title: How old genes make a new head: redeployment of Six and Eya genes during the evolution of vertebrate cranial placodes.
abstract: Cranial placodes give rise to many evolutionary novelties of the vertebrate head, such as its specialized paired sense organs and cranial ganglia. There is an increasing evidence that all placodes originate from a common primordium located around the anterior neural plate and defined by the expression of transcription factors of the Six1/2, Six4/5, and Eya families. These transcription factors continue to be expressed in the different placodes and appear to control similar developmental processes (e.g., proliferation, cell shape changes, and neurogenesis) in the different placodes suggesting that they play a central role for generic placodal development. Elucidating the central role of Six and Eya genes for placodal development requires an understanding of (1) how these genes are induced in the pre-placodal ectoderm at the right place and time and (2) how they subsequently affect and promote placodal development. The first part of this review gives a brief overview of what is currently known about these upstream and downstream regulatory linkages of Six and Eya genes. The second part of the review then discusses the distribution and function of Six and Eya genes in other deuterostomes in order to infer changes of upstream and downstream linkages in the course of deuterostome evolution by which Six and Eya genes adopted their new role in vertebrate placode development. It is argued that these genes were probably recruited to the neural plate border in the ancestor of urochordates and vertebrates, and adopted novel roles in the regulation of neuronal differentiation and possibly other pathways of cytodifferentiation as well in the vertebrate lineage.
authors.affiliation: Brain Research Institute, University of Bremen, FB2, PO Box 330440, 28334 Bremen, Germany.
authors.forename: Gerhard
authors.initials: G
authors.lastname: Schlosser