From so simple a beginning - what amphioxus can teach us about placode evolution

Cranial placodes are an evolutionary novelty of vertebrates that give rise to many cranial sense organs and ganglia, as well as to the neurosecretory anterior pituitary. Although amphioxus does not have placodes, it shares with vertebrates several of the ectodermal patterning mechanisms and cell types that are important in placode development. Comparisons between amphioxus, vertebrates and other groups provide us with important insights into what the last common chordate ancestor probably looked like and allow us to propose a scenario for how placodes evolved by rewiring of gene regulatory networks. After reviewing ectodermal patterning and the cytodifferentiation of neurosecretory and sensory cells in amphioxus, this review will argue that the evolutionary origin of cranial placodes involved 1) the concentration of sensory and neurosecretory cell types in the head by linking their development to ancient cranial ectodermal patterning mechanisms; and 2) the formation of high density arrays of sensorineural precursors by intercalating a progenitor expansion module into the gene regulatory network driving differentiation of sensory or neurosecretory cells.