Why is eyesight not important to life in a cave

When the study team introduced a disabling mutation into DNMT3B in zebrafish--a different type of fish with normal eyes--they found that the mutant zebrafish had more active eye genes and larger eyes. The results suggest that a genetic change resulting in elevated DNMT3B levels occurred during the evolution of cavefish, leading to epigenetic suppression of eye development genes. The content of this article does not necessarily reflect the views of this organization.

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DOI Maintaining eyes and the visual parts of the brain uses lots of energy , so the loss of eyes is a big advantage for animals living in the dark. It was assumed that these fish became blind because mutations disabled key genes involved in eye development. This has been shown to be the case for some other underground species that have lost their eyes. Instead, the genes have been switched off by the addition of chemical tags called methyl groups.

This is what is known as an epigenetic , rather than genetic, change. And for scientists, this built-in comparative power makes it a good choice for further exploration. They have two populations to study that can interbreed and are polar opposites for physical traits. So Cartwright's group decided to use computational power to investigate how multiple evolutionary mechanisms interact to shape the fish that live in caves.

Unless selection was really, really, strong. How strong? In their model, the selection for blindness would need to be about 48 times stronger than the immigration rate for Mexican tetras to evolve blindness in caves. Cartwright's group estimates that a measure of fitness for blindness, called the selection coefficient, in the tetra is between 0. These coefficients are high enough that laboratory experiments should have detected a difference between surface and cave forms of the fish; however, none have to date.

Cartwright's team turned to a hypothesis going all the way back to a letter to the editor of Nature in by E. Ray Lankester, that essentially stated that the reason you have blindness in caves is because the fish that can see simply leave. They aren't trying to get to the light anymore because they can't see it. They did this by calculating the oxygen consumption of their eyes and vision-related parts of their brain.

The results, published September 11 in the journal Science Advances , showed that for young, developing fish, the energy cost of sight is 15 percent greater than if they were blind. Indeed, the findings also show that blind A.

These omnivorous fish compensate for lack of vision by eating just about anything they can find, including scavenging dead animals and plants. Energy-saving eye loss, or the expensive tissue hypothesis, is one of a number of theories to explain why sighted animals that took up life inside caves evolved to be blind.

For one thing, Jeffery says, it's unknown how the evolutionary pressure to save energy actually tampers with the fish's vision.

One such process in Mexican blind cavefish is a phenomenon called pleiotropy, in which genes usually involved in eye development are reassigned to features more useful to life in caves, such as increased numbers of taste cells for finding food in the dark. In fact, Moran says, the evidence suggests both pleiotrophy and energy savings are major drivers in the cavefish eye loss.

Clues gleaned from the fast-evolving fish species may help to explain eye loss in a host of other cave dwellers, such as spiders and crabs. Many cave insects and other invertebrates have large eyes, for instance.

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