Deuterostome Ancestors and Chordate Origins

Synopsis The Deuterostomia are a monophyletic group, consisting of the Ambulacraria, with two phyla, Hemichordata and Echinodermata, and the phylum Chordata, containing the subphyla Cephalochordata (lancelets or Amphioxus), Tunicata (Urochordata), and Vertebrata. Hemichordates and echinoderms are sister groups and are critical for understanding the deuterostome ancestor and the origin and evolution of the chordates within the deuterostomes. Enteropneusta, worm-like hemichordates, share many chordate features as adults, including a post-anal tail, gill slits, and a central nervous system (CNS) that deploys similar developmental genetic regulatory networks (GRNs). Genomic comparisons show that cephalochordates share synteny and a vermiform body plan similar to vertebrates, but phylogenomic analyses place tunicates as the sister group of vertebrates. Tunicates have a U-shaped gut and a very different adult body plan than the rest of the chordates, and all tunicates have small genomes and many gene losses, although the GRNs underlying specific tissues, such as notochord and muscle, are conserved. Echinoderms and vertebrates have extensive fossil records, with fewer speci