Early onset of urea synthesis and ammonia detoxification pathways in three terrestrially developing frogs

Frogs evolved terrestrial development multiple times, necessitating mechanisms to avoid ammonia toxicity at early stages. Urea synthesis from ammonia is a key adaptation that reduces water dependence after metamorphosis. We tested for early expression and plasticity of enzymatic mechanisms of ammonia detoxification in three terrestrial-breeding frogs: foam-nest-dwelling larvae of Leptodactylus fragilis (Lf) and arboreal embryos of Hyalinobatrachium fleischmanni (Hf) and Agalychnis callidryas (Ac). Activity of two ornithine-urea cycle (OUC) enzymes, arginase and CPSase 1, and levels of their products urea and CP in tissues were high in Lf regardless of nest hydration but reduced in experimental low- vs. high-ammonia environments. High OUC activity in wet and dry nests, comparable to that under experimental high ammonia, suggests terrestrial Lf larvae maintain high capacity for urea excretion regardless of their immediate risk of ammonia toxicity. This may aid survival through unpredictab..., We conducted experiments during the rainy seasons of 2018 and 2019 in Panama with early ontogenetic stages of three terrestrial-breeding frogs: Leptodactylus fragilis (which lay eggs in subterranean foam-nests), Agalychnis callidryas (which lay gelatinous clutches on leaves over ponds) and Hyalinobatrachium fleischmanni (which lay gelatinous clutches on leaves over streams, then provide paternal egg-care). We collected clutches from the field and moved them to an ambient-conditions laboratory (~26°C, ~85% RH) at the Smithsonian Tropical Research Institute (STRI) in Gamboa. For all three species, we exposed developing animals to species-specific well-hydrated (wet) and dry terrestrial conditions in the laboratory and sampled them at ages after the detection of ammonia or urea excretion in prior work (Méndez-Narváez and Warkentin, 2022). For L. fragilis and H. fleischmanni, we also exposed individuals from well-hydrated (wet) terrestrial conditions to species-specific low and high environ...,