Regeneration of the germline in the annelid Capitella teleta.

The germline is essential for sexual reproduction and survival of the species. In many metazoans, the developmental potential to generate a distinct germline is segregated from somatic cell lineages early in embryogenesis, suggesting that the unique features of the germline must be established from its onset. Previous studies suggest that germ cells cannot regenerate once removed from the embryo, but few animals have been experimentally tested. We investigated the ability of the germline to regenerate in a lophotrochozoan, the segmented worm Capitella teleta, which has a stereotyped cell lineage program by deleting the germline precursor (cell 3D) in early stage embryos using an infrared laser. Larvae and juveniles resulting from germline deletions were examined for presence of multipotent progenitor cells (MPCs), stem cells that form the germ cells and somatic stem cells. In contrast to control deletions of a non-germline macromere, most larvae resulting from deletion of cell 3D lacked MPCs as assayed by expression of germline markers CapI-vasa, CapI-nanos and Ct-piwi1, but showed persistent expression of these markers in the somatic posterior growth zone. However, approximately 13% of experimental larvae had MPCs, indicative of some germline regeneration. In contrast, by two weeks post-metamorphosis, all juveniles resulting from deletion of cell 3D had MPCs, as detected by CapI-vasa expression. Furthermore, when raised to adulthood, most animals developed reproductive structures and were fertile. In another set of deletions, both the D quadrant mesodermal and germline progenitors were removed. These juveniles also regenerated MPCs. Surprisingly, this deletion caused substantial ectopic expression of CapI-vasa and CapI-nanos in other larval tissues. Our results indicate that C. teleta can regenerate the germline following removal of the germline progenitors in the early embryo. The dramatic difference in ability to regenerate the germline between the larval and adult stages suggests that there are two distinct compensation events at two phases of the life cycle: a regulative event in the early stage larva and a stem cell transition event after metamorphosis, when the animals are capable of substantial body regeneration.