Reiterative expression of pax1 directs pharyngeal pouch segmentation in medaka

A striking characteristic of vertebrate development is the pharyngeal arches, which are a series of bulges on the lateral surface of the head of vertebrate embryos. Although each pharyngeal arch is segmented by the reiterative formation of endodermal outpocketings called pharyngeal pouches, the molecular network underlying the reiterative pattern remains unclear. Here, we show thatpax1plays critical roles in pouch segmentation in medaka embryos. Importantly,pax1expression in the endoderm prefigures the location of the next pouch before the cells bud from the epithelium. TALEN-generatedpax1mutants did not form pharyngeal pouches posterior to the second arch. Segmental expression oftbx1andfgf3, which play critical roles in pouch development, was almost nonexistent in the pharyngeal endoderm ofpax1mutants, with disturbance of the reiterative pattern ofpax1expression. These results suggest thatpax1plays a critical role in generating the primary pattern for segmentation in the pharyngeal endoderm by regulatingtbx1andfgf3expression. Our findings illustrate the critical roles ofpax1in vertebrate pharyngeal segmentation and provide insights into the evolutionary origin of the deuterostome gill slit.