How the Cavefish Lost its Eyes

In the 1990s, Suzanne Rutherford and Susan Lindquist discovered that mutation of a gene in fruit flies led to a wide range of developmental defects. The gene, Hsp90, isn't a developmental gene, but encodes a heat shock protein which helps organisms cope with stress. I wrote about their research last year, explaining how Hsp90 might provide a mechanism to translate environmental stress into developmental and evolutionary changes. It's an exciting and intriguing idea, but all the work I described in that post was done with model species in lab conditions. Researchers have now found an example of Hsp90-driven evolution in the field; in a paper appearing in Science, they showed how this process led to the loss of eyes in the cavefish Astyanax mexicanus.

The Hsp90 story has been one of my favourites since I first read about it. The basic idea is that Hsp90 hides mutations, stopping the genetic changes from having an effect and so protecting them from selection. By masking mutations, Hsp90 allows them to build up over generations, safe from selection's cleansing scalpel; when Hsp90 is missing or isn't working properly, all of the mutations get exposed at once. The way it works is that heat shock proteins bind to other proteins and act as a kind of molecular scaffolding, helping them stay the right shape in the face of stressful conditions (ie, heat). When it's not busy protecting proteins from the effect of heat, Hsp90 binds to various proteins that are inherently unstable (like signalling proteins, for example) and keeps them from deforming. Mutations often change a protein's shape, but with Hsp90 maintaining them these proteins can accumulate mutations without being exposed to their effect. The mutations still happen, but Hsp90 prevents them from changing the protein's shape, which effectively hides them from selection.

To show that this doesn't only happen in the lab, Susan Lindquist teamed up with Professor Clifford Tabin of Harvard Medical School and scientists from several other institutes to study the Mexican cavefish. It's an ordinary-looking fish when found in rivers and creeks, but the cave-dwelling populations have lost their eyes and become albino. When the reserachers treated embryos from surface fish with a chemical which blocks Hsp90, they grew into adults with a wide range of eye sizes. The team then selected the small-eyed fish and allowed them to breed; their offspring had small eyes even when Hsp90 wasn't inhibited. Clearly, Hsp90 was masking mutations that affected eye size in the surface fish; once these mutations were exposed selection could act on them and even liberate them from being hidden by Hsp90.

The next step was to show how something similar could have happened in the history of the cave-dwelling fish. The researchers suspected that it had to do with the stress of switching to a cave environment; Hsp90 might have become too busy dealing with stress-related changes to have the time to stabilize other proteins. The team measured the differences between the cave and river water. The biggest difference they found was much lower levels of salt in the cave water, so they tried growing embryos from surface fish in low salinity water. Just like the embryos grown with the Hsp90-blocker, these grew into adults with big eyes, small eyes, and everything in between. Coping with the salinity was overwhelming Hsp90, and mutations were getting exposed. Fish with smaller eyes may have been favoured by selection because eyes are energetically expensive but relatively useless in these dark environments. “This is the first time that we can see in a natural setting where the stress came from and observe the variation that results,” said Tabin in a press release.

"Hsp90 [...] produces a fulcrum point for evolutionary change," said Lindquist. By buffering development against the effect of mutations, Hsp90 allows them to accumulate unseen; when the environment changes and the developing organism is under stress, the accumulated mutations seem to appear all at once. Many of them will probably be harmful, but some might not be -- they might even be beneficial in the creature's new circumstances. There's definitely more to the story of evolution, and many characteristics aren't under Hsp90's control. Nevertheless, this study shows that it can be an important player, providing bursts of variation just when they are most needed.

Reference
Rohner, N. et al. Cryptic Variation in Morphological Evolution: HSP90 as a Capacitor for Loss of Eyes in Cavefish. Science 342(6164): 1372 (2013). doi:10.1126/science.1240276

Press release from the Whitehead Institute for Biomedical Research

Image credits
The cavefish photo is from the Whitehead press release. It was taken by Nicolas Rohner.