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First published online 24 August 2005
doi: 10.1242/dev.01973

Development 132, 4259-4272 (2005)
Published by The Company of Biologists 2005
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The evolutionary history of placodes: a molecular genetic investigation of the larvacean urochordate Oikopleura dioica

Susan Bassham and John H. Postlethwait*

Institute of Neuroscience, University of Oregon, Eugene, OR 97403, USA

* Author for correspondence (email: jpostle{at}uoneuro.uoregon.edu)

Accepted 7 July 2005

The evolutionary origin of vertebrate placodes remains controversial because divergent morphologies in urochordates, cephalochordates and vertebrates make it difficult to recognize organs that are clearly homologous to placode-derived features, including the olfactory organ, adenohypophysis, lens, inner ear, lateral line and cranial ganglia. The larvacean urochordate Oikopleura dioica possesses organs that morphologically resemble the vertebrate olfactory organ and adenohypophysis. We tested the hypothesis that orthologs of these vertebrate placodes exist in a larvacean urochordate by analyzing the developmental expression of larvacean homologs of the placode-marking gene families Eya, Pitx and Six. We conclude that extant chordates inherited olfactory and adenohypophyseal placodes from their last common ancestor, but additional independent proliferation and perhaps loss of placode types probably occurred among the three subphyla of Chordata.

Key words: Appendicularia, Larvacean, Tunicate, Chordate evolution




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