Developing a more tolerant tummy

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A bushy-tailed woodrat. Picture credit Oregon Caves

A bushy-tailed woodrat. Picture credit Oregon Caves

Woodrats living in the deserts of the US have a pretty poor selection of foods on offer. Juniper bushes and cactuses used to make up the majority of their diet. While I do love a bit of juniper once it’s made into gin, Monty Python’s life of Brian showed us, if any proof were needed, that a diet of just juniper bushes isn’t much fun (Profanity warning at the end of that clip). And cactus? No thanks.

But scientists looking at the genetics of woodrats have found that as creosote bushes have colonized the deserts where the woodrats lived, the animals managed to develop a tolerance to eating them, despite the fact that the bushes kill most odents of their size. That’s because creosote leaves are coated with resin contains a substance called NDGA, which most rodents’ livers and kidneys can’t process. But not Neotoma Bryanti and N.lepida. These woodrat species have evolved the ability to produce the right enzymes to detoxify the substance, and so a diet of creosote leaves is no trouble to them at all.

Creosote leaves. picture credit Homer Edward Price

Creosote leaves. picture credit Homer Edward Price

Surprisingly, the woodrats needed only a few genetic changes to make that switch to creosote tolerance, as shown in a study published in BMC Ecology. As developing tolerance to a new food is a complex process, the scientists from the University of Utah were expecting to find a lot more genes involved. And as the two species evolved tolerance at different times, they were expecting to find that they used different mechanisms. In fact, similar genes are involved for both species.

Evolutionarily speaking, developing the ability to withstand a chemical, is a pretty big change, and it’s unusual to find such a massive change in such a short time. There needs to be incredibly high selection pressure – that is, some kind of environmental factor means that tolerance is so beneficial, tolerant individuals begin to swamp the gene pool. Or that being unable to tolerate it is such a disadvantage that intolerant individuals die out, leaving only the tolerant ones.

For woodrats living in areas with lots of creosote bushes, it seems that being able to eat them in a landscape with so little food on offer provided a massive selection pressure in favour of tolerance.

But woodrats aren’t the only animal to have developed this resistance to a toxic chemical. To many of us, drinking milk causes no problem at all, but until a short time ago, all adult humans got a nasty tummy upset from dairy. This was because once they were weaned, their bodies switched off production of the enzyme lactase, which breaks up the milk sugar lactose.

Tasty milk. Picture credit Stefan Kühn

Tasty milk. Picture credit Stefan Kühn

While scientists looking at woodrats’ tolerance to creosote can only go on genetic evidence, we’ve got another bank of evidence for looking at human evolution, in the form of remains of ancient pots, which seem to have the residues of cheese and yoghurt.  Experts think Neolithic folk were able to eat these because the process of producing them removed some of the lactose.Now when I say a short time ago, I actually mean between 2,000 and 20,000 years ago. That’s roughly when evolutionary scientists think the mutation occurred to keep lactase production on into adulthood. It might seem like a very vague estimate to us, but in evolutionary terms, it’s no time at all!

On several different occasions worldwide, it seems that mutations have arisen that made adults tolerant to lactose. We know this from evidence of Turkish dairy farming cultures around 8,00 years ago, and lactose tolerance in Linearbandkeramik cultures around 5,000 years ago.

Adult milk drinkers clearly got a huge boost from the stuff and had many more kids to pass on those milk tolerance genes onto, so they became much more frequent. The nature of that boost is a bit unclear though – theories range from the ability to drink milk during winter famines in northern Europe, to milk being a safe source of water in areas where the natural water supply was contaminated.

So gaining tolerance to a new food can clearly provide a massive benefit to many species in hard times. First, the right mutations need to arise in the population, and they have to provide a benefit to the species in just the right way. But we’ve still got a lot more to learn about how the evolutionary dynamics work. I just know I won’t be trying creosote leaves anytime soon.