Evolution of gastrointestinal tract morphology and plasticity in cave-adapted Mexican tetra, Astyanax mexicanus

Abstract

The gastrointestinal tract has evolved in numerous ways to allow animals to optimally assimilate energy from different foods. The morphology and physiology of the gut is plastic and can be greatly altered by diet in some animals. In this study, we investigated the evolution and plasticity of gastrointestinal tract morphology by comparing laboratory-raised cave- and river-adapted forms of the Mexican tetra, Astyanax mexicanus, reared under different dietary conditions. In the wild, river-dwelling populations (surface fish) consume plants and insects throughout the year, while cave-dwelling populations (cavefish) live in a perpetually dark environment and depend on nutrient-poor food brought in by bats or seasonal floods. We found that multiple cave populations converged on a reduced number of digestive appendages called pyloric caeca and that some cave populations have a lengthened gut while others have a shortened gut. Moreover, we identified differences in how gut morphology and proliferation respond to diet between surface fish and cavefish. Using a combination of quantitative genetic mapping, population genetics, and RNA sequencing, we found that changes to the molecular and genetic pathways that influence cell proliferation, differentiation, and immune system function may underlie evolution of the cavefish gut.