December 19, 2013
Shubashree Desikan
The Mexican tetra, Astyanax mexicanus, is a type of fish found in northeastern Mexico. It is also found in a blind form and is known as blind cavefish. Many thousands of years ago some of these fish found their way into caves that were nearly completely dark. Over the years, they adapted to this environment by totally losing their eyes and colouring. There are however eyed forms of this fish which did not get into caves, and these can interbreed, making them members of the same species.
Studying this, Nicolas Rohner et al, state in Science an explanation of how this major evolutionary change happened in, relatively, such a short time-span.
Tying up with an idea by Susan Lindquist, they propose that the heat shock protein 90 (HSP90), found in the cells, plays a major role in this quick evolution to eyeless and depigmented forms. When HSP90 is present, any mutation happening at the level of the genes is masked and prevented from manifesting as traits. If for some reason, HSP90 is depleted from the cell, the mutation shows up as physical changes. In the case of the cavefish, they say that the environmental change acts as a stress that causes depletion of HSP90 in the cells of the cavefish.
The authors tested this theory by an experiment involving surface and cavefish. When surface fish (the form with eyes) were raised in the presence of a drug that blocks HSP90 activity, they developed smaller eyes over a few generations. Cavefish, on the other hand, under the same conditions did not show increase in the eye-orbits. (Though they do not have eyes, they have eye-orbits in their skulls.)
The next step was to effect the depletion of HSP90 through external, environmental stress. They identified that the caves in which the fish were reared had water with low conductivity. They repeated the experiments with a low-conductivity environment, and found compatible results.
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