Cell Lineage and Fate Map of the Primary Somatoblast of the Polychaete Annelid Capitella teleta

Like most polychaete annelids, Capitella teleta (formerly Capitella sp. I) exhibits a highly stereotypic program of early development known as spiral cleavage. Animals with spiral cleavage have diverse body plans, and homologous embryonic cells can be readily identified among distantly related animals. Spiralian embryos are particularly amenable to studies of fate-mapping, and larval fates of identified cells are conserved among diverse taxa. One cell of particular importance in spiralian development is 2d, or the primary somatoblast, which generates ectoderm of the body posterior to the prototroch. We are interested in the evolution of the primary somatoblast, and thus far, the 2d sublineage has only been analyzed in a few species. In Capitella teleta, 2d generates ectoderm of the segmented trunk and post-segmental pygidium. In this study, development of the 2d lineage was characterized in detail through intracellular injections of DiI, and time-lapse as well as confocal microscopy to analyze cleavage patterns and the fates of larval cells. Analysis of cleavage patterns reveals that the first bilateral division in the 2d sublineage occurs with the division of 2d(112), the same 2d daughter cell that first divides bilaterally in the polychaete Platynereis dumerilii. Larval fates of blastomeres 2d(1), 2d(2), 2d(11), 2d(12), 2d(112), 2d(1121), and 2d(1122) were determined. All cells show stereotypic descendant clones that are consistent with segregation within sublineages. In the first few divisions of the 2d sublineage, larval-specific structures (neurotroch and telotroch) and pygidial ectoderm are segregated from segmental ectoderm and ventral nerve cord. The daughters of the first bilateral division, 2d(1121) and 2d(1122), generate the right and left halves of the segmental ectoderm and ventral nerve cord respectively, although the clones are consistently asymmetric across the dorsal midline. The pattern of cleavage divisions and the fates of the 2d daughters in Capitella teleta are compared to those in other spiralians with special attention to annelids.