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Animal EvolutionGenomes, Fossils, and Trees$
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Maximilian J. Telford and D.T.J. Littlewood

Print publication date: 2009

Print ISBN-13: 9780199549429

Published to Oxford Scholarship Online: September 2009

DOI: 10.1093/acprof:oso/9780199549429.001.0001

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Origins of metazoan body plans: the larval revolution

Origins of metazoan body plans: the larval revolution

(p.43) CHAPTER 5 Origins of metazoan body plans: the larval revolution
Animal Evolution

Rudolf A. Raff

Oxford University Press

Bilaterian animal body plan origins are generally thought about in terms of adult forms. However, most animals have larvae with body plans, ontogenies, and ecologies distinct from their adults. The first of two primary hypotheses for larval origins suggests the earliest animals were small pelagic forms similar to modern larvae, with adult bilaterian body plans evolved subsequently. The second suggests that adult bilaterian body plans evolved first and that larval body plans arose by interpolations of features into direct-developing ontogenies. The two hypotheses have different consequences for understanding parsimony in evolution of larvae and of developmental genetic mechanisms. If primitive metazoans were like modern larvae and distinct adult forms evolved independently, there should be little commonality of patterning genes among adult body plans. However, sharing of patterning genes in adults is observed. If larvae arose by co-option of adult bilaterian-expressed genes into independently evolved larval forms, larvae may show morphological convergence, but with distinct patterning genes, as is observed. Thus, comparative studies of gene expression support independent origins of larval features. Precambrian and Cambrian embryonic fossils are also consistent with direct development of the adult as primitive, with planktonic larval forms arising during the Cambrian. Larvae have continued to co-opt genes and evolve new features, allowing study of developmental evolution.

Keywords:   metazoan origins, larval evolution, body plans, Cambrian radiation, regulatory gene evolution, evolutionary developmental biology

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