Evolutionary and homeostatic changes in morphology of visual dendrites of Mauthner cells in<scp>Astyanax</scp>blind cavefish

Mauthner cells are the largest neurons in the hindbrain of teleost fish and most amphibians. Each cell has two major dendrites thought to receive segregated streams of sensory input: the lateral dendrite receives mechanosensory input while the ventral dendrite receives visual input. These inputs, which mediate escape responses to sudden stimuli, may be modulated by the availability of sensory information to the animal. To understand the impact of the absence of visual information on the morphologies of Mauthner cells during development and evolutionary time scales, we examined Astyanax mexicanus. This species of tetra is found in two morphs: a seeing surface fish and a blind cavefish. We compared the structure of Mauthner cells in surface fish raised under daily light conditions, surface fish that raised in constant darkness, and two independent lineages of cave populations. The length of ventral dendrites of Mauthner cells in dark-raised surface larvae were longer and more branched, while in both cave morphs the ventral dendrites were smaller or absent. The absence of visual input in surface fish with normal eye development leads to a homeostatic increase in dendrite size, whereas over evolution, the absence of light led to the loss of eyes and a phylogenetic reduction in dendrite size. Consequently, homeostatic mechanisms are under natural selection that provide adaptation to constant darkness.