The Dawn of a New Age Interrelationships of Acoela and Nemertodermatida and the Early Evolution of Bilateria
Abstract: 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.
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