Development and evolution of the larval stomach of the Budgett’s Frog, Lepidobatrachus laevis

Abstract

Anurans have a highly conserved biphasic life history with morphologically- and ecologically- divergent tadpole and adult phases. Tadpoles are typically aquatic, filter-feeding herbivores that transform dramatically during metamorphosis into semi-terrestrial, bulk-feeding carnivores. The gut morphology of these life phases is very constrained–tadpoles have a reduced, derived stomach while adult frogs have a typical vertebrate gut with a large digestive stomach. Lepidobatrachus laevis tadpoles evolved extreme larval carnivory, even cannibalism, with nearly complete ecomorphological overlap with adult frogs. Their stomachs develop peramorphically and are adult-like at hatching. This dissertation investigates the extent of the heterochronic shift and the developmental mechanisms generating the adult-like tadpole stomach in L. laevis. In Chapter 1, using histological examinations of microanatomy, I find that the developmental trajectory of the stomach of L. laevis is highly derived from the plesiomorphic state, and that the development of an adult stomach morphology is completely peramorphically shifted into larval development. The stomach of L. laevis does not remodel at metamorphosis. In Chapter 2, I use RNA-seq to investigate the genetic mechanisms by which development of the adult-like stomach occurs, with particular focus on the role of decreased retinoic acid in mediating this developmental shift. The findings in this chapter show that modules of stomach development in L. laevis, including those regulated by retinoic acid, are temporally shifted to resemble stomach metamorphosis in Xenopus tropicalis. The evidence presented in these two chapters provides evidence of a heterochronic shift in the development of L. laevis tadpole stomach morphology.