New Insights into Vertebrate Origins

p0010 Vertebrates share several distinct morphological characters with three invertebrate groups: lancelets, tunicates, and hemichordates (Figure 6.1). Tunicates, lancelets, and vertebrates have traditionally been considered to be a monophyletic group—the chordates—that shares five morphologic features: a notochord, a dorsal neural tube, an endostyle, a muscular postanal tail, and pharyngeal gill slits. Hemichordates share some of these chordate features; the pharyngeal gill slits (Figure 6.2), an endostyle, and a postanal tail. Previously, hemichordates were thought to contain a notochord homolog (the stomochord) and a dorsal neural tube in the neck region, but recent evidence from developmental genetics has questioned these homologies. Developmental genetics and genomics have allowed for the reexamination of the question of chordate origins by comparing developmental gene expression in embryos of different phyla. This powerful approach has allowed new insights into the molecular mechanisms underlying morphologic changes. Genomics has allowed for investigations into the phylogenetic relationships of the chordates and their invertebrate relatives and for the comparison of the shared genetic pathways in related embryos. We review current research on this topic and show that our view of the chordate ancestor has changed during the past 10 years. For years, the chordate ancestor has been considered to be a filter-feeding, tunicate-like animal with a tiny chordate tadpole larva. However, recent evidence from my laboratory and others has shown that the chordate ancestor was more likely a benthic worm with a mouth and pharyngeal gill slits supported by cartilaginous gill bars (Cameron et al., 2000; Gerhart et al., 2005; Rychel et al., 2006). Further research in developmental genetics and genomics is likely to be fruitful