Fig. 6.

Scenario for evolution of the chordate body plan. All diagrams depict cross-sections of the gastrula stage of extant deuterostome species and their hypothetical ancestors. Enhanced BMP activity during gastrulation could have resulted in loss of the ancestral mouth and centralization of neurogenic ectoderm on the ancestral ventral side of a hypothetical intermediate to chordates. Opening of a new mouth on the ancestral dorsal side could account for the inverse DV orientation of chordates relative to the ambulacraria. The colored lines outside the embryos indicate the ectodermal expression domains of transcription factors (33, 50, 60–62). Note that the sea urchin pax6 gene has no ectodermal expression at the gastrula stage, although another Pax factor gene, pax2/5/8, is expressed in the lateral ectoderm, marking a subdomain of the presumptive ciliary band (33, 63). Asterisks indicate the sites of future mouth openings. Based on the similarity of the larval patterning mechanisms between the hemichordates and sea urchins, we propose that the deuterostome ancestor was an indirect developer with a ciliary band nervous system. During early chordate evolution, changes in the BMP level during gastrulation could have occurred, resulting in the loss of the larval mouth and centralization of the neurogenic ectoderm on the ancestral ventral side devoid of BMP activity. A new mouth opening on the ancestral dorsal side could then redefine an inverse DV orientation relative to the Ambulacraria.