Méndez-Narváez Javier, Warkentin Karen M
Department of Biology Boston University Boston Massachusetts USA.
Calima Fundación para la Investigación de la Biodiversidad y Conservación en el Trópico Cali Colombia.
Ecol Evol. 2022 Feb 14;12(2):e8570. doi: 10.1002/ece3.8570. eCollection 2022 Feb.
Vertebrate colonization of land has occurred multiple times, including over 50 origins of terrestrial eggs in frogs. Some environmental factors and phenotypic responses that facilitated these transitions are known, but responses to water constraints and risk of ammonia toxicity during early development are poorly understood. We tested if ammonia accumulation and dehydration risk induce a shift from ammonia to urea excretion during early stages of four anurans, from three origins of terrestrial development. We quantified ammonia and urea concentrations during early development on land, under well-hydrated and dry conditions. Where we found urea excretion, we tested for a plastic increase under dry conditions and with ammonia accumulation in developmental environments. We assessed the potential adaptive role of urea excretion by comparing ammonia tolerance measured in 96h-LC tests with ammonia levels in developmental environments. Ammonia accumulated in foam nests and perivitelline fluid, increasing over development and reaching higher concentrations under dry conditions. All four species showed high ammonia tolerance, compared to fishes and aquatic-breeding frogs. Both nest-dwelling larvae of and late embryos of excreted urea, showing a plastic increase under dry conditions. These two species can develop the longest on land and urea excretion appears adaptive, preventing their exposure to potentially lethal levels of ammonia. Neither late embryos of nor nest-dwelling larvae of experienced toxic ammonia levels under dry conditions, and neither excreted urea. Our results suggest that an early onset of urea excretion, its increase under dry conditions, and elevated ammonia tolerance can all help prevent ammonia toxicity during terrestrial development. High ammonia represents a general risk for development which may be exacerbated as climate change increases dehydration risk for terrestrial-breeding frogs. It may also be a cue that elicits adaptive physiological responses during early development.
脊椎动物向陆地的殖民化已经发生过多次，包括青蛙中超过50次陆生卵的起源。一些促进这些转变的环境因素和表型反应是已知的，但对早期发育过程中水分限制和氨毒性风险的反应却知之甚少。我们测试了氨积累和脱水风险是否会在来自三个陆生发育起源的四种无尾两栖动物的早期阶段诱导从氨排泄向尿素排泄的转变。我们在水分充足和干燥条件下，对陆地上早期发育过程中的氨和尿素浓度进行了量化。在发现有尿素排泄的情况下，我们测试了在干燥条件下以及发育环境中氨积累时尿素排泄是否会适应性增加。我们通过比较在96小时半数致死浓度试验中测得的氨耐受性与发育环境中的氨水平，评估了尿素排泄的潜在适应性作用。氨在泡沫巢和卵周液中积累，随着发育过程增加，并在干燥条件下达到更高浓度。与鱼类和水生繁殖的青蛙相比，所有这四个物种都表现出对氨的高耐受性。[某物种]的巢居幼体和[另一物种]的晚期胚胎都排泄尿素，在干燥条件下排泄量有适应性增加。这两个物种在陆地上发育的时间最长，尿素排泄似乎具有适应性，可防止它们接触可能致命的氨水平。[某物种]的晚期胚胎和[另一物种]的巢居幼体在干燥条件下都没有经历有毒的氨水平，也都没有排泄尿素。我们的结果表明，早期开始排泄尿素、在干燥条件下尿素排泄增加以及提高氨耐受性，都有助于预防陆地发育过程中的氨毒性。高氨水平是发育过程中的一个普遍风险，随着气候变化增加陆生繁殖青蛙的脱水风险，这种风险可能会加剧。它也可能是一种在早期发育过程中引发适应性生理反应的信号。
