Subterranean Fishes of the World

3248

2017

AIM 2017

Drastic changes in life style and habitat through vertebrates’ evolution have brought along functional and morphological brain modifications. During colonization of caves, Astyanax cavefish have evolved behavioral adaptations in order to ensure their survival in such extreme conditions. Most of these behaviors, important for maintaining body homeostasis, are controlled by complex circuits within the hypothalamus. Here we performed a comparative study on the ontogeny and organization of peptidergic neurons in the hypothalamus and neurosecretory preoptic area (NPO) of Astyanax surface and cave morphotypes. Six out of the nine neuropeptides analyzed by in situ hybridization showed quantitative differences in terms of cell numbers in specific neuronal clusters. We found that variations where mainly restricted to clusters in the basal hypothalamus, while cell numbers in the NPO where similar in both morphs. Our interpretation of the gene expression data in the context of the prosomeric model highlights great conservation in terms of alar-basal forebrain organization from fish to mammals. Using knock-down experiments, we then showed that the increase in Hypocretin and NPY neurons in cavefish results from expanded expression domains of the transcription factors Lhx9 and Lhx7, respectively. Additionally, using pharmacological experiments, we found that early SHH and FGF signaling impact the fate specification and/or the differentiation of specific NPY and Hypocretin clusters. Finally, we found that manipulating Lhx9 expression has long lasting effect on Hypocretin neuron numbers in the hypothalamus, and increases sleep scores in cavefish. Thus, we propose a link between early developmental changes in brain patterning and the evolution of behavioral adaptations.

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