The blind cavefish (Astyanax mexicanus) is a sightless version of a popular aquarium species, the Mexican tetra. They live in 29 deep caves scattered throughout Mexico, which their sighted ancestors colonised in the middle of the Pleistocene era. In this environment of perpetual darkness, the eyes of these forerunners were of little use and as generations passed, they disappeared entirely. They now navigate through the pitch-blackness by using their lateral lines to sense changes in water pressure.
But there is a deceptively simple way of restoring both the eyes and sight that evolution has taken, and Richard Borowsky from New York University’s Cave Biology Research Group has found it. You merely cross-bred fish from different caves.
Unlike their completely eyeless parents, the hybrids develop eyes albeit ones that are smaller than those of their relatives on the surface. More amazingly still, many of them could actually see, as shown by their ability to reflexively follow a series of moving stripes.
In the most successful inter-cave cross, over a third of the offspring had working eyes. And if the blind fish were bred with surface ones, every single one of their offspring could see. Not bad for a lineage that hasn’t seen light for over a million years!
The hybrids’ restored eyes are a reflection of the genetic changes of their parents. Eyes are very complicated structures and their development is governed by a whole suite of genes. In a previous study, Borowsky found eye genes in twelve different places around the genome of one cavefish population. Mutating any of these could interfere with the production of a working eye, which means that there are many ways of evolving blindness.
Fish populations from different caves have each taken their own individual route, involving changes to different combinations of genes. Based on his new data, Borowsky thinks that this happened on at least three independent occasions, with each group losing their eyes through changes in three or four of the twelve key sites. But in the hybrids, every faulty gene from one parent was compensated for by the working version from the other.
Borowsky also found that two fish had a greater chance of producing a hybrids if they hailed from closer caves. That suggests that fish from neighbouring caves are more closely related than those from distant ones and have more similar genes underlying their blindness.
More on hybrids:
Image courtesy or Richard Borowsky
Reference: Borowsky, R. (2008). Restoring sight in blind cavefish. Current Biology, 18(1), R23-R24.