Genetic analysis of cavefish reveals more about evolution

A multi-institutional study offers additional insight into the evolutionary process by examining how albinism evolves in cavefish. Researchers, including New York University Biology Professor Richard Borowsky, examined two populations of Mexican cavefish and found that albinism in both populations was linked to Oca2–a pigmentation gene also responsible for the most common form of albinism in humans. They observed different deletions in the gene in each population and found that both deletions cause a loss of Oca2’s functionality, demonstrating that the albinism in the two groups evolved independently. The findings are reported in the latest issue of the journal Nature Genetics.

The study also included researchers from the Harvard Medical School, the Howard Hughes Medical Institute at the Children’s Hospital of Boston, the University of Hamburg, and the University of Maryland’s Department of Biology. The research was supported by a grant from the National Science Foundation.

The replicated experiment is a powerful tool for experimental science, but typically unavailable in the study of evolution. Cave adaptations have evolved in many species independently, however, and each cave species can be considered a replicate of the same evolutionary experiment that asks how species change in perpetual darkness. A frequent outcome is that the species lose pigmentation or become albino. Cavefish, therefore, are a rich source for the examination of the evolutionary process.

In order to isolate genes’ role in the evolutionary process, the researchers examined two distinct cave-dwelling fish populations. To provide a framework in which to study the genetics of this species, they made a microsatellite linkage map, which allowed them to locate the regions of the chromosomes that had genes controlling cave related traits.

The researchers found that genetic markers for albinism in the two groups appeared in the same location, suggesting three possibilities: the two cave populations had the same mutation in the same gene, they showed different mutations in the same gene, or they had mutations in distinct but closely linked genes. To clarify their results, the researchers performed a complementation test fish in both caves, which yielded only albino offspring. They concluded that albinism in these two cave populations and in a third population not yet as well studied is caused by mutations in the same gene. Subsequent analyses revealed that Oca2 is the gene responsible for pigmentation in the cavefish and that the deletion of a specific exon, or protein- coding DNA sequence, produced albinism.

What remains a mystery is why the same gene should be mutated independently in all three populations when other genes are also known to cause albinism and why it should be the same gene that causes the most common form of albinism in humans. One possibility, suggested by the researchers, is that it is a large gene presenting a big target for mutations, and it seems to have no other functions besides helping to make melanin. Therefore, it doesn’t diminish other aspects of fitness when it is mutated.