Evolutionary relationships between the amphibian, avian, and mammalian stomachs

Although the gut is homologous among different vertebrates, morphological d ifferences exist between different species. The most obvious variation in t he guts of extant vertebrates appears in the stomach. To investigate the ev olution of this structure, we compared the histology of the stomach and gas trointestinal tract in amphibian (Xenopus laevis), avian (Gallus gallus), a nd mammalian (Mus musculus) organisms, and defined the expression patterns of several genes within the developing guts of these lineages. In all three groups, we find that the anterior portion of the stomach has a similar gla ndular histology as well as a common embryonic expression of the secreted f actors Wnt5a and BMP-4. Likewise, within the amniote lineages, the posterio r nonglandular stomach and pyloric sphincter regions are also comparable in both histological and molecular phenotypes. The posterior stomach expresse s Six2, BMPR1B, and Barx1, whereas the pyloric sphincter expresses Nkx2.5. Although the adult Xenopus stomach exhibits both glandular and aglandular r egions and a distinct pyloric sphincter similar to that of the amniotic ver tebrates, the histology of the Xenopus tadpole gut shows less distinct vari ation in differentiation in this region, which is most likely a derived con dition. The molecular signature of the embryonic Xenopus gut correlates wit h the more derived morphology of the larval phase. We conclude that the glo bal patterning of the gut is remarkably similar among the different vertebr ate lineages. The distinct compartments of gene expression that we find in the gut be necessary for the unique morphological specializations that dist inguish the stomachs from terrestrial vertebrates.