The phylogenetic position of the comb jellies (Ctenophora) and the importance of taxonomic sampling

Abstract

The transition to a vermiform body shape is one of the most important events in animal evolution, having led to the impressive radiation of Bilateria. However, the sister group of Bilateria has remained obscure. Cladistic analyses of morphology indicate that Ctenophora is the sister group of Bilateria. Previous analyses of SSU rRNA sequences have yielded conflicting results; in many studies Ctenophora forms the sister group of Cnidaria + Bilateria, but in others the ctenophores group with poriferans. Here we re-examine the SSU sequence by analyzing a dataset with 528 metazoan + outgroup sequences, including almost 120 poriferan and diploblast sequences. We use parsimony ratchet and jackknife methods, as well as Bayesian methods, to analyze the data. The results indicate strong phylogenetic signals for a cnidarian + bilaterian group and for the comb jellies to have branched off early within a group uniting all epithelial animals [(Ct,(Cn,Bi))]. We demonstrate the importance of inclusive taxonomic coverage of ribosomal sequences for resolving this problematic part of the metazoan tree: topological stability increases dramatically with the addition of taxa, and the jackknife frequencies of the internal nodes uniting the lineages [(Cn,Bi) and ((Ct,(Cn,Bi))] also increase. We consider the reconstructed topology to represent the current best hypothesis of the interrelationships of these old lineages. Some morphological features supporting alternative hypotheses are discussed in the light of this result.

FGF signaling in gastrulation and neural development in Nematostella vectensis, an anthozoan cnidarian

“…Modern molecular phylogenetics suggest that cnidarians, and possibly placozoans, represent the closest extant animals related to the triploblastic, bilaterally symmetric bilaterians (protostomes and deuterostomes; Collins et al 2005;Wallberg et al 2004). Recent molecular evidence suggests that these diverse groups use many of the same signal transduction pathways and transcription factors during early development (Kusserow et al 2005;Miller et al 2005;Technau et al 2005;Technau and Scholz 2003).…”

Terminal addition, the Cambrian radiation and the Phanerozoic evolution of bilaterian form

“…Thus, the resolution of superphyletic bilaterian taxa has improved over the last 20 years (see Halanych 2004 for review), although questions still remain; for example, placement of chaetognaths. The exact relationship of bilaterians with more basal metazoan groups has also improved (e.g., Medina et al 2001;Wallberg et al 2004). Some uncertainty remains in the placement of aceol flatworms (although support seems to be firming for a basal placement as sister to the rest of the Bilateria (e.g., Ruiz-Trillo et al 2004), the position of cnidarians and ctenophores relative to the Bilateria, and the class level relationships within the sponges.…”

Back in time: a new systematic proposal for the Bilateria

“…slit-shaped mouth, internal mesenteries and asymmetric siphonoglyphs in the polyp form) were noted in the past (Stephenson 1926;Hyman 1951;Salvini-Plawen 1978;Willmer 1990), they were not taken as evidence for bilaterality because anthozoans were considered derived cnidarians and hence their internal bilateral features as secondarily evolved. Recent molecular phylogenies, however, have shown that Cnidaria and Bilateria are sister groups (Medina et al 2001;Wallberg et al 2004) and, importantly, that the Anthozoa is not a derived cnidarian clade but a basal group rendering its bilaterally symmetric features as possible plesiomorphies for the cnidarians (Collins 2002). Hence, cnidarians could originally be truly bilaterian ( Finnerty et 3B 4B Figure 4.…”

“…The name Planulozoa was recently proposed by Wallberg et al (2004) to define a clade comprising Cnidaria and Bilateria. As such, Planulozoa is formally equivalent to the name Bilateria, here proposed for the more inclusive clade (CnidariaCTriploblastica).…”