The same fish apart from a few neurons

One is colourful and lives in the rivers of Latin America, the other one is blind, depigmented and lives in the darkness of Mexican caves, yet they are the same fish: Astyanax mexicanus. Researchers at the Paris-Saclay Institute of Neuroscience (CNRS/Université Paris-Sud) have investigated the embryonic mechanisms that cause their morphological and behavioural differences.

The evolution of brain development and its behavioural consequences is a major topic in understanding how vertebrates colonise new environments. Astyanax mexicanus is a model of choice to address this issue. 

This fish has two morphotypes: one surface-dwelling form which inhabits the rivers of Central and South America, and a cave-dwelling form made up of several populations living in the total and permanent darkness of Mexican caves. These two forms separated from an ancestor similar to the surface fish less than 30,000 years ago.

The cave fish have evolved to regressive traits – the most dramatic being the loss of eyes and pigmentation – but they have also developed several constructive traits such as a larger jaw, more papillae and neuromasts or larger olfactory epithelia.

How have the cave-dwelling specimens evolved to survive in such an environment?
Researchers at the Paris-Saclay Institute of Neuroscience (CNRS/Université Paris-Sud) observed that a different number of certain neurons developed in the hypothalamus in the embryos of both specimens. This natural variation in brain development impacts not only on the morphology of the cave fish, but also on their behaviour.

By intervening in the neuronal development of the cave fish larvae, the researchers succeeded in making them mimic the behaviour of surface fish. This study therefore demonstrates new variations underlying the evolution and adaptation of cave fish to their extreme environment. These variations in the number of neurons originate in the very early embryonic processes, which occur during the first ten hours after fertilisation. These results were published in the journal eLife.

References:
  Developmental evolution of the forebrain in cavefish, from natural variations in neuropeptides to behavior, Alexandre Alié, Lucie Devos, Jorge Torres-Paz, Lise Prunier, Fanny Boulet, Maryline Blin, Yannick Elipot and Sylvie Rétaux, eLife, 6 February 2018. doi.org/10.7554/eLife.32808
And a second study which appeared in the journal on 6 February 2018
  Hypocretin underlies the evolution of sleep loss in the Mexican cavefish, James B Jaggard Bethany A Stahl Evan Lloyd David A Prober Erik R Duboue Alex C Keene, eLife, 6 February 2018. doi.org/10.7554/eLife.32637
 
Contact: Sylvie Rétaux | retaux @ inaf.cnrs-gif.fr