Early embryonic transcription of Vasa in the Mediterranean mussel reveals divergent routes to germline specification in bivalves

Background The process of germline specification is a central topic in biology, and recent findings have revealed a diverse spectrum of mechanisms that extend beyond the traditional dichotomy of epigenesis and preformation. In bivalves, germline biology is particularly intriguing, as gonads are formed anew at every reproductive season. Nonetheless, the origin of primordial germ cells (PGCs) remains largely unexplored. Results Here, we investigate PGC specification during the first 48 hours of development in Mytilus galloprovincialis . Combining hybridization chain reaction and bulk transcriptomics, we show that the transcriptional profile of the germline marker Vasa markedly differs from that reported in the Pacific oyster Crassostrea gigas . Given the unique doubly uniparental inheritance of mitochondria in M. galloprovincialis , we further examined the early transcription of three putative sex-determination related genes— Fox-L2 , Dmrt-1L , and Sox-H —in order to assess whether they had any sex biased transcription. Only Fox-L2 was transcriptionally active, although without any clear evidence of sex-specific patterns. Conclusions Together, our results reveal a high diversity in PGC specification among bivalves and challenge the assumption of a conserved developmental program, even among closely related species. Additionally, our results consolidate M. galloprovincialis as a valuable system for investigating the evolution of PGC specification and sex-related developmental pathways, and highlight the importance of adopting the comparative approach for understudied animal groups.