How the Worm Got its Pharynx: Phylogeny, Classification and Bayesian Assessment of Character Evolution in Acoela

Abstract

Acoela are marine microscopic worms currently thought to be the sister taxon of all other bilaterians. Acoels have long been used as models in evolutionary scenarios, and generalized conclusions about acoel and bilaterian ancestral features are frequently drawn from studies of single acoel species. There is no extensive phylogenetic study of Acoela and the taxonomy of the 380 species is chaotic. Here we use two nuclear ribosomal genes and one mitochondrial gene in combination with 37 morphological characters in an analysis of 126 acoel terminals (about one-third of the described species) to estimate the phylogeny and character evolution of Acoela. We present an estimate of posterior probabilities for ancestral character states at 31 control nodes in the phylogeny. The overall reconstruction signal based on the shape of the posterior distribution of character states was computed for all morphological characters and control nodes to assess how well these were reconstructed. The body-wall musculature appears more clearly reconstructed than the reproductive organs. Posterior similarity to the root was calculated by averaging the divergence between the posterior distributions at the nodes and the root over all morphological characters. Diopisthoporidae is the sister group to all other acoels and has the highest posterior similarity to the root. Convolutidae, including several "model" acoels, is most divergent. Finally, we present a phylogenetic classification of Acoela down to the family level where six previous family level taxa are synonymized.

A cultivable acoel species from the Mediterranean, Aphanostoma pisae sp. nov. (Acoela, Acoelomorpha)

“…The 18S rDNA sequence attributed to A. virescens on GenBank is more similar to that of many species of Isodiametra, and less like that of the other Aphanostoma species (see Jondelius et al, 2011;Nilsson et al, 2011). Second, there is hardly any difference in the generic diagnoses of Aphanostoma and Praeconvoluta, other than an elliptical penis sheath in Praeconvoluta, as opposed to a spherical penis sheath in Aphanostoma.…”

“…Second, there is hardly any difference in the generic diagnoses of Aphanostoma and Praeconvoluta, other than an elliptical penis sheath in Praeconvoluta, as opposed to a spherical penis sheath in Aphanostoma. Molecular analyses so far suggest both genera should be merged (see Jondelius et al, 2011;Nilsson et al, 2011). To help with this decision, it seems to be worthwhile to make an effort constructing a phylogenetic tree of the Isodiametridae with as many species as possible.…”

Xenacoelomorpha: a case of independent nervous system centralization?

“…In fact, the acoel NS is characterized by its high plasticity, which is evident in the very different neuronal arrangements that the acoel species show. A quick comparison of, for instance, acoels belonging to the most basal family (Diopisthoporidae) and those of the most divergent families within the class Crucimusculata reveals very striking differences in neural system arrangement [4,6,[48][49][50][51]. Using the knowledge obtained from several studies, old and new, we are in a position to understand, for the first time, the evolution of the NS within this group (see also [6]), always in accordance with the latest available phylogeny of the Acoela [48].…”

The nervous system of Xenacoelomorpha: a genomic perspective

“…1) (Crezée, 1975). The sister group of Prosopharyngida is Crucimusculata (Jondelius et al, 2011), a clade that includes all the named 'higher acoels'. This clade, which includes some of the most studied acoels (e.g.…”