Animal behaviors vary immensely throughout the animal kingdom, yet in limited cases are the
underlying changes in neuroanatomy or neuronal function understood. The blind Mexican
cavefish, Astyanax mexicanus, is an emerging model system to address questions in evolutionary
biology, but the relationship between evolved differences in behaviors and neuroanatomy is
poorly understood. To gain insight into how changes in neuroanatomy may contribute to evolved
differences behaviors, we have constructed a brain atlas for adult surface fish, and for three
separate populations of cavefish. We have dissected brains from adult animal, stained them with
Nissl, and performed serial sectioning throughout the entire brain. We then performed volumetric
reconstruction of serial sections, and morphometric analysis of distinct neuroanatomical areas.
Consistent with previously published data, we find a reduction in the size of the optic tectum and
an expanded telencephalon in all three cave populations relative to surface conspecifics.
Furthermore, sub-nuclei within regions known to regulate diverse behaviors such as the thalamus,
epithalamus and hypothalamus were investigated. Our data reveal that the overall volume of the
hypothalamus was larger in cavefish populations relative to surface fish, though changes in size
are present in some, but not all nuclei. Additionally, the thalamus and epithalamus, regions
associated with diverse behaviors, were modestly larger for all cave populations, with Molino
having the largest of all morphs examined. To begin utilizing this atlas, we are investigating
neuroanatomical regions underlying stress. We have previously demonstrated that adult cavefish
have reduced stress responses relative to surface forms. Our data demonstrate that reduced
stress is also found in larval forms. We are now examining whole brain activity maps in adult
brains in surface and cavefish subjected to stress, and will map these to the brain atlas generated.
These studies will provide the community with a detailed neuroanatomical description of
differences in the brains of surface and multiple cavefish populations, and highlight the utility in
examining how millions of years of evolution lead to divergence of fundamental behaviors.