title: Mitochondrial genome and nuclear sequence data support myzostomida as part of the annelid radiation.
abstract: The echinoderm symbionts Myzostomida are marine worms that show an enigmatic lophotrochozoan body plan. Historically, their phylogenetic origins were obscured due to disagreement about which morphological features are evolutionarily conserved, but now most morphological evidence points to annelid origins. In contrast, recent phylogenetic analyses using different molecular markers produced variable results regarding the position of myzostomids, but all suggested these worms are not derived annelids. To reexamine this issue, we analyzed data from nuclear genes (18S rDNA, 28S rDNA, Myosin II, and Elongation Factor-1alpha), and a nearly complete myzostomid mitochondrial genome. Here, we show that the molecular data are in agreement with the morphological evidence that myzostomids are part of the annelid radiation. This result is robustly supported by mitochondrial (gene order and sequence data) and nuclear data, as well as by recent ultrastructural investigations. Using Bayes factor comparison, alternative hypotheses are shown to lack support. Thus, myzostomids probably evolved from a segmented ancestor and gained a derived anatomy during their long evolutionary history as echinoderm symbionts.
authors.affiliation: Unit of Evolutionary Biology/Systematic Zoology, Institute of Biochemistry and Biology, University of Potsdam, Potsdam-Golm, Germany. bleidorn@uni-potsdam.de
authors.forename: Christoph
authors.initials: C
authors.lastname: Bleidorn
authors.forename: Igor
authors.initials: I
authors.lastname: Eeckhaut
authors.forename: Lars
authors.initials: L
authors.lastname: Podsiadlowski
authors.forename: Nancy
authors.initials: N
authors.lastname: Schult
authors.forename: Damhnait
authors.initials: D
authors.lastname: McHugh
authors.forename: Kenneth M
authors.initials: KM
authors.lastname: Halanych
authors.forename: Michel C
authors.initials: MC
authors.lastname: Milinkovitch
authors.forename: Ralph
authors.initials: R
authors.lastname: Tiedemann
mesh_terms: D000818:Animals; D000845:Annelida / Q000145:classification / Q000235:genetics*; D001499:Bayes Theorem; D002467:Cell Nucleus / Q000235:genetics*; D004272:DNA, Mitochondrial / Q000235:genetics*; D004275:DNA, Ribosomal / Q000235:genetics; D004447:Echinodermata / Q000469:parasitology*; D019143:Evolution, Molecular; D050259:Genes, Mitochondrial / Q000235:genetics; D005819:Genetic Markers; D016678:Genome; D006790:Host-Parasite Interactions; D016013:Likelihood Functions; D024462:Myosin Type II / Q000235:genetics; D020648:Peptide Elongation Factor 1 / Q000235:genetics; D010802:Phylogeny; D013559:Symbiosis
chemical_list: D004272:DNA, Mitochondrial; D004275:DNA, Ribosomal; D005819:Genetic Markers; D020648:Peptide Elongation Factor 1; D024462:Myosin Type II