Researchers from the Harvard Medical School and Whitehead Institute explained in detail how the cavefish adapted to its environment and lost its eyesight in the process.
Their study focused on a certain species of cave fish and pointed the heat-shock protein HSP90 as the change agent.
According to Harvard Medical School postdoctoral researcher and co-lead author Nicolas Rohner, thousands of years ago a group of Astyanax mexicanus, fish that are inherently found in Northeastern parts of Mexico, was forced to leave their river habitat and reside in the harsh underwater caves.
With such unfriendly and dangerous living conditions, these fish had no choice but to adapt in their new environment. Because of the extreme darkness, they lost their pigmentations since protection is no longer required, gained highly sensitive senses to avoid water pressures and identify prey movement, and lost their eyesight to channel their body’s limited resources towards more useful biological functions as they survive in the caves underwater.
This evolutionary process manifested in the loss of eyesight of these particular fish species is called “standing genetic variation.” The concept advocates that there are genetic mutations that are held back by any species population and this can only be triggered when the group experiences very extreme tension in dangerous situations.
Biology professor and Whitehead Institute member Susan Lindquist said that these mutations are held in check by the heat-shock protein HSP90. Her research proved the HSP90’s keeping genetic disparities from occurring in different types of organisms. Her studies show that during times of extreme anxiety and stress, the abundant stock of HSP90 quickly diminish, which in turn facilitates the phenotypic alterations to unfold without control.
Lindquist explained, “The delicate balance of protein folding—especially that controlled by HSP90—holds the key. Moderate changes in the environment create stresses on protein folding, causing minor changes in the genome to have much larger effects. Because HSP90 governs the folding of the key regulators of growth and development it produces a fulcrum point for evolutionary change.”
To test their theories, the researchers from Harvard Medical School and Whitehead Institute conducted a test on same species of fish with one group thriving on the surface and the other in underwater caves.
The surface fish, when put into a simulated stressful situation by blocking HSP90 activity, had developed changes in the size of its eyes in varying levels. On the other hand, similar tests on the cave fish showed no remarkable variation in eye size. The researchers however found that they come out with small orbits, evidence that they still respond to HSP90 changes.
According to Rohner, “It’s a very cool story in terms of the speed of evolution. This is the first study showing that this HSP90-mediated mechanism can be applied to vertebrates for real morphological adaptive traits.”
The study was published in the December 13 issue of Science.
