AsianScientist (Dec. 18, 2015) – Scientists have uncovered the genetic secret behind the crab-eating frog’s (Fejervarya cancrivora) status as the only frog to survive in salty environments. The findings, published in Scientific Reports, suggest that the frog’s genes associated with ion transport have evolved rapidly and are significantly enriched in the negative regulation of renal sodium excretion.
Among the over 4,000 amphibian species worldwide, most live in rain forests, creeks, mountains, plateaus, or deserts, but almost none survive in highly salty environments, except one: the crab-eating frog, a species native to Southeast Asia. It is the only known modern amphibian tolerates hyperosmotic external conditions.
Compared with other species which cannot survive in environments with salinity higher that ten percent, the crab-eating frog inhabits coastal lowland, marshes and mangrove swamps. It even tolerates brief excursions into sea water or brackish water.
For several decades ago, scientists have tried to figure out why the crab-eating frog can live in such an exceptional environment. Scientists have measured their physiological characters (i.e., kidney function, tissue osmolyte levels) and the levels protein such as hepatic carbamoyl phosphate synthetase I, but the underlying molecular mechanisms were barely known.
A team led by Dr. Che Jing, of the Kunming Institute of Zoology, Chinese Academy of Sciences, conducted a comparative study on the transcriptomes and genomics of the crab-eating frog and its closely related saline-intolerant species, F. limnocharis and Hoplobatrachus rugulosus to explore the molecular basis of adaptations to hyperosmotic external environments.
The results showed that in F. cancrivora, genes associated with ion transport (i.e., sodium ion transport) have evolved rapidly. The positively selective genes ANPEP and AVPR2 were significantly enriched in the negative regulation of renal sodium excretion. Analyses of orthologous genes suggested rapid evolution occurred in genes potentially involved in the adaptation to seawater.
Both positively selected and differentially expressed genes exhibit enrichment in the GO category regulation of renal sodium excretion. More differentially expressed rapidly evolved genes occur in the kidney of F. cancrivora than in F. limnocharis. Four genes involved in the regulation of body fluid levels show signs of positive selection and increased expression.
These findings suggest that compared with the skin and brain, the kidney plays vital roles in the adaptations to hyperosmotic environments. Moreover, this study also emphasized the importance of the renin-angiotensin-aldosterone system and aldosterone-regulated sodium reabsorption pathway in osmotic regulation through evaluating the significant up-regulations of several genes in F. cancrivora.
These findings have elucidated the roles of the positively selected and differentially expressed genes play in the adaptation of crab-eating frogs to salty environments. The candidate genes and variants are promising for future studies on high salinity adaptation in poikilothermic animals.
The article can be found at: Shao et al. (2015) Transcriptomes Reveal the Genetic Mechanisms Underlying Ionic Regulatory Adaptations to Salt in the Crab-eating Frog.
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Source: Chinese Academy of Sciences; Photo: Bernard Dupont/Flickr/CC.
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