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Convergent evolution of bilaterian nerve cords
José M Martín-Durán1, Kevin Pang1, Aina Børve1
1Sars International Centre for Marine Molecular Biology, University of Bergen, Thørmohlensgate 55, 5006 Bergen, Norway.
Nature
|December 14, 2017
Summary
The evolutionary history of the nervous system is debated. This study found no conserved molecular patterns in nerve cords across diverse bilaterians, suggesting independent evolution of nervous system traits.
Area of Science:
- Zoology
- Developmental Biology
- Evolutionary Biology
Background:
- A medial ventral nerve cord is hypothesized as an ancestral Bilateria trait.
- Similar dorsoventral molecular patterns in vertebrates, flies, and annelids support this hypothesis.
- The conservation of these patterns across diverse bilaterian neuroanatomies remains unclear.
Purpose of the Study:
- To investigate the conservation of dorsoventral nerve cord patterning across diverse bilaterian phyla.
- To reassess the hypothesis of a conserved medial ventral nerve cord as an ancestral Bilateria character.
Main Methods:
- Studied neuroanatomies and molecular patterns in representatives of Xenacoelomorpha, Rotifera, Nemertea, Brachiopoda, and Annelida.
- Assessed the conservation of dorsoventral nerve cord patterning.
Main Results:
- No conserved dorsoventral molecular regionalization was found in the nerve cords of any studied species.
- This lack of conservation was observed even in the annelid Owenia fusiformis, which has a trunk neuroanatomy similar to vertebrates and flies.
- Similarities in dorsoventral patterning and trunk neuroanatomies appear to have evolved independently.
Conclusions:
- Molecular patterns are not universally conserved across bilaterian nerve cords.
- The hypothesis of a condensed nervous system with a medial ventral nerve cord as an ancestral Bilateria character is not supported by conserved molecular patterning.
- Dorsoventral patterning and trunk neuroanatomies in Bilateria likely evolved independently.
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