Blind cave fish undergo eye exams
September 28, 2010 1 Comment
Blind cavefish have plenty to show us
It’s rare for an ancestral species to still be alive at the same time as its descendent species, but one example of this phenomenon is the blind cavefish. As its name implies, it is sightless and dwells in caves in northeastern Mexico. Aboveland, biologists can also find the ancestral species to these fish. The ancestral versions are all sighted, providing a perfect scenario for examining some of the genes involved in eye development and sight.
Warning: Bad pun ahead
Not blind to such opportunities, plenty of investigators have turned to Astyanax mexicanus and its ancestor as an animal model. These researchers focus on a field known as “evo devo,” or evolutionary developmental biology. With species such as A. mexicanus, biologists can examine genetic changes in developmental processes that lead to differences between organisms.
One group reported a few years ago that the rudimentary eye in the blind cavefish stopped developing because of overexpression of a pair of genes in the hedgehog family. The two genes, sonic hedgehog (shh) and tiggy winkle hedgehog (twhh) (evo devo folks have a little too much fun with gene names) influence many developmental processes, and in humans, appropriate shh expression results in the formation of two separate eyes, rather than a single, central eye. These researchers took the ancestral species of A. mexicanus and triggered production of extra shh and twhh during development on only one side of the head. On that side of the head, the fish exhibited arrested eye development as embryos, and the adults had no eye there.
Shh: Pleiotropic effects
Genes like shh have what we call “pleiotropic effects,” meaning that they influence several traits. As it happens, even as the blind cavefish lost sight, they gained in other adaptations to living in the dark, including reduced pigmentation, super-refined taste buds, and the ability to navigate using water pressure changes.
Many mutations + three evolutionary events = one outcome
Investigators also realized that for the 29 different populations of A. mexicanus dwelling caves in Mexico, each group might have a different suite of mutations that led to sightlessness. In other words, mutations in different genes in each population led to the same endpoint of blindness. Researchers knew for certain that sightlessness had evolved at least three times among these separated groups. The fish thus also provide an example of convergent evolution, when similar adaptations arise in separated populations or species because environmental pressures are the same.
The fish also offer a lesson in genetics. In studies of bacterial genetics, we learn about a process called complementation. It starts with mixing together two kinds of bacteria, one that has resistance to one chemical and another with resistance to a second chemical. Bacteria that end up with both genes will have resistance to both compounds, something easily tested by growing them on medium containing the two chemicals. This process is called complementation because the two bacterial gene sets complement one another when combined under these conditions.
Cavefish complementation experiments
Operating on a similar principle, a team of researchers crossed blind cavefish from different geographical areas. They thought that because different genes for sight had changed in different populations, the fish might also exhibit complementation. As predicted, in many cases, the genes of the father complemented the genes of the mother, resulting in a complete suite of genes appropriate for eye development in their offspring. The new fish, instead of experiencing truncated eye development, actually had eyes. The researchers had restored sight to the blind cavefish in a single generation. The effect was most pronounced between populations that were most distant from each other, reflecting the standard thinking that geographical separation often reflects genetic separation.
Fish eye exams
The same researchers also devised a clever test to discern whether or not a fish can see. They immobilized the fish and placed them inside a cylinder with flashing, shifting patterns of stripes in black and white. If the fish could see, their eyes would move with the patterns. Thanks to the blind cavefish, we have an excellent example of pleiotropy, convergent evolution, complementation, and natural selection principles, along with the first-ever test for determining whether or not a fish has sight.