Fig. 12.

Retinoic acid (RA) signaling dependence of ectodermal sensory neuron (ESN) formation in amphioxus. Schematic representation of an amphioxus larva at 30 h post fertilization (hpf) with the anterior end directed towards the right. The central nervous system (CNS) is drawn in orange, the gut is drawn in gray, and a posterior-high gradient of RA signaling is indicated in magenta. ESN progenitors (ESNPs) are depicted as small ovals. Ectodermal expression of neural marker genes is shown as a black outline for hu/elav, as green filling for tlx, and as blue filling for soxb1c. Initially, tlx is widely expressed in the ventral ectoderm, but gradually becomes restricted to individual ESNPs that migrate dorsally. During early embryogenesis, RA signaling levels mildly influence the distribution of tlx-expressing ESNPs. The hu/elav gene is likely expressed in all specified ESNPs. In contrast, soxb1c expression is only detected in a small population of late developing ESNPs, located at mid-trunk levels in the dorso-lateral ectoderm. The single amphioxus rar gene is also most strongly expressed in this ectodermal domain [29, 41], as indicated by the black-outlined rhombus shown above the larva. Our data suggest that medium levels of RA signaling in the mid-trunk ectoderm contribute to specification of the soxb1c-expressing ESNP population and regulate its neurogenic activity