Evolutionary origin of vertebrate cranial placodes

Schlosser, Gerhard. (2021). Evolutionary origin of vertebrate cranial placodes. In Gerhard Schlosser, Evolutionary origin of sensory and neurosecretory cell types: Vertebrate cranial placodes, Volume 2. Boca Raton: CRC Press.

Abstract

Chapter 6 addresses the question of how cranial placodes evolved as novel structures in vertebrates by redeploying pre-existing and sometimes evolutionarily ancient cell types. The chapter integrates insights from comparative studies with our knowledge on vertebrate placode development. After briefly summarizing the evolutionary history of sensory and neurosecretory cells, the chapter discusses how these cell types may have become concentrated in non-neural ectoderm adjacent to the neural plate and how they may have become segregated into an anterior and posterior proto-placodal territory in the last common tunicate-vertebrate ancestor. It is then discussed how an increase in progenitor expansion may have converted these proto-placodal territories into proper placodes, which give rise to larger and complex sense organs, in ancestral vertebrates. It is argued that rewiring of the gene regulatory network upstream and downstream of the transcriptional regulators Six1/2, Six4/5 and Eya probably played a central role for the evolution of placodes. A detailed scenario of placode evolution is then presented, which summarizes the proposed regulatory changes and links them to functional changes of life style during chordate and vertebrate evolution. The chapter ends with general conclusions on the evolution of novelties as illustrated by placodes and placodal cell types.

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CRC Press

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Attribution-NonCommercial-NoDerivatives 4.0 International

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Zoology (Book Chapters)