RYCHEL, A.L.*; SWALLA, B.J.; University of Washington; University of Washington: Evolution of Pharyngeal Gill Bars in the Deuterostomes

Vertebrate pharyngeal gills contain structural elements that are cartilages elaborated from neural crest cells. The invertebrate chordate group, the Cephalochordates, also have pharyngeal gills containing structural elements, although the origin of these structures are not known. The other major invertebrate chordate group, the Tunicata, also have pharyngeal gills, but they do not have major structural elements within them. The chordates mentioned above are part of a larger monophyletic group, the Deuterostomes, which is a group that also contains the sister taxa echinoderms and hemichordates. The hemichordates, like their chordate relatives, also have pharyngeal gills supported by structural elements, very similar in shape and structure to those of cephalochordates. Since hemichordates and chordates possess pharyngeal gills, it is likely that the deuterostome ancestor also possessed pharyngeal gills and that these structures were later lost in echinoderms. We are investigating the structure and development of hemichordate gill bars and comparing them with cephalochordate and vertebrate gill bars. This study shows that the pharyngeal cartilage of hemichordates and cephalochordates are composed of fibrillar collagen and sulfated proteoglycans, similar to vertebrate pharyngeal cartilages. However, unlike vertebrate pharyngeal cartilage, hemichordate and cephalochordate pharyngeal cartilage is acellular and secreted from endodermally derived epithelia, as an extra-thickened basal lamina. This type of pharyngeal cartilage is likely the ancestral mode of constructing pharyngeal cartilage in the deuterostomes. The evolution of neural crest allowed the elaboration of these structures from a new cell population.