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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}

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Genes & Dev. 28, 34-43
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Developmental plasticity, genetic assimilation, and the evolutionary diversification of sexual expression in Solanum.
P. K. Diggle and J. S. Miller (2013)
Am. J. Botany 100, 1050-1060
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Mouth asymmetry in the textbook example of scale-eating cichlid fish is not a discrete dimorphism after all.
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Proc R Soc B 279, 4715-4723
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J. Mollus. Stud. 78, 308-311
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{beta}-Catenin 1 and {beta}-catenin 2 play similar and distinct roles in left-right asymmetric development of zebrafish embryos.
M. Zhang, J. Zhang, S.-C. Lin, and A. Meng (2012)
Development 139, 2009-2019
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PNAS 109, E326-E335
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Curious Chiral Cases of Caddisfly Larvae: Handed Behavior, Asymmetric Forms, Evolutionary History.
R. Hinchliffe and A. R. Palmer (2010)
Integr. Comp. Biol. 50, 606-618
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Lophotrochozoa Get into the Game: The Nodal Pathway and Left/Right Asymmetry in Bilateria.
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Cold Spring Harb Symp Quant Biol
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Development 136, 207-217
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Congenital Unilateral Absence of the Corticospinal Tract in a Siamese Cat.
A. C. Palmer, N. D. Jeffery, and R. J. M. Franklin (2007)
Veterinary Pathology 44, 949-951
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Courtship and genetic quality: asymmetric males show their best side.
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Proc R Soc B 274, 2115-2122
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The ecology of invasion: acquisition and loss of the siphonal canal in gastropods.
G. J. Vermeij (2007)
Paleobiology 33, 469-493
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Right-handed snakes: convergent evolution of asymmetry for functional specialization.
M. Hoso, T. Asami, and M. Hori (2007)
Biol Lett 3, 169-173
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Phenotypic plasticity and evolution by genetic assimilation.
M. Pigliucci, C. J. Murren, and C. D. Schlichting (2006)
J. Exp. Biol. 209, 2362-2367
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Early Asymmetry of Gene Transcription in Embryonic Human Left and Right Cerebral Cortex.
T. Sun, C. Patoine, A. Abu-Khalil, J. Visvader, E. Sum, T. J. Cherry, S. H. Orkin, D. H. Geschwind, and C. A. Walsh (2005)
Science 308, 1794-1798
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Stress-induced variation in evolution: from behavioural plasticity to genetic assimilation.
A. V Badyaev (2005)
Proc R Soc B 272, 877-886
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Developmental plasticity and the origin of species differences.
M. J. West-Eberhard (2005)
PNAS 102, 6543-6549
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