Gut microbes keep species apart


Mountain ranges and rivers can act as physical barriers that separate closely related species and keep them from cross-breeding. But the trillions of microbes in an animal’s guts could have the same role.

Robert Brucker and Seth Bordenstein, biologists at Vanderbilt University in Nashville, Tennessee, have found that the gut bacteria of two recently diverged wasp species act as a living barrier that stops their evolutionary paths from reuniting. The wasps have subtly different collections of gut microbes, and when they cross-breed, the hybrids develop a distorted microbiome that causes their untimely deaths.

“This is the most convincing evidence that the microbiome evolves with hosts over long time periods and might affect the speciation process,” says Bordenstein. The results are published in Science.

Jürgen Gadau, an evolutionary biologist at Arizona State University in Tempe, says that the microbiome is just one of many factors that drive the origin of species. “The important point is that microbes can change very rapidly,” he says — so they could very quickly enforce the separation of nascent species.

Written By: Ed Yong
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  1. This might be related to an earlier posted news item:

    Immune-killing cells switch found

    Gut microbes are likely to have an influence on what ingested proteins are broken down or remain partially intact. E.g. There’s a theory about different kinds of milk processing that has led to a new kind of cows’ milk being marketed in supermarkets. So the idea that proteins or fragments might occasionally enter blood circulation more or less intact directly from the gut is becoming more widely accepted.

    The earlier immune cell switch item indicates that foreign proteins in foods, which might resemble the native switching proteins, could potentially disrupt the immune system, sometimes becoming too aggressive. (With autoimmune disease consequences.) Same thing might happen in the presence of molecules that are known to disrupt proteins – like fructose overdosing or slightly but chronically elevated blood glucose among people eat normally but don’t exercise.

    Differences in gut micro biota might have contributed to the divergence of the human species from our common ancestors with the chimpanzees. An environmental change generating significant areas of dry grassland compared to forest would have enabled the ancestors of chimps and humans to exploit relatively rapid growing grass as a more easily concentrated energy resource. Maybe there’s outright strategic advantages in this from more solar energy per hectare of land available from grass than from forests (less energy incorporated into forms of wooden biomass and only subsequently available to humans after fossilisation).

    Human ancestors may have become more specialised in exploiting these very extensive grassland resources by preying on the wide-ranging grazing animals which already possessed the right kind of gut biota for converting grass cellulose to animal fat and protein. So early human gut biota evolved to become simpler and less diverse, effectively by offloading and outsourcing gut micro biota functionality to grazing ruminants. And even more so after humans incorporated fire technology – possibly both aspects may have simultaneously been crucial.

    While some of our non-human ancestors maintained their dependence on retaining their own sufficient gut biota to continue as omnivores, but more with the vege and fruit menu of the forest environment. (Interesting aside being that humans eventually developed agricultural practises to allow the consumption of industrial quantities of grass products directly – with possible implications for autoimmune diseases. Which is one of the basis of the paleo diet idea.)

    On the boundaries of these widely different forest vs grassland habitats you’d expect inter-breeding between what might eventually become divergent species. Offspring that inherited more limited gut micro biota in their infancy would be unlikely to subsequently thrive in the forest. While those offspring with full sets of gut biota would be capable of surviving in either habitat.

    But there’s a natural asymmetry: it’s conceivable that evolutionary changes might be quicker when it involves losing capabilities by suppressing genes or changing the internal gut environment to alter the balance of gut micro biota, than in creating new capabilities from existing structure or functions, or available micro biota. Perhaps the outcome of diverging gut micro biota withing the same species but occupying slightly different habitats led to a net movement of population into areas and lifestyles where there is easier meat. i.e. away from the forest. So you’d get all of those with limited gut micro biota into the grasslands and breeding with each other, but only some of those retaining their broader gut biota also living in the grasslands. Eventually inter-breeding would become increasingly unsuccessful for the forest lifestyle. Ultimately leading to their being more human-like chimps than chimp-like chimps.

    Plus there’s the fire issue. Cooking might be essential to enable a relatively tiny gut with limited micro biota. You need intelligence to play with fire. It might have been a little less dangerous to be a chimp with a cigarette lighter in grassland than in a forest.

  2. Beautifully written article, even I understood most of it! It seems so obvious that the first changes to take place when the forests fell down on our ancestors, was for the digestive system to adapt to eating the products of savannah and that the micro biota would have to carry the burden of the change. The fact that even the wize haven’t noticed this very much previously, gives hope to the rest of us. Another piece of the jig-saw, and an example of how evolutionary theory might have effects in the development of medicine, in terms of shedding new light on auto-immune diseases.

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