Animal phylogeny and the ancestry of bilaterians: inferences from morphology and 18S rDNA gene sequences

Abstract

Insight into the origin and early evolution of the animal phyla requires an understanding of how animal groups are related to one another. Thus, we set out to explore animal phylogeny by analyzing with maximum parsimony 138 morphological characters from 40 metazoan groups, and 304 18S rDNA sequences, both separately and together. Both types of data agree that arthropods are not closely related to annelids: the former group with nematodes and other molting animals (Ecdysozoa), and the latter group with molluscs and other taxa with spiral cleavage. Furthermore, neither brachiopods nor chaetognaths group with deuterostomes; brachiopods are allied with the molluscs and annelids (Lophotrochozoa), whereas chaetognaths are allied with the ecdysozoans. The major discordance between the two types of data concerns the rooting of the bilaterians, and the bilaterian sister-taxon. Morphology suggests that the root is between deuterostomes and protostomes, with ctenophores the bilaterian sister-group, whereas 18S rDNA suggests that the root is within the Lophotrochozoa with acoel flatworms and gnathostomulids as basal bilaterians, and with cnidarians the bilaterian sister-group. We suggest that this basal position of acoels and gnathostomulids is artifactal because for 1,000 replicate phylogenetic analyses with one random sequence as outgroup, the majority root with an acoel flatworm or gnathostomulid as the basal ingroup lineage. When these problematic taxa are eliminated from the matrix, the combined analysis suggests that the root lies between the deuterostomes and protostomes, and Ctenophora is the bilaterian sister-group. We suggest that because chaetognaths and lophophorates, taxa traditionally allied with deuterostomes, occupy basal positions within their respective protostomian clades, deuterostomy most likely represents a suite of characters plesiomorphic for bilaterians.

Phylogeny of sipunculan worms: A combined analysis of four gene regions and morphology

“…The included sipunculan sequence grouped with the seven annelids, as the sister taxon to a sabellid. Peterson and Eernisse (2001) and Staton (2003) also concluded that Sipuncula were more closely related to annelids than to molluscs contradicting the hypothesis of Scheltema (1996). On the other hand, total evidence analyses of ribosomal genes and morphology have suggested that sipunculans are the sister group to a clade including annelids and molluscs .…”

Evolution of arthropod visual systems: Development of the eyes and central visual pathways in the horseshoe crabLimulus polyphemusLinnaeus, 1758 (Chelicerata, Xiphosura)

“…However, studies from the field of molecular phylogeny (recent contributions, e.g., Shultz and Regier, 2000;Cook et al, 2001;Friedrich and Tautz, 2001;Hwang et al, 2001;Peterson and Eernisse, 2001;Regier and Shultz, 2001;Pisani et al, 2004) as well as morphological studies on the architecture of the nervous system recently have provided evidence for a close relationship of Hexapoda and Crustacea (reviews, e.g., Strausfeld, 1998Strausfeld, , 2005Schram and Koenemann, 2004;Harzsch et al, 2005a;Harzsch, 2006). The name "Tetraconata" has been suggested for a taxon that embraces the hexapods and crustaceans (Dohle, 2001;Richter, 2002) in reference to the tetrapartite crystalline cone in the ommatidia as a synapomorphy of these groups.…”

Molecular evidence that phoronids are a subtaxon of brachiopods (Brachiopoda: Phoronata) and that genetic divergence of metazoan phyla began long before the early Cambrian

“…The molecular affinity of both phoronids and brachiopods with protostomes has been less controversial, being supported by all SSU rDNA evidence and by additional molecular data from mitochondrial sequences and from other nuclear genes (Cohen et al 1998a;de Rosa et al 1999;Cohen 2000;Helfenbein 2000;Saito et al 2000;Helfenbein and Boore 2003;Ruiz-Trillo et al 2004). Although the (phoronid+brachiopod) clade has been questioned (Peterson and Eernisse 2001), existing evidence from SSU sequences supports the following conclusions: (1) both brachiopods and phoronids belong among the protostomes, their closest affinity being with molluscs and annelids; (2) no well-supported sistergroup relationship links brachiopods and phoronids with any other phylum; (3) brachiopods and phoronids form a generally well-supported clade, within which (4) a clade of (phoronids+inarticulate brachiopods) is moderately well supported; and (5) there is weak evidence for phoronids being the sister-group of craniiform inarticulate brachiopods.…”