Evolved differences in microglial cell biology between surface and cave populations of Astyanax mexicanus

Microglia govern multiple aspects of brain architecture and function by eliminating dying neurons and glia, stimulating neurogenesis, refining neural connections, and orchestrating immune responses. The Mexican tetra, Astyanax mexicanus, is a powerful model system for investigating the evolution of brain function, yet microglia have not been investigated in this system. A. mexicanus exists as surface- and cave-dwelling morphotypes with prominent behavioral and physiological differences. Notably, these evolved behavioral and physiological changes in the cave morphotype, including diminished immune responses and alterations in sleep, circadian rhythms, and sensory processing, are directly linked to known microglial functions in other systems, suggesting that evolved differences in microglia may shape brain circuitry adaptations in cavefish. Here we develop an experimental toolbox to examine microglial specification, dynamics, and function in A. mexicanus and perform comparative analysis of microglial cell biology between the surface and cave morphotypes. We find that the cave populations show increased numbers of microglia over developmental time relative to their surface counterparts. Microglia in A. mexicanus exhibit a rapid expansion in response to inflammatory cues, distinct from the inflammatory response observed in the related teleost zebrafish. Furthermore, microglia in cave populations exhibit lower lysosomal pH and expanded proteolytic compartments relative to surface morphotypes. Together, our observations reveal evolved differences in microglial cell biology between surface and cave populations of A. mexicanus and provide a framework to uncover novel neuroimmune mechanisms underlying the remarkable adaptations of the Mexican tetra.