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Dismissal of Acoelomorpha: Acoela and Nemertodermatida are separate early bilaterian clades
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, Systematic Zoology.
Uppsala University, Disciplinary Domain of Science and Technology, Biology, Department of Evolution, Genomics and Systematics, Systematic Zoology.
2007 (English)In: Zoologica Scripta, ISSN 0300-3256, E-ISSN 1463-6409, Vol. 36, no 5, 509-523 p.Article in journal (Refereed) Published
Abstract [en]

We used new 18S and 28S rRNA sequences analysed with parsimony, maximum likelihood and Bayesian methods of phylogenetic reconstruction to show that Nemertodermatida, generally classified as the sister group of Acoela within the recently proposed Phylum Acoelomorpha, are a separate basal bilaterian lineage. We used several analytical approaches to control for possible long branch attraction (LBA) artefacts in our results. Parsimony and the model based phylogenetic reconstruction methods that incorporate 'corrections' for substitution rate heterogenities yielded concordant results. When putative long branch taxa were experimentally removed the resulting topologies were consistent with our total evidence analysis. Deletion of fast-evolving nucleotide sites decreased resolution and clade support, but did not support a topology conflicting with the total evidence analysis. Establishment of Acoela and Nemertodermatida as two early lineages facilitates reconstruction of ancestral bilaterian features. The ancestor of extant Bilateria was a small, benthic direct developer without coelom or a planktonic larval stage. The previously proposed Phylum Acoelomorpha is dismissed as paraphyletic.

Place, publisher, year, edition, pages
2007. Vol. 36, no 5, 509-523 p.
National Category
Biological Sciences
Identifiers
URN: urn:nbn:se:uu:diva-17553DOI: 10.1111/j.1463-6409.2007.00295.xISI: 000249207700007OAI: oai:DiVA.org:uu-17553DiVA: diva2:45324
Available from: 2008-07-09 Created: 2008-07-09 Last updated: 2017-12-08Bibliographically 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.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 667
Keyword
Acoela, Nemertodermatida, Bilateria, Metazoa, evolution, phylogenetic inferrence, ancestral reconstruction, taxon sampling, microRNA
National Category
Biological Systematics
Research subject
Systematic Zoology
Identifiers
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)
Opponent
Supervisors
Available from: 2009-09-04 Created: 2009-08-16 Last updated: 2009-09-04Bibliographically approved

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Wallberg, AndreasAhmadzadeh, AfsanehJondelius, Ulf

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