Meeting Abstract
41.3 Wednesday, Jan. 5 Developmental basis of novel gut morphology in frogs BLOOM, S.K.; INFANTE, C.R.; EVERLY, A.W.; HANKEN, J.; NASCONE-YODER, N.M.*; North Carolina State University College of Veterinary Medicine; Harvard University; Harvard University; Harvard University; North Carolina State University College of Veterinary Medicine nmnascon@ncsu.edu
Evolutionary adaptation frequently occurs by selection on morphological variation, yet the developmental mechanisms that give rise to novel morphologies are poorly understood. Most anuran tadpoles are omnivores with a rudimentary stomach and elongated intestine; however, the unusual tadpoles of the South American frog Lepidobatrachus laevis are obligate carnivores with an enlarged stomach and short intestine. In all tadpoles the foregut loops rightward to orient the liver and pancreas asymmetrically; however, in Lepidobatrachus this loop is shifted distally, which dramatically alters the final anatomical positions of the digestive organs. To identify the molecular mechanisms that could lead to the evolution of these and other novel trophic features, we conducted a phenotypic screen for small molecules capable of inducing the development of carnivore-like gut looping in the omnivorous tadpoles of Xenopus laevis. Remarkably, exposing Xenopus embryos to multiple retinoic acid (RA) signaling inhibitors results in the formation of a shifted gut loop and reorganized foregut anatomy reminiscent of a carnivorous digestive tract. In the reciprocal experiment, in which Lepidobatrachus embryos are exposed to ectopic RA, the gut loop and foregut anatomy more closely resemble those of omnivorous tadpoles. Interestingly, Pitx2, a determinant of left-right asymmetric gut looping, is expressed in a distinct spatial pattern in Lebidobatrachus embryos; in both species, exposure to RA signaling modulators shifts the domain of Pitx2 expression in correlation with the resultant changes in looping. Modifications of RA signaling may have altered left-right asymmetric morphogenesis during anuran gut evolution, enabling the evolution of a novel larval feeding strategy.
