Among the over 4,000 amphibian species worldwide, most distribute in rain forests, creeks, mountains, plateaus, or deserts, but almost no one survives in environments with high salinity. As an exception, the crab-eating frog, Fejervarya cancrivora, a species native to south-eastern Asia, is the only known modern amphibian tolerates hyperosmotic external conditions. Compared with other species which cannot survive in environments with salinity higher that 10‰, the crab-eating frog inhabits coastal lowland, marshes and mangrove swamps. It even tolerates brief excursions into sea water or brackish water.
Since several decades ago, scientists try to figure out the reasons behind this specialty of the crab-eating frogs. Some physiological characters (i.e., kidney function, tissue osmolyte levels) and certain protein levels (i.e., hepatic carbamoyl phosphate synthetase I) were measured, However, the underlying molecular mechanisms are barely known. Therefore, recently, Dr. CHE Jing (Kunming Institute of Zoology, Chinese Academy of Sciences) and her colleagues 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 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.
In summary, this study elucidated the roles of the positively selected and differentially expressed genes play in the adaptation of crab-eating frogs to salty environment. The candidate genes and variants are promising for future studies on high salinity adaptation in poikilothermic animals. The main findings of this study have been published on Scientific Reports (http://www.nature.com/articles/srep17551).
State Key Laboratory of Genetic Resources and Evolution, Kunming Institute of Zoology (http://sourcedb.kiz.cas.cn/yw/zjrc/groups/201212/t20121210_3702475.html)