Evolution of brain development

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The paper in eLife is here: https://elifesciences.org/articles/32808

Colonization of extreme environments implies the acquisition of adaptive behaviors necessary to succeed in that endeavor. These adaptations often involve massive shaping in brain morphology, specifically in regions processing information related to particular ecological features. Astyanax mexicanus species is becoming a popular fish model to study developmental causes leading to morphological and behavioral adaptations, since it comprises “wild-type” river-dwelling fish as well as several “natural mutant” populations living in caves that have evolved outstanding troglomorphic features, as the eyeless phenotype.    

After coming from a lab working in the well-established model zebrafish, working in a non-model organism like Astyanax mexicanus has been challenging and frustrating at some points to me. However, after some adaptation and plasticity from my part to deal with the particularities and constraints of Astyanax biology, it is now very satisfying and rewarding to see the project’s advancement over time and to enjoy the biological insights and data provided by this natural variant fish.

I joined the lab of Sylvie Rétaux to study the evolution of hypothalamus development in Astyanax mexicanus cavefish. The hypothalamus is a homeostatic sensor involved in the regulation of vital functions that may be different for the two types of habitats in which Astyanax resides. At first, I was impressed by the fact that lab members were able to identify to what morphotype an embryonic brain belongs just by looking at the expression of some genes. In our work we focused in the development of neuropeptidergic neurons which are the functional units regulating behaviors like sleep, food intake, reproduction, among others. All these behaviors are central to cavefish survival, in the darkness of their caves. We found that cavefish hypothalamus contained more neurons expressing hypocretin and NPY and less neurons expressing POMCb when compared to the surface counterpart. Interestingly, the differences were obvious from the moment when neurons are born in the early embryos, and we attributed the variations to subtle changes in the expression of homeotic “LIM” genes, specifically Lhx7 and Lhx9, as well as expression of signaling molecules Shh and Fgf8. It means that that the differences in brain anatomy between the two Astyanax morphotypes arise during gastrulation, between 6 and 10 hours post-fertilization, when the embryo looks like a ball of cells! We went further and, by genetic manipulations, we showed that numeric differences in hypocretin neurons contribute to behavioral adaptations exhibited in cavefish.  

Every other year our laboratory engages in a fieldtrip to Mexico, in order to study the fish in their natural environment. During lab meetings, Sylvie mentioned to us that it is a tradition in the team that all members must go at least once to the caves, to experience the extreme conditions where the fish live. So, we went to Mexico last year and the fieldtrip was nothing else than an enriching experience. I think that only by being in that dense darkness it is possible to appreciate the impressive selective pressure overcame by the fish thriving in those caves. I learnt that, in different ways, these “scientific retreats” have an important impact in the maturation of every project, since it helped us understanding the natural behavior of the fish and the ecological characteristics that are unique to the different caves and somehow affect the subsistence of cavefish populations.

Fieldtrip to Mexico. Entrance of Sabinos cave seen from the inside (left). Cavefish containing pool in Sabinos cave (center). Decomposing dead bat found in the floor of this cave (right). 

We took advantage of this travel to Mexico to attend to the 5th Astyanax meeting, where we spent some good time with other members of the Astyanax community. Among the presentations, the team of Alex Keene showed by a different approach the involvement of the hypocretinergic system in the sleep loss phenotype described in cavefish. Since the findings of both teams supported each other’s, we decided to co-submit our works in the same journal. The two papers came out together on February 6th, 2018, in eLife.

In this moment many diverse projects are under development in our laboratory, combining population genetics, behavior, morphogenesis, comparative neuroanatomy and genome evolution. Regarding brain evolution, we are now going further backwards in developmental time to study the mechanisms that can account for differences in brain patterning that we observed, specifically focusing in the establishment of embryonic axis during gastrulation and maternal contribution during the formation of the embryonic organizer. Astyanax mexicanus offers an ideal model species to study the different levels of complexity, from development to behavior, which can be target of natural selection in independently evolving populations leading at the same time to convergent phenotypes. These outstanding features of Astyanax mexicanus motivate me to continue in cavefish research in the future.  The difficulties experienced by working in a non-model organism are largely overcome by the exceptional results and insights cavefish can offer.