Article

Convergent evolution of bilaterian nerve cords

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Abstract

It has been hypothesized that a condensed nervous system with a medial ventral nerve cord is an ancestral character of Bilateria. The presence of similar dorsoventral molecular patterns along the nerve cords of vertebrates, flies, and an annelid has been interpreted as support for this scenario. Whether these similarities are generally found across the diversity of bilaterian neuroanatomies is unclear, and thus the evolutionary history of the nervous system is still contentious. Here we study representatives of Xenacoelomorpha, Rotifera, Nemertea, Brachiopoda, and Annelida to assess the conservation of the dorsoventral nerve cord patterning. None of the studied species show a conserved dorsoventral molecular regionalization of their nerve cords, not even the annelid Owenia fusiformis, whose trunk neuroanatomy parallels that of vertebrates and flies. Our findings restrict the use of molecular patterns to explain nervous system evolution, and suggest that the similarities in dorsoventral patterning and trunk neuroanatomies evolved independently in Bilateria.

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Acknowledgements

We thank the staff at the marine stations, current and former members of the Hejnol laboratory, and C. Dunn. The Sars Core Budget, the FP7-PEOPLE-2009-RG, and the European Research Council Community’s Framework Program Horizon 2020 to A.H. funded this work. A National Science Foundation International Research Fellowship Program Postdoctoral Fellowship funded K.P. The Carlsberg Foundation funded H.S.L. The Swedish Research Council funded U.J. and J.T.C.

Author information

Author notes

    • José M. Martín-Durán
    •  & Kevin Pang

    These authors contributed equally to this work.

Affiliations

  1. Sars International Centre for Marine Molecular Biology, University of Bergen, Thørmohlensgate 55, 5006 Bergen, Norway

    • José M. Martín-Durán
    • , Kevin Pang
    • , Aina Børve
    • , Henrike Semmler Lê
    • , Anlaug Furu
    •  & Andreas Hejnol
  2. Natural History Museum of Denmark, Biosystematics Section, Universitetsparken 15, DK-2100 Copenhagen, Denmark

    • Henrike Semmler Lê
  3. Naturhistoriska Riksmuseet, PO Box 50007, SE-104 05 Stockholm, Sweden

    • Johanna Taylor Cannon
    •  & Ulf Jondelius

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Contributions

J.M.M.-D., K.P., H.S.L., and A.H. designed the study. J.M.M.-D., K.P., A.B., A.F., A.H., U.J., and J.T.C. collected the animals. J.M.M.-D., K.P., A.B., H.S.L., A.F., and A.H. performed the experiments. J.M.M.-D., K.P., and A.H. wrote the manuscript. All authors read and approved the final manuscript.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Andreas Hejnol.

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Extended data

Supplementary information

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    Life Sciences Reporting Summary

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    Supplementary Information

    This file contains supplementary figure 1 - orthology analyses, supplementary table 1 - gene complement in M. stichopi and I. pulchra, supplementary table 2 - referenced summary of the expression of dorsoventral transcription factors in major lineages of Bilateria, and supplementary references.

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