The mechanisms of primordial germ cell determination during embryogenesis in molluscan species

Primordial germ cells (PGCs) are the first germ cells distinguishable from surrounding somatic cells during embryogenesis. In many animal species PGC specification is generally classified into two modes: preformation, in which PGCs are determined by maternally inherited components in early development, and epigenesis, in which PGCs arise from proximal somatic cells by inductive signal in late development. In this review we focused on the process of PGC formation in molluscan bivalves and gastropods and compared the PGC specification modes of these two classes. Several reports indicated that bivalves tend to adopt preformation, as maternally inherited germline-specific genes are transcribed in specific blastomeres differentiating into PGCs. In gastropods, maternal germline-specific genes are transcribed in mesodermal stem cells. PGCs seem to be epigenetically determined from mesodermal stem cells by inductive signals after the veliger larval stage, which indicates that PGC precursor cells not only generate germline tissues but also generate mesodermal somatic tissues. The common origin of germline and mesodermal somatic tissues is observed in the annelids Platyhelminthes and Cnidaria, and is considered to be an ancient mode of germ cell determination. We suggest that gastropods retain the ancient PGC specification mode, while bivalves switch their PGC specification mode to preformation.