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The phylogenetic position of the comb jellies (Ctenophora) and the importance of taxonomic sampling
Uppsala University, Disciplinary Domain of Science and Technology, Biology, Department of Evolution, Genomics and Systematics, Systematic Zoology.
Uppsala University, Disciplinary Domain of Science and Technology, Biology, Department of Evolution, Genomics and Systematics, Molecular Evolution.
Uppsala University, Disciplinary Domain of Science and Technology, Biology, Department of Evolution, Genomics and Systematics, Systematic Zoology.
2004 (English)In: Cladistics, ISSN 0748-3007, E-ISSN 1096-0031, Vol. 20, no 6, 558-578 p.Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
2004. Vol. 20, no 6, 558-578 p.
National Category
Biological Systematics
URN: urn:nbn:se:uu:diva-70317DOI: 10.1111/j.1096-0031.2004.00041.xOAI: oai:DiVA.org:uu-70317DiVA: diva2:98228
Available from: 2008-07-09 Created: 2008-07-09 Last updated: 2011-02-18Bibliographically approved
In thesis
1. The Dawn of a New Age: Interrelationships of Acoela and Nemertodermatida and the Early Evolution of Bilateria
Open this publication in new window or tab >>The Dawn of a New Age: Interrelationships of Acoela and Nemertodermatida and the Early Evolution of Bilateria
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Deciphering the rapid emergence of bilaterian animals around the time of the Cambrian Explosion and reconstructing the interrelationships of animal groups have long been two of the most elusive problems in Zoology. This thesis concerns the phylogenetic interrelationships within and among Acoela and Nemertodermatida, two groups of small worms that are believed to be basal bilaterians and which may provide important clues for understanding the early evolution of animals. In addition to trying to resolve the phylogenetic positions of these groups, major focus is put on inferring how ancestral animals might have looked, given the phylogenetic hypotheses put forward. The data used to infer phylogenies include nuclear ribosomal DNA, the mitochondrial COI gene and microRNAs.

Based on phylogenetic analyses of a large number of 18S SSU ribosomal DNA sequences, it is proposed that Cnidaria is the sister taxon to Bilateria. Poor taxon sampling is suggested to be one of the reasons for why earlier assessments of the interrelationships among the most basal animal groups have yielded many conflicting results using the same gene.

Analyses of new 18S SSU rDNA and 28S LSU rDNA sequences from six of the nine known species of nemertodermatids corroborate earlier indications that Acoela and Nemertodermatida are not sister taxa, as once thought. Being separate basal bilaterian animal groups, it is suggested that the last common ancestor of all bilaterians shared much of their comparatively simple morphology. Many methods are deployed to assess whether the phylogenetic results are mainly due to long-branch attraction, but no indication of this artifact is detected.

The first comprehensive phylogenetic framework of Acoela is reconstructed from the 18S SSU, 28S LSU and COI genes, in combination with morphological data. The ancestral acoel worm is reconstructed using Bayesian methods and morphological observations in extant species. Two indeces, posterior similarity and reconstruction signal, are implemented to assess how similar different species are to the last common ancestor of all acoels and illustrate how clearly different characters or nodes are reconstructed. It is suggested that the ancestral acoel looked much like extant species of Diopisthoporus.

The phylogenetic positions of Acoela and Nemertodermatida are assessed using new data on microRNAs in the acoel Hofstenia miamia and the nemertodermatid Meara stichopi. Acoela and Nemertodermatida are again found to be basal bilaterians, in congruence with earlier results. Using the work-flow and indeces developed earlier, it is concluded that the bilaterian ancestral microRNA repertoire can not yet be reconstructed with high confidence.

All papers stress the importance of inclusive taxon sampling for making generalized inferences about ancestral features in animals.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2009. 44 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 667
Acoela, Nemertodermatida, Bilateria, Metazoa, evolution, phylogenetic inferrence, ancestral reconstruction, taxon sampling, microRNA
National Category
Biological Systematics
Research subject
Systematic Zoology
urn:nbn:se:uu:diva-107550 (URN)978-91-554-7591-8 (ISBN)
Public defence
2009-09-25, Ekmansalen, Evolutionary Biology Centre, Uppsala University, Norbyvägen 18 SE-752 36, Uppsala, 13:00 (English)
Available from: 2009-09-04 Created: 2009-08-16 Last updated: 2009-09-04Bibliographically approved

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