Development and evolution of chordate cartilage.

Deuterostomes are a monophyletic group of animals containing vertebrates, lancelets, tunicates, hemichordates, echinoderms, and xenoturbellids. Four out of these six extant groups-vertebrates, lancelets, tunicates, and hemichordates-have pharyngeal gill slits. All groups of deuterostome animals that have pharyngeal gill slits also have a pharyngeal skeleton supporting the pharyngeal openings, except tunicates. We previously found that pharyngeal cartilage in hemichordates and cephalochordates contains a fibrillar collagen protein similar to vertebrate type II collagen, but unlike vertebrate cartilage, the invertebrate deuterostome cartilages are acellular. We found SoxE and fibrillar collagen expression in the pharyngeal endodermal cells adjacent to where the cartilages form. These same endodermal epithelial cells also express Pax1/9, a marker of pharyngeal endoderm in vertebrates, lancelets, tunicates, and hemichordates. In situ experiments with a cephalochordate fibrillar collagen also showed expression in pharyngeal endoderm, as well as the ectoderm and the mesodermal coelomic pouches lining the gill bars. These results indicate that the pharyngeal endodermal cells are responsible for secretion of the cartilage in hemichordates, whereas in lancelets, all the pharyngeal cells surrounding the gill bars, ectodermal, endodermal, and mesodermal may be responsible for cartilage formation. We propose that endoderm secretion was primarily the ancestral mode of making pharyngeal cartilages in deuterostomes. Later the evolutionary origin of neural crest allowed co-option of the gene network for the secretion of pharyngeal cartilage matrix in the new migratory neural crest cell populations found in vertebrates.