Meeting Abstract
Evolving from filter feeding chordate ancestors, vertebrates adopted a more active life style. These ecological and behavioral changes went along with an elaboration of the vertebrate head including novel paired sense organs which develop from novel embryonic tissues, the cranial placodes. To understand how the new cranial sense organs of vertebrates evolved, we thus need to elucidate how their embryonic precursors, the cranial placodes emerged in evolution. Interestingly, many of the genes encoding signalling molecules or transcription factors, which govern placode development are evolutionary ancient and also play important roles for spatial patterning or neuronal or sensory differentiation in invertebrates. Furthermore, cell types that are homologous to placodally derived cells in vertebrates (e.g. the hair cells of inner ear and lateral line) have been described not only in other deuterostomes but also in more distantly related invertebrate taxa. Nevertheless, proper placodes – specialized ectodermal domains with a rapidly expanding population of multipotent sensorineural progenitors giving rise to high density arrays of neurons and receptor cells – evolved as new structures only in the vertebrate lineage. This suggests that the evolutionary origin of placodes involved recruitment of pre-existing cell types into new sense organs, due to changes in the connectivity between evolutionarily ancient regulatory genes – i.e. the rewiring of gene regulatory networks (GRN). In a recent RNA-Seq screen in Xenopus, we have begun to identify direct target genes of the transcription factor Six1 and its cofactor Eya1 – key regulators of placode development in vertebrates providing us with first insights into the GRN underlying vertebrate placode formation. Comparisons with other chordates and beyond are now required to reconstruct how this GRN was established by evolutionary rewiring of more ancestral GRNs.
