Desert woodrats in the western North American desert frequently feed on highly toxic plants. A new study shows that their gut bacteria make it possible for them to digest those toxins, and a microbial transplant from these woodrats confers that ability to other rodents.
Many plants are very capable of defending themselves against herbivores: a toxic resin coats the leaves of the creosote bush, while juniper toxins are found inside their needles. Some desert woodrats (Neotoma lepida), also called packrats, specialize on creosote bush: Not only can they degrade the toxic plants, they survive on them. About 17,000 years ago, at the end of the last glacial period, the Southwest dried out and Mexican-native creosote moved north into the Mojave Desert, replacing juniper. But it didn’t make it farther north into the Great Basin deserts. So, packrats in the Mojave started eating creosote bushes, while those in the Great Basin kept eating juniper.
Liver enzymes help detoxify ingested chemicals, but gut microbes are thought to take at least some of the burden off the liver. Previous work showed how creosote-eating woodrats and juniper-eating woodrats have different sets of gut microbes. Now, in three separate experiments, a University of Utah team led by Kevin Kohl show how crucial these bacteria are.
First, they compared gut-microbe genes in two sets of creosote-eating Mojave woodrats: one was fed only rabbit chow and the other was fed chow mixed with a bit of creosote resin. Diet, they found, determines the microbial composition. The guts of creosote-fed woodrats teemed with creosote-metabolizing microbes, while the guts of creosote-free woodrats had only one-fourth the levels of those same microbes.
Next, drinking water laced with antibiotics killed about 90 percent of the gut microbes, rendering the rodents unable to digest toxins. Woodrats placed on rabbit chow were fine, but those fed creosote lost 10 percent of their body weight within two weeks.
In third experiment, the team sped up evolution by transplanted feces from creosote-eating Mojave woodrats into juniper-eating Great Basin woodrats. Ancient juniper eaters in the Mojave were likely poorly equipped to eat the invading creosote. Slow, evolutionary genetic changes played a big role in adapting naïve hosts to new diets, but the “transfer of toxin-degrading microbes from one organism to the other is much more rapid,” study coauthor Denise Dearing of Utah explains in a news release. Microbes can be acquired through contact with their mothers, for example.
Since woodrats naturally eat their own and others’ feces, the team simply mixed it in with the rabbit chow. With fecal gut microbes from creosote-eaters, the juniper-eaters gained the ability to digest creosote bush, persisting on that diet without losing weight. Furthermore, juniper-eaters who didn’t receive fecal transplants lost 10 percent of their weight by day 11 -- but it wasn’t because they rejected the creosote-laced food. Their urine was more acidic, suggesting that their livers expended a lot more energy to degrade those toxins.
The work was published in Ecology Letters this week.