Endoderm Morphogenesis Reveals Integration of Distinct Processes in the Development and Evolution of Pharyngeal Arches

Abstract

The vertebrate pharyngeal arches (PAs) are established by a combination of two styles of segmentation; the most anterior 2 PAs are simultaneously but the others are sequentially formed. However, the mechanism underlying their coexistence is unclear. Here, we show that the simultaneous and sequential segmentation discretely proceeded, respectively, but were finally integrated at the second PP (PP2), by dynamic morphogenesis of pharyngeal endoderm in the zebrafish. The coordination of these 2 distinct processes appears to be common in the PA development of many vertebrates, in which specific developmental defects posterior to the PP2 are caused by mutations of particular genes or perturbation of retinoic acid signaling. Surprisingly, comparative analysis of PA segmentation showed that the combinatorial styles of PA development is present in shark but not in lamprey, suggesting that PA segmentation was modified in the stem gnathostomes corresponding to the drastic pharyngeal innovations, such as PA2-derived opercular.