A comparative molecular approach to mesodermal patterning in basal deuterostomes: the expression pattern of Brachyury in the enteropneust hemichordate Ptychodera flava

This work concerns the formation of mesoderm in the development of an enter opneust hemichordate, Ptychodera flava, and the expression of the Brachyury gene during this process. Brachyury expression occurs in two distinct phas es. In the embryo, Brachyury is transcribed during gastrulation in the futu re oral and anal regions of the gut, but transcripts are no longer detected by 2 weeks of development. Brachyury expression is not detected during the 5 months of larval planktonic existence. During this time, the adult coelo ms begin to develop, originating as coalescences of cells that appear to de laminate from the wall of the gut. Brachyury expression cannot be detected again until metamorphosis, when transcripts appear in the mesoderm of the a dult proboscis, collar and the very posterior region of the trunk. It is al so expressed in the posterior end of the gut. At no time is Brachyury expre ssed in the stomochord, the putative homologue of the chordate notochord, T hese observations illuminate the process of maximal indirect development in Ptychodera and, by comparison with patterns of Brachyury expression in the indirect development of echinoderms, their sister group, they reveal the e volutionary history of Brachyury utilization in deuterostomes.