S2-1.5 Jan. 4 How old genes make a new head – recent insights into development and evolution of neural crest and placodes in vertebrates SCHLOSSER, Gerhard; University of Bremen firstname.lastname@example.org
Two embryonic tissues - neural crest and cranial placodes - give rise to most evolutionary novelties of the vertebrate head. While many cellular and molecular components of neural crest and placodes predate the origin of vertebrates, the integration of these components into true neural crest and placodes appears to have evolved only in the vertebrate lineage. Both tissues develop similarly in several respects: they originate from ectoderm at the neural plate border; they undergo pronounced cell shape changes; and they give rise to multiple specialized cell fates. Due to these similarities and their joint appearance in the vertebrate lineage, it is often thought that neural crest and placodes have a common evolutionary origin from a specialized population of neural plate border cells. However, pronounced developmental differences between the two tissues make this scenario questionable. First, development of neural crest and placodes depends on largely non-overlapping sets of transcription factors. In addition, recent data show that each tissue is induced at a different time of development and relies on partly different inductive tissues and signals. Moreover, our own experiments in Xenopus suggest that competence to respond to placode induction is confined to non-neural ectoderm, while competence to respond to neural crest induction is confined to neural ectoderm. Taking these various lines of evidence together, I propose that both tissues evolved independent from each other, that they recruited different sets of genes encoding for transcription factors and signaling molecules, and that neural crest cells originated from cells on the neural side of the neural plate border in protochordates, while placodes originated from cells on the non-neural side of the neural plate border.