To elucidate the evolution of germ cell specification in Metazoa, recent comparative studies focus on ancestral animal groups. Here, we followed the germline throughout the life cycle of the polychaete annelid Platynereis dumerilii, by examining mRNA and protein expression of vasa and other germline-specific factors in combination with lineage tracing experiments. In the fertilised egg, maternal Vasa protein localises to the yolk-free cytoplasm at the animal pole. It then asymmetrically segregates first into the micromeres, then into the founder cells of the mesodermal posterior growth zone (MPGZ). Vasa transcripts initially show ubiquitous distribution, but then become progressively restricted to the MPGZ. The cells of the MPGZ are highly proliferative, as evidenced by BrdU pulse labelling experiments. Besides vasa, they express nanos along with the stem cell-specific genes piwi, and PL10. At 4 days of development, four primordial germ cells are singled out from within the MPGZ, and migrate into the anterior segments to colonise a newly discovered "primary gonad". Our data suggest a common origin of germ cells and of somatic stem cells, similar to the situation found in planarians and cnidarians, which may constitute the ancestral mode of germ cell specification in Metazoa.

BackgroundIn the polychaete Platynereis, the primordial germ cells (PGCs) emerge from the vasa, piwi, and PL10 expressing mesodermal posterior growth zone (MPGZ) at the end of larval development, suggesting a post-embryonic formation from stem cells.MethodsIn order to verify this hypothesis, embryos and larvae were pulse labeled with the proliferation marker 5-ethynyl-2'-deoxyuridine (EdU) at different stages of development. Subsequently, the PGCs were visualized in 7-day-old young worms using antibodies against the Vasa protein.ResultsSurprisingly, the primordial germ cells of Platynereis incorporate EdU only shortly before gastrulation (6-8 hours post fertilization (hpf)), which coincides with the emergence of four small blastomeres from the mesoblast lineage. We conclude that these so-called 'secondary mesoblast cells' constitute the definitive PGCs in Platynereis. In contrast, the cells of the MPGZ incorporate EdU only from the pre-trochophore stage onward (14 hpf).ConclusionWhile PGCs and the cells of the MPGZ in Platynereis are indistinguishable in morphology and both express the germline markers vasa, nanos, and piwi, a distinct cluster of PGCs is detectable anterior of the MPGZ following EdU pulse-labeling. Indeed the PGCs form independently from the stem cells of the MPGZ prior to gastrulation. Our data suggest an early PGC formation in the polychaete by preformation rather than by epigenesis.

Germ cells in hydrozoans arise lifelong from multipotent interstitial stem cells. To discover if a true germline-soma segregation exists in these species, we studied gametogenesis in Hydractinia echinata using in situ hybridization and immunohistochemistry for the germ cell marker Vasa. We could show that Hevas is a zygotic transcript, present in embryos from the gastrula stage onward. In the planula larva, Hevas is expressed in the interstitial stem cells located in the endoderm. During metamorphosis, Hevas-expressing cells appear in the ectoderm in the lower half of the polyp. While the Hevas transcript is not detectable in developing gametes, the protein accumulates during oogenesis. Vasa containing granules are detectable at the polar-body-forming pole after fertilization. These results suggest that, in Hydractinia, maternal Vasa protein, but not the mRNA, is a maternal constituent of a germ plasm and might be involved in the specification and maintenance of interstitial stem cells.

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Guidance of cells and tissue sheets is an essential function in developing and differentiating animal tissues. In Hydra, where cells and tissue move dynamically due to constant cell proliferation towards the termini or into lateral, vegetative buds, factors essential for guidance are still unknown. Good candidates to take over this function are fibroblast growth factors (FGFs). We present the phylogeny of several Hydra FGFs and analysis of their expression patterns. One of the FGFs is expressed in all terminal regions targeted by tissue movement and at boundaries crossed by moving tissue and cells with an expression pattern slightly differing in two Hydra strains. A model addressing an involvement of this FGF in cell movement and morphogenesis is proposed: Hydra FGFf-expressing cells might serve as sources to attract tissue and cells towards the termini of the body column and across morphological boundaries. Moreover, a function in morphogenesis and/or differentiation of cells and tissue is suggested.