Blind cavefish species evolved separately

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A loss of sight, albinism and altered sleep patterns are all traits that the blind Mexican cavefish has evolved in order to thrive in perpetual darkness. New research shows that this popular aquarium tetra is an example of convergent evolution, with several independent populations continually losing their sight and pigmentation.

Despite the obvious differences, blind cavefish are the same species as the surface dwelling version of Astyanax mexicanus, and they can interbreed.   

In a study published in the open-access journal BMC Evolutionary Biology, A team of researchers studied the DNA for 11 populations of cavefish, taken from three different regions, and 10 populations of the surface dwelling fish in order to study the evolutionary origin of their physical differences.

The results showed that the surface populations were very similar genetically, but it was very different for the cave populations. The cavefish had a much reduced genetic diversity, most likely as a result of limited space and food.

However, some of the cave populations received migrants from the surface, and in those populations there was a (expected) higher diversity. The report notes that migrations appeared to be a two-way affair.

It was always thought that originally there were at least two groups of fish that lived in the rivers of Sierra de El Abra, Mexico, one group colonising the caves and then becoming extinct on the surface, and then the rivers were restocked by a different population which also invaded the caves.

"We were fortunate in being able to use A. mexicanus as a kind of 'natural' experiment where nature has already provided the crosses and isolation events between populations for us."

Explains Prof Richard Borowsky, from the Cave Biology Group at New York University. "Our genotyping results have provided evidence that the cave variant had at least five separate evolutionary origins from these two ancestral stocks."

Lead researcher Dr Martina Bradic states: "Despite interbreeding and gene flow from the surface populations the eyeless 'cave phenotype' has been maintained in the caves. This indicates that there must be strong selection pressure against eyes in the cave environment. Whatever the advantage of the eyeless condition, it may explain why different populations of A. mexicanus cave fish have independently evolved the same eyeless condition, a striking example of convergent evolution."

For further information, see the open-access paper: Martina Bradic, Peter Beerli, Francisco García-de León, Sarai Esquivel-Bobadilla and Richard Borowsky. Gene flow and population structure in the Mexican blind cavefish complex (Astyanax mexicanus). BMC Evolutionary Biology, January 2012.

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