A long-standing question in biology is how evolution tinkers with genes without mucking things up. The prevailing theory is that the genome has copies of critical genes, so that if mutations spoil one, there's a backup. Now researchers have new proof that evolution can work this way.
The scientists tracked down a duplicated gene that made possible so-called mirror fish, which have large, reflective scales. "This is a valuable proof of concept that beautifully illustrates how gene and genome duplications can facilitate morphological evolution," says R. Craig Albertson, an evolutionary biologist at Syracuse University in New York state.
Biologists Matthew Harris and Nicholas Rohner of the Max Planck Institute for Developmental Biology in Tübingen, Germany, started their experiment by using chemicals to mutate zebrafish genes. They then scanned the mutant fish for unusual traits. One mutant was almost scaleless, reminiscent of domesticated mirror carp bred to lack many scales to be easy to clean. The researchers tracked the mutation down to a gene called fibroblast growth factor receptor 1 (fgfr1).
Because mutations in this gene typically cause mammal and fish embryos to die, Harris and Rohner suspected it must have a twin somewhere else in the zebrafish genome. It did. The researchers found an intact version of fgfr1, in addition to the mutated copy that leads to mirror scales.
To see if the process of duplication and mutation happens outside the lab, Harris and Rohner scanned for fgfr1 in mirror carp. They found duplicate fgfr1 genes in two varieties of mirror carp: One had lost 111 bases from the twin gene, and another had a base substitution in the twin gene, both of which had presumably led to fewer scales. The team reports  its findings online today in Current Biology.
This is the first duplicated gene that Harris and Rohner have discovered, but they expect to find many more. About 25% of the zebrafish's 17,000 genes are duplicated, and those that are mutated may also exist in wild animals as well as domesticated ones.