When the only thing you can eat can kill you, having more gene copies can help

According to research supported by the U.S. National Science Foundation, gene duplications being selected over evolutionary time — not the specialization of certain enzymes — allows wood rats to feed on creosote bush, a highly toxic desert shrub. The findings could aid in understanding genetic adaptations to poisonous foods in other mammals and even why individual humans metabolize drugs differently.

Around the end of the ice age, two species of wood rats in certain locations were forced to change their diet from juniper to creosote bush when the former died out in their native habitat and creosote invaded. The team of researchers, led by Denise Dearing, a distinguished professor in the School of Biological Sciences at the University of Utah, set out to understand just how these wood rats were able to switch to eating the toxic plant when so many other animals in the area couldn't. They found that natural selection favored genetic changes leading to the duplication of genes that increased levels of existing detoxification enzymes to clear creosote toxins rather than modifying these enzymes to break down the toxic creosote faster.

"Rather than new tools specially designed for metabolizing this toxin, evolution made use of existing machinery — just by making more of it," said Dearing. "That's not to say it wasn't a massive change. There wasn't just an increase in the numbers of a single gene; many genes were duplicated across multiple categories known to have a role in detoxification."

Humans also vary in the number of detoxification-related genes, potentially related to certain groups having toxic plants in their diet. "It's possible that the variation we see in human detoxification gene number was driven by a similar necessity to eat toxic plants," Dearing added. "These genes also help break down drugs and medicine, so the variation could drive differences in how quickly people break down drugs."

The changes in wood rats also involved enhancing the expression of at least one protein associated with one of the genes. Additionally, the researchers found that the change, which occurred independently in two different species of wood rat, took place over a short time relative to the evolutionary history of the species. The work was published in the journal Science.