In most metazoans, germline precursors irreversibly segregate from somatic lineages during embryonic development. However, in animals that propagate asexually, germline precursors can originate in adults. Anthony De Tomaso and colleagues have been studying germline specification in Botryllus schlosseri, an emerging model of development and regeneration that propagates both sexually and asexually. They now report that self-renewing,lineage-restricted germline progenitors are embryonically specified in this colonial ascidian (see ). B. schlosseri begin life as free-living larvae that metamorphose into sedentary, colonial filter-feeders. B. schlossericolonies grow by asexual reproduction, regenerating their entire bodies,including their germline tissues, every week. The researchers show that, as in many metazoans, the expression of vasa (which encodes an ATP-dependent RNA helicase) marks a population of embryonically specified,long-lived germline progenitors in B. schlosseri. Interestingly,however, transient vasa knockdown disrupts somatic growth in the colony, which suggests that vasa might play a role in the asexual, as well as the sexual, development of this intriguing organism.
