Research Article Open Access

Xenacoelomorpha Flatworms are Basal Deuterostome

Abstract

Background: Whether position Xenacoelomorpha as an early branch of Bilateria (Protostomes+Deuterostomes) has been intensely debated during last several decades. Considering Darwin’s “tree of life”, with the “Phylogenetic Species Concept”, we choose mitochondrial genome as the subject to predict phyloenetic position of Xenacoelomorpha, by genes geneology.

Results: Herein, we sequence Heterochaerus australis mitochondrial genome and infer intrinsic relationships of metazoan with Xenacoelomorpha. The optimal tree under the popular Maximum Likelihood (ML) and Bayesian phylogenetic reconstructions are consensus with each other being strongly supported. The relationship between Chordates, Ambulacrarians and Xenoturbella/Acoelomorph is resolved. To avoid previous query about alignment process, the datasets are alignmented and trimmed automatically. Reducing taxon or cutting outgroups cannot affect the relationship between Xenacoelomorpha and other metazoan. Meanwhile, analysis using CAT model and Dayhoff groups also supporting the prediction made by mtZOA, relaxing the restriction of alignment criteria (MAFFT, strategy G–ins–1, BLOSUM 62, 45, 30) introducing potential misleading signals cannot challenge the tree topology indicating our auto-alignment mitochondrial dataset is not artificially restricted one.

Conclusion: Finally, a repeatable prediction of the genes geneology with reliable statistical support places Xenacoelomorpha as a basal Deuterostome.

Yi Wang

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