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Orthonectids Are Highly Degenerate Annelid Worms

Schiffer, PH; Robertson, HE; Telford, MJ; (2018) Orthonectids Are Highly Degenerate Annelid Worms. Current Biology , 28 (12) 1970-1974.e3. 10.1016/j.cub.2018.04.088. Green open access

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Abstract

The animal groups of Orthonectida and Dicyemida are tiny, extremely simple, vermiform endoparasites of various marine animals and have been linked in the Mesozoa (Figure 1). The Orthonectida (Figures 1A and 1B) have a few hundred cells, including a nervous system of just ten cells, and the Dicyemida (Figure 1C) are even simpler, with ∼40 cells. They are classic “Problematica”—the name Mesozoa suggests an evolutionary position intermediate between Protozoa and Metazoa (animals) and implies that their simplicity is a primitive state, but molecular data have shown they are members of Lophotrochozoa within Bilateria, which means that they derive from a more complex ancestor. Their precise affinities remain uncertain, however, and it is disputed whether they even constitute a clade. Ascertaining their affinities is complicated by the very fast evolution observed in their genes, potentially leading to the common systematic error of long-branch attraction (LBA). Here, we use mitochondrial and nuclear gene sequence data and show that both dicyemids and orthonectids are members of the Lophotrochozoa. Carefully addressing the effects of unequal rates of evolution, we show that the Mesozoa is polyphyletic. While the precise position of dicyemids remains unresolved within Lophotrochozoa, we identify orthonectids as members of the phylum Annelida. This result reveals one of the most extreme cases of body-plan simplification in the animal kingdom; our finding makes sense of an annelid-like cuticle in orthonectids and suggests that the circular muscle cells repeated along their body may be segmental in origin.

Type: Article
Title: Orthonectids Are Highly Degenerate Annelid Worms
Open access status: An open access version is available from UCL Discovery
DOI: 10.1016/j.cub.2018.04.088
Publisher version: https://doi.org/10.1016/j.cub.2018.04.088
Language: English
Additional information: This version is the author accepted manuscript. For information on re-use, please refer to the publisher’s terms and conditions.
Keywords: Mesozoa, Dicyemida, Rhombozoa, Orthonectida, Annelida, phylogeny
UCL classification: UCL
UCL > Provost and Vice Provost Offices > School of Education
UCL > Provost and Vice Provost Offices > School of Education > UCL Institute of Education
UCL > Provost and Vice Provost Offices > School of Education > UCL Institute of Education > IOE - Psychology and Human Development
UCL > Provost and Vice Provost Offices > School of Life and Medical Sciences
UCL > Provost and Vice Provost Offices > School of Life and Medical Sciences > Faculty of Life Sciences
UCL > Provost and Vice Provost Offices > School of Life and Medical Sciences > Faculty of Life Sciences > Div of Biosciences
UCL > Provost and Vice Provost Offices > School of Life and Medical Sciences > Faculty of Life Sciences > Div of Biosciences > Genetics, Evolution and Environment
URI: https://discovery.ucl.ac.uk/id/eprint/10047628
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