Establishing the germline in spiralian embyos.

Elucidating the origin of germ cells in embryos and larvae is often obscured by the fact that the typical germ cell markers vasa, nanos and piwi are not exclusively expressed in primordial germ cells (PGCs), but are also commonly found in undifferentiated somatic tissues and stem cells as part of an evolutionary conserved 'germline multipotency program' (Juliano et al., 2010). Hidden in the crowd of undifferentiated cells, the PGCs have occasionally been overlooked and their formation during early embryogenesis was only revealed recently by new methodological approaches (e.g. Wu et al., 2011). Spiralians are excellent model organisms to deepen our understanding of PGC formation, given the highly stereotypical cleavage that occurs during embryogenesis. In these species, detailed cell lineage studies enable the tracing of single cells up to gastrulation stages. Here, I review our knowledge of the origin of PGCs in these invertebrates. Similarities in PGC formation among spiralian phyla as well as peculiarities of the highly derived clitellates are discussed with respect to developmental mode and evolution. Furthermore, the issue of gonad regeneration in platyhelminths and the asexually reproducing oligochaete Enchytraeus japonensis is addressed. An alternative strategy of compensating for caudal regeneration is presented for the polychaete Platynereis dumerilli. Finally, the molecular bases of PGC specification and the question of germplasm are discussed.