“Rapid Evolution” Method Found in Eyeless Fish
The controversial idea that vertebrate evolution can happen rapidly, in the merest handful of generations, has been given a boost.
In the journal Science, Harvard University evolutionary geneticist Nicolas Rohner and colleagues recently reported finding the mechanism by which some cavefish are born eyeless after the species moves from surface waters to dark caves.
In dark caves, the team found, conductivity (as measured by salinity) is low. This inhibits heat shock protein 90 (HSP90) production in the fish. As HSP90 is a key regulator of development, its inhibition unmasks rich genetic variations. Such variations include sudden eyelessness.
As eye function generates a metabolic cost, eyelessness is a favorable adaptation in a dark cave habitat.
“Obviously we were surprised about our findings that HSP90 can reveal cryptic variation in an adaptive trait in a vertebrate species, two findings that never have been reported before,” Rohner tells Bioscience Technology. “However, we think that the model system we used was perfect to find such signatures of selection. For if such a mechanism plays a role in evolution, it would most likely play a role in a case where members of a species are suddenly confronted with drastically different, and therefore stressful, conditions. This is the case for cave animals. And this is probably not restricted to such cases, as many speciation events are caused by changing environments.”
Traditional Darwin evolution dictates that spontaneous genetic mutations produce novel traits. Nature can select for the most beneficial of those traits, which take root over countless generations, from many hundreds to many thousands of years.
Scientists have long been bewildered by the cavefish Astyanax mexicanus, which thousands of years ago found itself removed from its surface river habitat and released into dark underwater caves.
In the darkness, the fish shed their color, becoming albinos. They developed heightened sensory abilities to identify both prey, and water pressure alterations. And most significantly, they lost their eyes.
All of the former were adaptive or beneficial traits. The eyelessness was particularly adaptive as maintaining a complex—but suddenly useless—organ comes at a high metabolic cost. Going eyeless lets fish better allocate resources.
Eye loss in such fish is an example of "standing genetic variation," a theory that posits that pools of genetic mutations can exist, but are normally silent. The impact of these mutations on detectable phenotypes don't become obvious until stressful conditions arise.
Whitehead member, and study co-author, Susan Lindquist previously reported that HSP90 inhibits such genetic variation in organisms from fruit flies to yeast. She discovered HSP90 levels decrease due to stress. These decreases let phenotypic changes occur rapidly.
Lindquist had seen the work of Harvard geneticist, and senior author, Clifford Tabin on the genetics of cavefish eye-loss. She and Tabin set out to discover whether HSP90 is an evolutionary role-player in cavefish by looking at both the cavefish, and their surface relatives.
Surface fish raised in the presence of a drug blocking HSP90 (mimicking a stressful environment) displayed great variation in eye size, which pointed to the role of HSP90. Cavefish raised in the same conditions showed no increase in variation in the size of their eye orbits. But they emerged with small orbits, showing that the genetics governing eye size remains responsive to HSP90.
The researchers next examined many natural conditions—from pH to oxygen content to temperature—found in surface and cave waters. There was a major difference in conductivity between cave and surface. The team raised surface fish in water whose conductivity equaled that of native caves.
Fish raised in low conductivity showed major variation in eye size. It was clear that an environmental stressor can have the same effects as chemical HSP90 blocking.
Some scientists believe the team’s paper settles the issue of whether large, rapid environmental change can alter outcomes of evolution in nature. Others think more study is needed.
The team is already working on it. “Our next steps will be to find the actual genes and alleles that are involved in the variation, and which are the targets of HSP90,” says Rohner. “We are in possession of quantitative trait data for the loss of eyes in cavefish. We will screen the genes in these intervals for HSP90 interactors and targets to find the genetic basis of our observation. Long term goals are to find evidence for such a mechanism in other species, eventually in mammals.”