Blind Cavefish Evidence of Expedited Evolution Triggered by Stress

A tiny, eyeless fish found deep within Mexican caves may be on its way to becoming the next big star of evolutionary research. According to the authors of a new study published in the journal Science, they have used the cavefish, known as Astyanas mexicanus, to demonstrate for the first time in nature the unveiling of so-called cryptic genetic variations in an animal.

Cryptic - or standing - genetic variation is an idea that dates back to the mid-19th century when the scientist Conrad Waddington applied stress to fruit fly pupae (the stage between larva and adult), and watched as they appeared with unfamiliar features such as broken veins in their wings.

Seeing this, Waddington argued organisms inherit a host of genetic variations that never see the light of day due to a mysterious mechanism masking them.

For decades, the mechanism went unidentified, with scientists affectionately nicknaming it "Waddington's Widget." It wasn't until 1998 that biologists from the University of Chicago published a paper in the journal Nature establishing the protein Hsp90 as a likely candidate.

However, as co-author of the new study and Harvard Medical School researcher Nick Rohner told Nature World News, "It's not like you have a small horse, then a hot summer and a modern-sized large horse is born." 

The process, he said, is much more involved than that.

In order to understand exactly how the phenomenon works in animals outside the lab, the researchers looked to the eyeless cavefish, comparing it to its surface-dwelling counterparts that retained their eyes and pigmentation.

The researchers estimate that a population of the A. mexicanus moved into the caves between 2 million to 3 million years ago, at which point they suddenly found themselves in a pitch black world. Whether they went as colonizers or begrudging victims, swept away against their will, is unclear. Either way, said co-researcher William Jeffrey from the University of Maryland, it was "a very, very recent event" as far as evolution is concerned.

"We are talking about a rapid evolutionary process here, as opposed to the 500 million years of natural selection that have unfolded since most animal [groups] appeared during the Cambrian Period," Jeffrey, who also works as a scientist at the Marine Biological Laboratory, said. "The fact that these eyeless cavefish are so young makes them very attractive to understand evolutionary processes at their beginning."

The fish would have encountered more than just darkness in their new home. Food is more sparse in caves and the water there is purer, meaning it conducts far less electricity. All of this together would have been enough, the researchers argue, to suppress the Hsp90 protein and give the suppressed genetic variants a chance to shine - a finding they corroborated through a series of tests.

The results, Rohner said, marks "the first ecological setting where Hsp90 also played a role in evolution."

The new study is also unique in that, unlike prior research, it suggests Hsp90 suppression can result in adaptive, or beneficial, morphological traits. Eyes sap a lot of energy - getting rid of them would have meant conserving it for things the fish needed more in the dark caves.

"On the surface, in an environment of light, bigger eyes are more helpful than tiny eyes," Jeffrey said. "But once the fish gets into the cave, if that (genetic variation) is unmasked, it doesn't matter if you have small eyes or big eyes because it is completely dark."

At this point it's still not clear how widespread the revealing of hidden genetic material due to stress is in nature, though there are a lot of places with extreme environments as well as quick changes in environment to examine, Rohner said.

Before they go looking in such places, however, there is still more work yet surrounding the cavefish. "The most imporant thing now is looking for the particular genes driving eye loss," the researcher said.