VASA expression suggests shared germ line dynamics in bivalve molluscs

Abstract

Germ line segregation can occur during embryogenesis or after embryogenesis completion, with multipotent cells able to give rise to both germ and somatic cells in the developing juvenile or even in adulthood. These undifferentiated cells, in some animals, are self-renewing stem cells. In all these cell lineages, the same set of genes, among which vasa, appears to be expressed. We traced VASA expression during the peculiar gonad rebuilding of bivalves to verify its presence from undifferentiated germ cells to mature gametes in an animal taxon in which the mechanism of germ line establishment is still under investigation. We utilized antibodies produced against VASPH, VASA homolog of Ruditapes philippinarum (Subclass Heterodonta), to compare the known expression pattern of R. philippinarum to two species of the Subclass Pteriomorphia, Anadara kagoshimensis and Crassostrea gigas, and another species of the Subclass Heterodonta, Mya arenaria. The immunohistological data obtained support a conserved mechanism of proliferation of “primordial stem cells” among the simple columnar epithelium of the gut, as well as in the connective tissue, contributing to the seasonal gonad reconstitution. Given the taxonomic separation of the analyzed species, we suggest that the process could be shared in bivalve molluscs. The presence of germ cell precursors in the gut epithelium appears to be a feature in common with model organisms, such as mouse, fruit fly, and human. Thus, the comparative study of germ line establishment can add details on bivalve development, but can also help to clarify the role that VASA plays during germ cell specification.Electronic supplementary materialThe online version of this article (doi:10.1007/s00418-017-1560-x) contains supplementary material, which is available to authorized users.

Early gametogenesis in the Pacific oyster: new insights using stem cell and mitotic markers

“…Likewise, Obata et al (2010) identified vasa-positive cells in Mytilus galloprovincialis, again suggesting the existence of GSCs in connective tissue during the non-reproductive season in mussel. Interestingly, as will be further discussed below, Milani et al (2015Milani et al ( , 2017 confirmed through confocal microscopy that Vasa is a germ cell marker in several species of bivalve.…”

“…Gametogenesis has long been of interest in bivalves (reviewed in Galtsoff, 1964), and has received renewed attention following the development of genomics in bivalves (Fabioux et al, 2004a(Fabioux et al, ,b, 2009Llera-Herrera et al, 2013;Milani et al, 2015Milani et al, , 2017Obata et al, 2010;Teaniniuraitemoana et al, 2017) and, more recently, because of current environmental concerns (Barranger et al, 2014;Cubero-Leon et al, 2012;de Sousa et al, 2014).…”

Germ cell line during the seasonal sexual rest of clams: finding niches of cells for gonad renewal

“…Recently it was hypothesized that during the gonad reconstituting phase, VASA-stained primordial stem cells (PriSCs) coming from the intestine may contribute to the rebuilding of reproductive tissue [14]. Actually, we suppose that the digestive epithelium could be a reservoir of multipotent cells-PriSCs-that can be deputed to multiple cell fate [14].…”

“…Actually, we suppose that the digestive epithelium could be a reservoir of multipotent cells-PriSCs-that can be deputed to multiple cell fate [14]. VASA-stained cells increase in number during gonad rebuilding and are present both in the intestine epithelium and in the connective tissue [13,14]. On the one hand, it is easy to assume that the VASA-positive cells in the connective tissue-were the new gonadic tissue is usually formed-are source of new gametogenic cells (see Online Resource 1 for confocal images of the reconstituting phase).…”

“…The spent phase is the less investigated stage of the cycle, but recent studies have pinpointed its potential value for the understanding of gonad rebuilding mechanisms in clams [13,14]. In this work, we analyzed such phase in the Manila clam Ruditapes philippinarum (Adams and Reeve, 1850), that in Po River Delta (Italy) occurs during the winter [15].…”