| Summary: | <p>Abstract</p> <p>Background</p> <p>Stem cells have a critical role during adult growth and regeneration. Germline stem cells are specialized stem cells that produce gametes during sexual reproduction. <it>Capitella teleta </it>(formerly <it>Capitella </it>sp. I) is a polychaete annelid that reproduces sexually, exhibits adult growth and regeneration, and thus, is a good model to study the relationship between somatic and germline stem cells.</p> <p>Results</p> <p>We characterize expression of the two <it>C. teleta </it>orthologs of <it>piwi</it>, genes with roles in germline development in diverse organisms. <it>Ct-piwi1 </it>and <it>Ct-piwi2 </it>are expressed throughout the life cycle in a dynamic pattern that includes both somatic and germline cells, and show nearly identical expression patterns at all stages examined. Both genes are broadly expressed during embryonic and larval development, gradually becoming restricted to putative primordial germ cells (PGCs) and the posterior growth zone. In juveniles, <it>Ct-piwi1 </it>is expressed in the presumptive gonads, and in reproductive adults, it is detected in gonads and the posterior growth zone. In addition, <it>Ct-piwi1 </it>is expressed in a population of putative PGCs that persist in sexually mature adults, likely in a stem cell niche. <it>Ct-piwi1 </it>is expressed in regenerating tissue, and once segments differentiate, it becomes most prominent in the posterior growth zone and immature oocytes in regenerating ovaries of regenerating segments.</p> <p>Conclusions</p> <p>In <it>C. teleta, piwi </it>genes may have retained an ancestral role as genetic regulators of both somatic and germline stem cells. It is likely that <it>piwi </it>genes, and associated stem cell co-regulators, became restricted to the germline in some taxa during the course of evolution.</p>
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