Somatic and germline expression of piwi during development and regeneration in the marine polychaete annelid Capitella teleta

We characterize expression of the two C. teleta orthologs of piwi, genes with roles in germline development in diverse organisms. Ct-piwi1 and Ct-piwi2 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, Ct-piwi1 is expressed in the presumptive gonads, and in reproductive adults, it is detected in gonads and the posterior growth zone. In addition, Ct-piwi1 is expressed in a population of putative PGCs that persist in sexually mature adults, likely in a stem cell niche. Ct-piwi1 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.In C. teleta, piwi genes may have retained an ancestral role as genetic regulators of both somatic and germline stem cells. It is likely that piwi genes, and associated stem cell co-regulators, became restricted to the germline in some taxa during the course of evolution.Stem cells are essential for animal development and adult tissue homeostasis, and they can presumably differentiate into many specialized cell types. Specialized stem cells called primordial germ cells (PGCs) are populations of undifferentiated stem cells in sexually reproducing animals that will exclusively give rise to the germ cells, either spermatocytes or oocytes [1]. These germline stem cells insure that genetic information is passed to the next generation. In some animals, germline stem cells are segregated from somatic cells during embryonic development. Two distinct mechanisms of germline specification have been described: preformation and epigenesis [2]. According to the preformationist mode, germ cells are specifi