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Assumptions that may seem self-evident are not always accurate when it comes to the evolution of vertebrate brains. Researchers at the University of Bayreuth have now demonstrated this by examining the largest neuron in the brains of blind Mexican cavefish. Their findings have been in the Proceedings of the National Academy of Sciences.

In the course of evolution, vertebrate brains can change in response to adaptation to a new habitat or altered living conditions. Until now, our understanding of brain evolution in vertebrates has mainly been based on changes in larger brain structures and an awareness that certain cell types have also undergone changes.

However, the selection factors that shape the characteristics of individual neurons over evolutionary time are challenging to study and remain poorly understood. Such insights require detailed knowledge of the shape and function of the neurons under investigation, as well as a significant alteration in a critical environmental factor that could serve as selective pressure.

Understanding how environmental changes have modified vertebrate brains over time contributes to our knowledge of biodiversity and the evolution of behaviors in vertebrates, including humans.

The Mexican tetra (Astyanax mexicanus) is a freshwater fish that naturally exists in a river-dwelling surface form and several independently evolved cave-dwelling forms. Unlike the surface form, some of the cave-dwelling forms lack functional eyes, which regressed over evolutionary time since, in the absence of light, the blind fish in caves faced no disadvantage and could reproduce.

However, a shared feature between the surface and cave forms is the Mauthner neuron, the largest neuron in the hindbrain of fish. As a so-called multisensory command neuron, it receives information from various sensory organs (including the eyes and ears) and, when necessary, triggers an escape response within milliseconds.

"The lack of predators in the cave environment and the loss of vision in the cave-dwelling form we studied suggest a drastic reduction in the Mauthner neuron and the loss of the dendritic branch that processes visual information in sighted species. However, we found that this seemingly self-evident assumption is incorrect," says Dr. Peter Machnik, head of the study at the University of Bayreuth's Department of Animal Â鶹ÒùÔºiology.

To gain these insights, the Bayreuth researchers examined and compared the Mauthner cells of the surface and cave forms of the Mexican tetra. The result: In the cave-dwelling form, all structural and functional features of the neuron remain intact, despite the significant environmental shift.

"Our study suggests that the Mauthner neuron in the blind Mexican tetra is subject to a complex form of stabilizing selection, which is not yet understood, but which conserves the characteristic features of the Mauthner neuron. The neuron's dendritic branch, which processes visual information in sighted species, still plays a role and may now be processing other sensory inputs," explains Machnik.

"What is particularly special about this study is that we have linked the advantages of the various forms of the Mexican tetra to those of the Mauthner neuron as an individually identifiable, multisensory neuron. This is a clear and verifiable example of how predictions about the evolution of neuron types in vertebrate brains can be far from self-evident," says Professor Dr. Stefan Schuster, Head of the Department of Animal Â鶹ÒùÔºiology.