Blind Mexican tetra cavefish have adapted to their extreme, food-scarce environment by storing fat and evolving insulin resistance and dysregulated blood glucose homeostasis, reports a paper published online this week in Nature. These findings suggest the evolution of extreme physiological measures to accommodate exceptional environmental challenges.
Owing to the absence of photosynthesizing plants and algae, cave-dwelling animals must withstand long periods of nutrient deprivation. The Mexican tetra, Astyanax mexicanus, can be found in river-dwelling (surface) populations and cave-dwelling (cave) populations that experience markedly different nutrient availability. Cavefish are starvation resistant and lose a smaller fraction of their bodyweight when food deprived, as compared to surface fish. Several factors have been identified that contribute to starvation resistance, including reduced metabolic circadian rhythm, decreased metabolic rate and elevated body fat. The genetic changes underlying these adaptations, however, remain largely unknown.
Nicholas Rohner and colleagues compared the blood glucose levels of laboratory-raised tetra surface fish and tetra cavefish. They found that the cave populations exhibited significantly higher blood glucose levels after feeding. The authors then examined the dynamics of glucose homeostasis during short- and long-term fasting. Cavefish showed significantly higher blood glucose levels compared to a minor decrease in surface fish; however, after 21 days, the authors observed a marked decrease in blood glucose levels in cavefish compared to a minor decrease in surface fish, suggesting that dysregulated glucose homeostasis is a feature of cave populations. In addition, the authors found a difference in the insulin receptor gene between surface fish and cavefish, indicating a mutation associated with insulin resistance.