Deciphering deuterostome phylogeny: molecular, morphological and palaeontological perspectives

Abstract

Deuterostomes are a monophyletic group of animals that include the vertebrates, invertebrate chordates, ambulacrarians and xenoturbellids. Fossil representatives from most major deuterostome groups, including some phylum-level crown groups, are found in the Lower Cambrian, suggesting that evolutionary divergence occurred in the Late Precambrian, in agreement with some molecular clock estimates. Molecular phylogenies, larval morphology and the adult heart/kidney complex all support echinoderms and hemichordates as a sister grouping (Ambulacraria). Xenoturbellids are a relatively newly discovered phylum of worm-like deuterostomes that lacks a fossil record, but molecular evidence suggests that these animals are a sister group to the Ambulacraria. Within the chordates, cephalochordates share large stretches of chromosomal synteny with the vertebrates, have a complete Hox complex and are sister group to the vertebrates based on ribosomal and mitochondrial gene evidence. In contrast, tunicates have a highly derived adult body plan and are sister group to the vertebrates based on the analyses of concatenated genomic sequences. Cephalochordates and hemichordates share gill slits and an acellular cartilage, suggesting that the ancestral deuterostome also shared these features. Gene network data suggest that the deuterostome ancestor had an anterior-posterior body axis specified by Hox and Wnt genes, a dorsoventral axis specified by a BMP/chordin gradient, and was bilaterally symmetrical with left-right asymmetry determined by expression of nodal.

Chordate origins and evolution

“…Unfortunately, this is when developmental biologists find themselves in uncharted territory. At this stage, they will need to become familiar not only with a vast number of animal body plans, but also with various phylogenetic schemes that suggest competing evolutionary hypotheses (Swalla and Smith, 2008). Fortunately, the exponential accumulation of molecular data from genomes and animal models comes to the rescue.…”

“…Max Telford has pioneered the molecular work that placed Xenoturbellids as deuterostomes (Bourlat et al, 2006) and brings us up to date on how this worm may answer questions of chordate origins (Telford, 2008). Then Atsuko Sato, working with John Bishop and Peter Holland, contributes a paper with photos of larvae of Rhabdopleura normani that summarizes the importance that pterobranchs have played in classic views of deuterostome evolution (Sato et al, 2008), due to the extensive fossil record of Pterobranchs or graptolites (Swalla and Smith, 2008).…”

“…These novel ideas have been subjected to thorough testing and the Olfactores hypothesis has received further support (Bourlat et al, 2006;Delsuc et al, 2006Delsuc et al, , 2008Dunn et al, 2008;Swalla and Smith, 2008). In contrast, the unorthodox assembly of cephalochordates and echinoderms, which initially raised suspicions about chordate monophyly (Delsuc et al, 2006), was not confirmed (Bourlat et al, 2006;Delsuc et al, 2008).…”

“…Also influenced by the new view of chordate evolution, Satoh (2008) and Swalla and Smith (2008) both enumerate the major ontogenetic and morphological features that place cephalochordates between all other nonchordate deuterostomes and vertebrates plus tunicates. Accordingly, Satoh (2008) suggests that the chordate ancestor was comparable to extant cephalochordates such as amphioxus and then proposes that this amphioxus-like animal evolved from a free-living ancestor similar to enteropneust hemichordates (Cameron et al, 2000;Swalla and Smith, 2008) through changes that fashioned a swimming, skeletal muscle driven, larva out of a ciliated larva similar to those of hemichordates and echinoderms.…”

“…Accordingly, Satoh (2008) suggests that the chordate ancestor was comparable to extant cephalochordates such as amphioxus and then proposes that this amphioxus-like animal evolved from a free-living ancestor similar to enteropneust hemichordates (Cameron et al, 2000;Swalla and Smith, 2008) through changes that fashioned a swimming, skeletal muscle driven, larva out of a ciliated larva similar to those of hemichordates and echinoderms. Satoh pictures this swimming larva as tadpole-like, but different, more primitive, than the tunicate tadpole, with only a trunk and a tail region.…”