Regeneration of the acorn worm pygochord with the implication for its convergent evolution with the notochord.

The origin of the notochord is a central issue in chordate evolution. This study examined the development of the acorn worm pygochord, a putative homologue of the notochord. Because the pygochord differentiates only after metamorphosis, the developmental was followed process by inducing regeneration after artificial amputation in Ptychodera flava. It was found that although the regeneration of the posterior part of the body did not proceed via formation of an obvious regeneration bud, pygochord regeneration was observed within a few weeks, possibly via trans-differentiation of endoderm cells. The expression of the fibrillary collagen gene (Fcol) and elav in the pygochord during regeneration was detected. This indicates that pygochord cells are not part of gut epithelial cells, but that they differentiated as a distinct cell type. Our gene expression analyses do not provide supporting evidence for the homology between the pygochord and notochord, but rather favored the convergent evolution between them.