The germline of multicellular animals is segregated from somatic tissues, which is an essential developmental process for the next generation. Although certain ecdysozoans and chordates segregate their germline during embryogenesis, animals from other taxa segregate their germline after embryogenesis from multipotent progenitor cells. An overlapping set of genes, including vasa, nanos and piwi, operate in both multipotent precursors and in the germline. As we propose here, this conservation implies the existence of an underlying germline multipotency program in these cell types that has a previously underappreciated and conserved function in maintaining multipotency.Key words: Germline, Stem cell, Vasa, Piwi, Nanos, Sea urchin Introduction Multicellular animals consist of specialized cells, most of which are differentiated and devoid of reproductive potential. However, some cells, such as stem cells, retain both the ability to self-renew and to create new differentiated cells. One such stem cell, the germline stem cell (GSC), gives rise to a continual supply of germ cells, which are specialized for the task of reproduction. Yet once egg and sperm fuse, the resulting zygote is totipotent, and thus can give rise to all cell types of the animal. Although GSCs normally give rise only to germ cells in vivo, and thus are unipotent, a significant body of evidence shows that these cells maintain the potential to acquire many cell fates. For example, mouse primordial germ cells (PGCs) acquire the morphological and cytological characteristics of embryonic stem (ES) cells when cultured in the presence of three growth factors: steel factor, leukemia inhibitory factor (LIF) and fibroblast growth factor (FGF) (Matsui et al., 1992). These cells create teratomas when injected into adult mice and chimeric pups when injected into blastocysts, thus demonstrating their pluripotency. Isolated human PGCs can also be converted to pluripotent stem cells without genetic manipulation (Shamblott et al., 1998). Furthermore, pluripotent cells spontaneously arise from isolated adult mouse spermatogonial stem cells under specific culture conditions (Guan et al., 2006). This phenomenon is not limited to vertebrates as mutations in the translational regulators mex-3 and gld-1 in Caenorhabditis elegans leads to transdifferentiation of germ cells into muscle, neurons and intestinal cells (Ciosk et al., 2006). Thus, although germ cells are highly specialized, they seemingly retain the capacity to give rise to all cell types. As such, it is possible that the developmental potential of GSCs in these organisms is not intrinsically limited but might instead be restricted by environment. What, then, is the underlying regulatory program that controls this highly potent cell type?The molecular regulation of the germline is best understood in a handful of animals, including in C. elegans, Drosophila melanogaster and the mouse. Here, we explore the diversity of germline origins more broadly by examining germline segregation mechanisms across diverse taxa f...

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A crucial event in animal development is the specification of primordial germ cells (PGCs), which become the stem cells that create sperm and eggs. How PGCs are created provides a valuable paradigm for understanding stem cells in general. We find that the PGCs of the sea urchin Strongylocentrotus purpuratus exhibit broad transcriptional repression, yet enrichment for a set of inherited mRNAs. Enrichment of several germline determinants in the PGCs requires the RNA-binding protein Nanos to target the transcript that encodes CNOT6, a deadenylase, for degradation in the PGCs, thereby creating a stable environment for RNA. Misexpression of CNOT6 in the PGCs results in their failure to retain Seawi transcripts and Vasa protein. Conversely, broad knockdown of CNOT6 expands the domain of Seawi RNA as well as exogenous reporters. Thus, Nanos-dependent spatially restricted CNOT6 differential expression is used to selectively localize germline RNAs to the PGCs. Our findings support a 'time capsule' model of germline determination, whereby the PGCs are insulated from differentiation by retaining the molecular characteristics of the totipotent egg and early embryo.