Woodrats duplicated their genes to survive eating poisonous plants

Woodrats are small desert rodents that have baffled scientists for years with their peculiar ability to eat the toxic creosote bush. Creosote leaves are laden with resin that would make most animals sick, but these tenacious creatures feast on it day after day. 

For decades, researchers wondered whether woodrats adapted by evolving highly specialized enzymes or if they simply produce an enormous amount of more general detoxification enzymes.

Woodrats eat toxic plants without harm

Scientists investigating this question observed that woodrats can live in areas where creosote has taken over.

Many species avoid the spindly green shrub because it’s coated with a chemical mix that can disrupt crucial processes in their bodies. Yet, these rodents thrive.

In a recent study, researchers discovered a twist in the woodrat enzyme story: it isn’t about better tools, but more of them.

Woodrat survival through gene duplication

This discovery was led by postdoctoral researcher Dylan Klure, along with a team of colleagues at the University of Utah. The experts zeroed in on how two woodrat species independently evolved to handle large doses of resin that would debilitate other mammals. 

After comparing both creosote-tolerant and creosote-sensitive woodrats, the results pointed toward a surge in gene copies rather than improved enzyme function.

A quick jump in evolutionary terms

Creosote may have entered the southwestern desert around the end of the last ice age, about 15,000 years ago. 

“These woodrats have only been exposed to creosote bush for about 15,000 years, and in an evolutionary timescale, that’s very little time,” said Klure, postdoctoral researcher at the University of Utah and lead author of the study. 

This short window hasn’t stopped woodrats from crafting a workable solution. When food is scarce, animals can either adapt to what’s available or starve.

Woodrats apparently adapted by copying existing detoxification genes and pumping out the related enzymes at high levels.

Woodrats churning out enzymes

According to the researchers, these detox genes are part of the glucuronidation pathway. In humans, this pathway helps metabolize a notable chunk of prescription drugs. 

By mapping which genes were active in woodrats’ livers, the scientists found an abundance of duplicates that churn out enzymes strong enough to process resin toxins. 

The presence of multiple copies means a single woodrat can manufacture waves of enzyme molecules to quickly flush harmful compounds out of its system.

An alternative to specialized enzymes

“Rather than new tools specially designed for metabolizing this toxin, evolution made use of existing machinery – just by making more of it,” said senior author Denise Dearing, professor of biology at the University of Utah. 

This stands in contrast to earlier findings in other species where smaller numbers of finely tuned enzymes handle toxins. In the woodrats’ case, the key isn’t building a fancy detox gadget – it’s mass-producing what they already have.

Why did woodrats shift to toxic plants?

Historically, woodrats dined on more abundant plants like juniper. However, climatic shifts dried up many parts of the Southwest, giving the creosote bush room to spread. This forced the local animals to either move or adapt. 

Some rodent populations picked the new leaves, survived, and reproduced – thus passing along their genetic advantage to the next generation.

Over time, these gene duplications reached surprisingly high levels in regions where creosote has been around the longest.

Woodrat nests reveal their changing diet

Paleontologists studying ancient woodrat nests found evidence that their diet mirrored changes in plant communities. Woodrats left behind “middens,” or nest sites stacked with plant fragments, seeds, and twigs. 

By comparing fossils from different time periods, the experts traced the shift from juniper to creosote.

The rodents’ DNA adapted alongside this changing meal plan, illustrating how an environmental upheaval can nudge animals down a new dietary path.

Enzyme gene duplication in woodrats

The genetic shift happened separately in two species of woodrats that faced similar conditions. This kind of parallel evolution suggests a powerful nudge from nature. 

“When creosote invades, it just takes over. It’s kind of an adapt-or-die situation,” said Klure. For woodrats, duplication of existing genes was apparently an easier workaround than redesigning detoxification enzymes from scratch.

Implications for humans

Scientists see a parallel between woodrats’ enzyme explosion and differences in detoxifying chemicals among humans.

People vary in the number of detoxification genes they carry, partly due to ancestral diets rich in tough-to-digest plant toxins. 

This might explain why individuals respond differently to medications or certain foods. Studying woodrats in controlled experiments could shine more light on these variations.

Gene expansions tell a bigger story

At first glance, the woodrats’ solution seems simple: copy genes until you can safely eat a toxic shrub. Yet, this adaptation demonstrates that quantity sometimes trumps refinement in the race to survive. 

Frequent duplications of existing detox genes allowed these rodents to remain in dry habitats overrun by a bush most mammals avoid. Such repeated expansions may be a more common strategy across the animal kingdom than previously thought.

The study is published in Science.

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