Frogs as integrative models for understanding digestive organ development and evolution

Abstract

The digestive system comprises numerous cells, tissues and organs that are essential for the proper assimilation of nutrients and energy. Many aspects of digestive organ function are highly conserved among vertebrates, yet the final anatomical configuration of the gut varies widely between species, especially those with different diets. Improved understanding of the complex molecular and cellular events that orchestrate digestive organ development is pertinent to many areas of biology and medicine, including the regeneration or replacement of diseased organs, the etiology of digestive organ birth defects, and the evolution of specialized features of digestive anatomy. In this review, we highlight specific examples of how investigations using Xenopus laevis frog embryos have revealed insight into the molecular and cellular dynamics of digestive organ patterning and morphogenesis that would have been difficult to obtain in other animal models. Additionally, we discuss recent studies of gut development in non-model frog species with unique feeding strategies, such as Lepidobatrachus laev is and Eleutherodactylouscoqui, which are beginning to provide glimpses of the evolutionary mechanisms that may generate morphological variation in the digestive tract. The unparalleled experimental versatility of frog embryos make them excellent, integrative models for studying digestive organ development across multiple disciplines.

Evolutionary and developmental considerations of the diet and gut morphology in ceratophryid tadpoles (Anura)

“…This includes a pepsinogen in early development that is electrophoretically indistinguishable from a form found in postmetamorphic Lepidobatrachus [14]. Evolutionary novelties in Lepidobatrachus larvae include a capacious stomach, the greater ratio of foregut to hindgut than any omnivorous tadpoles, and a gastroduodenal loop in a more caudal position [15,16].…”

Direct development or endotrophic tadpole? Morphological aspects of the early ontogeny of the plump toad Osornophryne occidentalis (Anura: Bufonidae)

“…The development of the midgut in exotrophic tadpoles reflects its correlation with trophic ecology. Omnivorous tadpoles often have long and coiled intestines when compared to tadpoles with specialized feeding habits like macrophagy, oophagy and carnivory; also, ontogenetic changes in diet (e.g., during metamorphosis) promote deep remodeling of the intestine (e.g., Wickramasinghe et al, 2007; Womble et al, 2016). In early embryos of E. coqui , the gut is an undifferentiated yolk‐filled sac, and subtle ontogenetic changes involve a length increase and a gradual twisting of the gut ends; after hatching, yolk provision decreases and a short, adult‐like gut develops (Langer, 2003).…”

Conserved chromatin and repetitive patterns reveal slow genome evolution in frogs

“…While genome evolution has been extensively studied in mammals 22 and birds 23 , 24 , the relative lack of phylogenetically diverse chromosome-scale frog genomes has limited the study of genome evolution in anuran amphibians. Here, we report a high-quality assembly for X. tropicalis and three new chromosome-scale genome assemblies for the Puerto Rican coquí ( Eleutherodactylus coqui ), a direct-developing frog without a tadpole stage 16 , 19 , the túngara frog ( Engystomops pustulosus ), which is a model for vocalization and mate choice 15 , 18 , 20 , and the Zaire dwarf clawed frog ( Hymenochirus boettgeri ), which has an unusually small embryo, is a model for regulation of cell and body sizes, and a source of potent host-defense peptides with therapeutic potential 13 , 17 , 21 . Genome assemblies are essential resources for further work to exploit the experimental possibilities of these diverse animals.…”