Subterranean Fishes of the World

3224

2013

AIm 2013

Astyanax mexicanus, with its surface dwelling (surface fish) and cave adapted (cavefish) morphs, is an important model system in evolutionary developmental biology. Astyanax cavefish differ from surface fish in numerous traits, including morphological, physiological, and behavioral traits. We will report on the developmental and neural bases for the evolution of social and feeding behavior in cavefish. We first focused on the loss of aggressive behavior in cavefish. We used an intruder-resident behavioral assay to compare aggressiveness quantitatively (attack counts) and qualitatively (pattern and nature of attacks) between the surface and Pachón cave morphs of Astyanax. Using this paradigm, we characterized aggressive behavior in surface fish, bring support for the genetic component of this trait, show that it is controlled by hindbrain raphe serotonergic neurons and that it corresponds to the establishment of dominance between fish. Cavefish have lost such aggressive/dominance behavior. The few attacks performed by cavefish during the behavioral test instead correspond to food searching behavior, driven by the developmental evolution of their hypothalamic serotonergic neurons, itself due to increased Hedgehog signaling during early forebrain embryogenesis. The latter result led us to focus on the developmental evolution of the hypothalamus, a forebrain region known to be involved in the control of homeostasis, including the control of feeding behavior and food intake. Previous analyses in our group on early embryos have shown that hypothalamic development and size is modified in cavefish. We therefore mapped and compared in surface fish and cavefish hypothalamus the neuropeptidergic (NPY, POMC, VP, OT, GnRH, SS) and dopaminergic neurons. We found significant differences in the sizes of some of these neuronal groups that may account, together with the difference in serotonin neurons reported above, in the differences of feeding behavior in the two Astyanax morphs. We propose that during evolution and adaptation to their cave habitat, cavefish have undergone a behavioral shift, due to modifications of their hypothalamic neuronal networks. They have lost the typical aggressive behavior of surface fish and evolved a food searching behavior that is probably more advantageous to survive in the dark. We have therefore demonstrated a link between the development of a neuronal network and the likely adaptive behaviors it controls. Work supported by ANR [ASTYCO] and [BLINDTEST].

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