Deciphering deuterostome phylogeny: molecular, morphological, and palaeontological perspectives
Deuterostomes are a monophyletic group of animals that include the vertebrates, invertebrate chordates, ambulacrarians, and xenoturbellids. Fossil representatives from most major deuterostome groups are found in the Lower Cambrian, suggesting that evolutionary divergence occurred in the late pre-Cambrian, in agreement with molecular clocks. Molecular phylogenies, larval morphology, and the adult heart/kidney complex all support echinoderms and hemichordates as a sister grouping (Ambulacraria). Xenoturbellids represent a relatively newly identified deuterostome phylum that lacks a fossil record, but molecular evidence suggests that these animals are a sister group to the Ambulacraria. Within the chordates, lancelets share large stretches of chromosomal synteny with the vertebrates, have an intact Hox complex and are sister group to the vertebrates according to ribosomal and mitochondrial gene evidence. In contrast, tunicates have a highly derived adult body plan and are sister group to the vertebrates by phylogenetic trees constructed from concatenated genomic sequences. Lancelets and hemichordates share gill slits and an acellular cartilage, suggesting that the ancestral deuterostome also shared these features. Gene network data suggests that the deuterostome ancestor had an A-P axis specified by Hox and Wnt genes, a D-V axis specified by a BMP/chordin gradient, and a L-R asymmetry determined by expression of nodal.
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