Ascidian neural crest-like cells: phylogenetic distribution, relationship to larval complexity, and pigment cell fate.

Migratory neural crest-like cells, which express the cell surface antigen HNK-1 and develop into pigment cells, have recently been identified in the ascidian Ecteinascidia turbinata. Here we use HNK-1 expression as a marker to determine whether neural crest-like cells are responsible for pigment development in diverse ascidian species. We surveyed HNK-1 expression and tyrosinase activity in 12 ascidian species, including those with different adult organizations, developmental modes, and larval sizes and complexities. We observed HNK-1 positive cells in every species, although the timing of HNK-1 expression varied according to the extent of larval complexity. HNK-1 expression was initiated during the late tailbud stage in species in which adult features are formed precociously in large complex larvae. In contrast, HNK-1 positive cells did not appear until the swimming tadpole or juvenile stage in species with small simple larvae in which most adult features appear after metamorphosis. Double labeling experiments indicated that HNK-1 and tyrosinase are expressed in the same subset of pigment-forming mesenchymal cells in species with complex or simple larvae. In addition, the absence of HNK-1 and tyrosinase expression in albino morphs of the colonial ascidian Botryllus schlosseri suggested that the major fate of neural crest-like cells is to become pigment cells. The results suggest that ascidian neural crest-like cells and vertebrate neural crest cells had a common origin during chordate evolution and that their primitive function was to generate body pigmentation.