To the classic case studies of evolution, such as Darwin’s finches and the peppered moth, a small group of researchers would now add the Mexican tetra. This small, freshwater fish is mostly found as a drably colored inhabitant of creeks and rivers. But at least a half dozen times, populations have taken up residence in caves, where they have been transformed. They remain the same species, but are now eyeless and nearly albino.
Nicolas Rohner, an evolutionary geneticist at Harvard Medical School in Boston, and his colleagues found that in the sighted tetras, a molecule called heat shock protein 90 (HSP90) masks variation in genes, such as those governing the size of the fish’s eye. The team proposes that the stresses of cave life disturbed the function of HSP90 and unmasked the genetic variation, providing a rich template upon which natural selection acted. The study bolsters those who have long believed HSP90 is an evolutionary force, says Kazuo Takahashi, a quantitative geneticist at Okayama University in Japan. “Its impact is huge.”
When Rohner treated fish embryos with an inhibitor of HSP90, some fish developed larger or smaller eyes and eye sockets than untreated fish, consistent with the idea that the chaperone masked phenotypic variation. Breeding the small-eye fish resulted in young that had small eyes as well, indicating that the size was genetically based.
Rohner then looked for an environmental stressor that might naturally release such variation and found that the underground water was much less salty than the surface water. When he placed surface fish embryos in water of such low salinity, the adults developed eyes and eye sockets that varied in size. In caves, smaller eyes should require less energy to grow and use, and thus would likely be favored by natural selection, Rohner suggests. “What is beautiful about this paper is that the HSP90-dependent change resembles a natural adaptation,” says Christine Queitsch, a geneticist at the University of Washington, Seattle.