The endoderm is the region of an early embryo that forms the digestive organs. Analysis of the frog model, Xenopus laevis, has provided a molecular framework of endoderm development, and it depends on VegT RNA. VegT codes for a transcription factor, and its RNA is localized during oogenesis to a small region of the oocyte, the vegetal cortex. After fertilization, VegT causes transcription of signaling molecules and transcription factors, necessary for development of both mesoderm and endoderm.
Many frogs, the anuran amphibians, have evolved reproductive modes involving great increases in the amount of yolk and the size of the egg. These natural experiments can be exploited to see the impact of increased yolk on developmental patterning. Such an increase likely occurred 350 million years ago with the evolution of the amniote egg, which allowed vertebrates to divorce themselves completely from the water. Derived changes in development of frogs with large eggs may mirror changes associated with that critical but obscure evolutionary event.
The impact of increased yolk on the molecular organization of three independently derived cases of large-egged anurans will be examined. The role of VegT in endoderm development in Eleutherodactylus coqui, whose egg is 20x larger in volume than that of X. laevis, will be a primary focus. Location of both EcVegT RNA and mesoderm formation has shifted towards the animal side of the egg and away from the very yolky vegetal side. The hypothesis will be tested that much of the yolk-rich vegetal part of the egg has withdrawn from active involvement in development. These vegetal cells would not be under VegT control and would not form definitive gut tissues. Their role would be solely to provide nutrition, much like the uncleaved yolk of the chicken egg.
With respect to broader impacts, most of the research will be done by undergraduates and MSc students. They will receive excellent training in molecular biology. The frogs under investigation are endemic to Latin America, so obtaining the animals provides opportunities for interactions with Hispanic colleagues and students through seminars, course lectures, and collaborations at the Universidad de Puerto Rico and the Pontificia Universidad Catolica del Ecuador. Finally, the experience gained by working with E. coqui will allow development of husbandry techniques that may prove valuable, given the decline in populations of several species related to E. coqui.
