Meeting Abstract
Ammonia excretion is cheap but requires water; excreting urea helps terrestrial vertebrates conserve water and avoid ammonia toxicity. The desiccation risk and waste-disposal problems created when frogs evolved terrestrial eggs offer opportunities to test the role of nitrogen (N) excretion plasticity in transitions to life on land. We hypothesize that terrestrial early life stages alter N-excretion in response to drying or high ammonia levels. We studied 3 frogs that independently evolved terrestrial development: gelatinous arboreal egg clutches of Agalychnis callidryas (Ac) and Hyalinobatrachium fleischmanni (Hf) and terrestrial foam nests of Leptodactylus fragilis (Lf). Dry conditions increased ammonia concentration in all 3 species. With extended terrestrial development, Hf embryos and tadpoles in Lf foam nests began excreting urea. In Lf this was crucial to avoid ammonia toxicity. We hypothesize these animals precociously upregulate the urea cycle enzymes CPS1 and Arginase; enzymatic analyses to date support Arginase upregulation in Lf. Urea was present in some Ac clutches at oviposition, but appears not to be synthesized by embryos. Still, ammonia remained surprisingly low in dry Ac clutches, well below toxic levels, suggesting an alternative pathway to avoid toxicity. Initial results suggest upregulation of glutamine synthesis by Ac embryos under dry conditions. Overall, our results suggest that terrestrial early life stages of frogs use different plastic mechanisms to avoid ammonia toxicity in different lineages.
