Roles of retinoic acid and Tbx1/10 in pharyngeal segmentation: amphioxus and the ancestral chordate condition

Background: Although chordates descend from a segmented ancestor, the evolution of head segmentation has been very controversial for over 150 years. Chordates generally possess a segmented pharynx, but even though anatomical evidence and gene expression analyses suggest homologies between the pharyngeal apparatus of invertebrate chordates, such as the cephalochordate amphioxus, and vertebrates, these homologies remain contested. We, therefore, decided to study the evolution of the chordate head by examining the molecular mechanisms underlying pharyngeal morphogenesis in amphioxus, an animal lacking definitive neural crest. Results: Focusing on the role of retinoic acid (RA) in post-gastrulation pharyngeal morphogenesis, we found that during gastrulation, RA signaling in the endoderm is required for defining pharyngeal and non-pharyngeal domains and that this process involves active degradation of RA anteriorly in the embryo. Subsequent extension of the pharyngeal territory depends on the creation of a low RA environment and is coupled to body elongation. RA further functions in pharyngeal segmentation in a regulatory network involving the mutual inhibition of RA- and Tbx1/10-dependent signaling. Conclusions: These results indicate that the involvement of RA signaling and its interactions with Tbx1/10 in head segmentation precede the evolution of neural crest and were thus likely present in the ancestral chordate. Furthermore, developmental comparisons between different deuterostome models indicate that the genetic mechanisms for pharyngeal segmentation are evolutionary ancient and very likely predate the origin of chordates.