New Insights into the Evolution of Neurons in Vertebrate Brains

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 Physiology.

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 stabilising 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 Physiology.

The study was funded by a Reinhart Koselleck Project of the German Research Foundation (DFG; Schu1470/8).