Turbellarian taxonomic database

Record # 21906
Mwinyi A, Bailly X, Bourlat SJ, Jondelius U, Littlewood DTJ, Podsiadlowsk L (2010)
The phylogenetic position of Acoela as revealed by the complete mitochondrial genome of Symsagittifera roscoffensis
BMC Evolutionary Biology 2010, 10:309 doi:10.1186/1471-2148-10-309

Abstract / Notes

"Background: Acoels are simply organized unsegmented worms, lacking hindgut and anus. Several publications
over recent years challenge the long-held view that acoels are early offshoots of the flatworms. Instead a
basal position as sister group to all other bilaterian animals was suggested, mainly based on molecular
evidence. This led to the view that features of acoels might reflect those of the last common ancestor of
Bilateria, and resulted in several evo-devo studies trying to interpret bilaterian evolution using acoels as a
proxy model for the "Urbilateria". Results: We describe the first complete mitochondrial genome sequence of a
member of the Acoela, Symsagittifera roscoffensis. Gene content and circular organization of the mitochondrial
genome does not significantly differ from other bilaterian animals. However, gene order shows no similarity to
any other mitochondrial genome within the Metazoa. Phylogenetic analyses of concatenated alignments of amino
acid sequences from protein coding genes support a position of Acoela and Nemertodermatida as the sister group
to all other Bilateria. Our data provided no support for a sister group relationship between Xenoturbellida
and Acoela or Acoelomorpha. The phylogenetic position of Xenoturbella bocki as sister group to or part of the
deuterostomes was also unstable. Conclusions: Our phylogenetic analysis supports the view that acoels and
nemertodermatids are the earliest divergent extant lineage of Bilateria. As such they remain a valid source
for seeking primitive characters present in the last common ancestor of Bilateria. Gene order of mitochondrial
genomes seems to be very variable among Acoela and Nemertodermatida and the groundplan for the metazoan
mitochondrial genome remains elusive. More data are needed to interpret mitochondrial genome evolution at the
base of Bilateria."

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