Early germline specification in bivalves: searching for OSKAR functional homolog

Germ cells play a unique role in heredity and evolution as carriers of genetic information across generations, but the study of the early stage of germ cell specification is still an overlooked issue.In this study, we documented the earliest stages of germ cell specification in Ruditapes philippinarum (the Manila clam), describing both the mechanism and timing. Initially, we focused our investigation on OSKAR, the necessary and sufficient factor involved in the earliest stage of Drosophila germline specification. On the basis of bioinformatic analyses we identified a Tudor domain-containing protein 7 (TDRD7) as the possible best candidate acting in the corresponding stage of germline specification in R. philippinarum. This hypothesis is supported by domain similarities between OSKAR and TDRD7, as well as in situ localization of the latter in putative germ cells in their earliest stage of differentiation from germline stem cells.In this light, germline specification appears to show evolutionarily conserved features, while maintaining some kind of plasticity, thus allowing differences among different taxa. Although OSKAR orthologues are not known outside insects, a corresponding role in other animals has to be carried out, possibly by different proteins with similar functions. We propose TDRD7 as a germline factor acting at early stages of germline specification in R. philippinarum, thus a good candidate as OSKAR functional-homolog.