent, respectively, the beginning of body architecture in the present generation, and the beginning of body architecture in the following generation. The separation of germ cells is a key point for the development of an organism and occurs by specification. This process has two main modes of action, the ancestral one, called epigenesis, and the derived one, called preformation. In this work we investigated bivalves (Mollusca Bivalvia), a clade in which preformation has evolved. Despite the relevance of these animals in marine ecosystems, their mechanism of germ line speci cation and seasonal gonad reconstitution lacks a detailed study. We thus analyzed bivalve germ line development by tracking the expression of VASA, a protein often used as germ line marker. We used specific antisera produced against the VASA homolog of Ruditapes philippinarum (Subclass Heterodonta, Family Veneridae). We compared the known developmental pattern of R. philippinarum to that of two species of the Subclass Pteriomorphia, Anadara kagoshimensis (Family Arcidae) and Crassostrea gigas (Family Ostreidae), and another species of the Subclass Heterodonta, Mya arenaria (Family Myidae). The immunohistological data obtained support for the two Subclasses a similar mechanism of primordial germ cells proliferation among the columnar epithelium of the gut. Indeed, given the taxonomic separation of the analyzed species, pertaining to two highly divergent clades, we suggest that the observed seasonal migration to the reconstituting gonad, associated with the timing of gonad maturation, could be a shared feature of bivalve molluscs. The comparative study of bivalve reproductive biology using the germ line determinant VASA can add details on the preformation mechanism and help to depict a more comprehensive picture of the role that this protein plays during cell specification.
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