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.

Regulative capacity for eye formation by first quartet micromeres of the polychaete Capitella teleta.

The stereotypic cleavage pattern shared by spiralian embryos provides unique opportunities to compare mechanisms of cell fate specification of homologous blastomeres, and can give insights into how changes in fate may have influenced the evolution of novel structures and morphological diversity. The potential of cells to undergo regulation and the timing of cell fate specification were investigated during early development in the polychaete annelid, Capitella teleta. Targeted laser deletions of the first quartet micromeres were performed, with a focus on the eye-forming cells 1a and 1c. Most of the larvae resulting from deletion of the 1a or 1c micromeres lack both the pigment cell and sensory cell of the eye as predicted by the C. teleta fate map. In a minority of cases, however, both left and right larval eye spots develop, suggesting that other blastomeres within the embryo regulate for loss of these cells. Deletion of the 1a and 1c derivatives, 1a(1) or 1c(1), also largely result in larvae with one pigment spot, although there are larvae with two eye spots, suggesting that the ability to regulate for loss of an eye-generating cell persists for an additional cell cycle. Cell deletion in conjunction with intracellular labeling indicates that all four quadrants retain the ability to generate eyes, including those that normally do not. Deletion of all four first quartet micromeres provides evidence that only the first quartet micromeres have eye-forming potential. Additionally, in contrast to the right side of the head where larval and adult eye sensory cells are derived from the same cell (1c), on the left side, the larval and adult eye sensory cells are generated by different embryonic lineages. We hypothesize that cell-cell interactions and cell position are important for regulative ability in Capitella. To our knowledge, this is one of the first detailed deletion studies of the first quartet micromeres and the first convincing example of regulation in polychaetes, which are often thought to be non-regulative in nature.