Coastal and inland American alligators (Alligator mississippiensis) diverge along physiological axes in response to a salinity gradient.

Ecotypes often occupy an environmental gradient, which can lead to divergence in physiological traits between ecotypes. We tested the hypothesis that alligators from inland and coastal environments have physiologically and genetically diverged from one another by exposing coastal and inland alligators to hypo-osmotic (0 PSU), iso-osmotic (10 PSU), and hyper-osmotic (20 PSU) salinities. For each alligator, we measured natremia before and after exposure, 1 behavioural trait, 10 histological traits, and gene expression levels in the liver and kidney. We found little evidence supporting population genetic differentiation between coastal and inland alligators, yet found significant physiological divergence between the 2 ecotypes. Coastal alligators exhibited slightly elevated natremia across salinity treatments both pre- and post-trial and there were large structural differences between the ecotypes in both the kidney and liver tissues. Broadly, the metabolic features of the liver were decreased and the osmoregulatory abilities of the kidney were increased to a greater extent in the coastal alligators than in the inland alligators, especially at high salinities. This was also reflected in the gene expression data, where most differentially expressed genes were involved in metabolic pathways. Together, these findings suggest that coastal alligators maintain sodium balance through a combination of increased renal processing capacity and a slightly elevated natremia set point. Overall, we found that the salinity gradient used in our study elicited contrasting physiological responses between alligators from coastal and inland environments.