Origin of Chordata and the “Upside-Down Theory”

Origin of Chordata and the “Upside-Down Theory” The hypothesis of dorso-ventral axis inversion in Chordata based on recent molecular biology data is discussed from the viewpoint of evolutionary morphology. It is supposed that, as a result of such an inversion, the nerve cord located ventrally in most Invertebrata found itself on the dorsal side of Chordata. However, an analysis of the contemporary hypotheses explaining chordate origin (dipleuruloid and hemichordate) shows that neither of them provides convincing evidence of dorso-ventral inversion. A new hypothesis agreeing with both the molecular and morphological data is suggested. According to this hypothesis, the divergence of Protostomia and Deuterostomia occurred early in the origin of Bilateria. The initial stage (for a common ancestor) is assumed to be some turbellarians, in which several nerve cords extend back from the apical organ in a radial-symmetrical pattern. In the process of subsequent oligomerization, the ventral nerve cords become more developed in Protostomia, while in Deuterostomia the dorsal nerve cords have priority. It is emphasized that reaching an understanding between molecular biologists and evolutionary morphologists is a necessity. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Russian Journal of Marine Biology Springer Journals

Origin of Chordata and the “Upside-Down Theory”

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Publisher
SP MAIK Nauka/Interperiodica
Copyright
Copyright © 2008 by MAIK Nauka
Subject
Life Sciences; Freshwater & Marine Ecology
ISSN
1063-0740
eISSN
1608-3377
D.O.I.
10.1134/S1063074008060072
Publisher site
See Article on Publisher Site

Abstract

The hypothesis of dorso-ventral axis inversion in Chordata based on recent molecular biology data is discussed from the viewpoint of evolutionary morphology. It is supposed that, as a result of such an inversion, the nerve cord located ventrally in most Invertebrata found itself on the dorsal side of Chordata. However, an analysis of the contemporary hypotheses explaining chordate origin (dipleuruloid and hemichordate) shows that neither of them provides convincing evidence of dorso-ventral inversion. A new hypothesis agreeing with both the molecular and morphological data is suggested. According to this hypothesis, the divergence of Protostomia and Deuterostomia occurred early in the origin of Bilateria. The initial stage (for a common ancestor) is assumed to be some turbellarians, in which several nerve cords extend back from the apical organ in a radial-symmetrical pattern. In the process of subsequent oligomerization, the ventral nerve cords become more developed in Protostomia, while in Deuterostomia the dorsal nerve cords have priority. It is emphasized that reaching an understanding between molecular biologists and evolutionary morphologists is a necessity.

Journal

Russian Journal of Marine BiologySpringer Journals

Published: Jan 25, 2009

References

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