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Science 306 (5697): 828-833

Copyright © 2004 by the American Association for the Advancement of Science

Symmetry Breaking and the Evolution of Development

A. Richard Palmer

Abstract: Because of its simplicity, the binary-switch nature of left-right asymmetry permits meaningful comparisons among many different organisms. Phylogenetic analyses of asymmetry variation, inheritance, and molecular mechanisms reveal unexpected insights into how development evolves. First, directional asymmetry, an evolutionary novelty, arose from nonheritable origins almost as often as from mutations, implying that genetic assimilation ("phenotype precedes genotype") is a common mode of evolution. Second, the molecular pathway directing hearts leftward—the nodal cascade—varies considerably among vertebrates (homology of form does not require homology of development) and was possibly co-opted from a preexisting asymmetrical chordate organ system. Finally, declining frequencies of spontaneous asymmetry reversal throughout vertebrate evolution suggest that heart development has become more canalized.

Systematics and Evolution Group, Department of Biological Sciences, University of Alberta, Edmonton, AB T6G 2E9, Canada, and Bamfield Marine Sciences Centre, Bamfield, BC V0R 1B0, Canada.

E-mail: rich.palmer{at}ualberta.ca

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