Conserved Genetic Mechanisms for Deuterostome Gut Regionalization
Casey, Elena S
The vertebrate digestive system is comprised of several discrete organs of varied function arranged in a stereotyped pattern along the anterior/posterior (A/P) axis. A tightly regulated transcriptional network regionalizes the developing gut along the A/P axis, but little is known of whether this network is a derived vertebrate trait or a variation on an ancestral endodermal patterning network. To answer this question and clarify how differences in organization and novelties in the gut arose during deuterostome evolution, we have characterized the temporal and spatial expression patterns of several members of the vertebrate endodermal patterning transcriptional network in the basal deuterostome, Saccoglossus kowalevskii. We determined that expression of the endodermal patterning genes foxA, gata4/5/6, nkx2.1, nkx2.2, gsc, and cdx are highly conserved across deuterostomes, suggesting a transcriptional network comprising these genes and controlling endodermal patterning may have been present in the last common deuterostome ancestor. After gut regionalization, organ primordia are established by the ordered expression of organ specification genes. To compare how functional regions are organized between vertebrates and Saccoglossus, we analyzed the expression of cell-type markers with a focus on markers of pancreatic cell types, and find that a key gene in vertebrate pancreatic development, ptf1a, is expressed in the 2nd partition of the developing Saccoglossus gut. We conclude that the second gut partition in Saccoglossus shares a homologous origin with the posterior foregut of vertebrates.
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