Microbiomes mediating microevolution

Bacteria. Is there anything they can’t do?

An forthchoming paper by Brucker and Bordenstein (B&B) argues that bacteria living inside larger eukaryotic organisms – the ones we can see think and think of as “real” organisms – could be having major impacts of their evolution.

Specifically, bacteria could be causing these larger organisms to speciate.
This sounded strange at first, but then I thought about the ever increasing attention to, and understanding of, the “microbiome.” Go back a few years, and you wouldn’t hear the statistic, “There are more bacterial cells in your body than human cells.” This is now common knowledge, at least among the science savvy crowd.

Brucker and Bordenstein point out that bacteria are ubiquitous, specific to particular host species, and drive changes in genes associated with immunity. We’re hearing more about treatments that pay attention to the microbial communities within us, partly because we’re finding out that disturbances in gut microbes could be related to all kinds of medical issues. And some people have been successfully cured by fecal transplants – which has a certain shock value.

The importance of the microbiome lends some plausibility to the claim that bacteria could affect speciation.

First, Brucker and Bordenstein argue that bacteria can create reproductive barriers that span multiple generations. Their main example is an experiment involving Wolbachia (pictured) infecting Drosophila. Wolbachia do all sorts of strange things to the hosts they affect, largely in messing up sex ratios of offspring and more. While interesting, I think it’s fair to say that Wolbachia are unusual in their interaction with their hosts. They also note, however, that bacteria are involved in generating cues important to mating, like odours. There are also cases of bacteria that can induce parthenogenetic reproduction. Again, Wolbachia is one, but it’s not the only one.

Having established the possibility that bacteria could split populations into species before mating, Brucker and Bordenstein turn their attention to how bacteria could exert selective pressure after mating.

One extremely interesting theoretical point is that symbiotic bacteria be more efficient at causing hybrid incompatibility than genes. Imagine that there are three genes (version 1: X, Y, Z, version 2: x, y, z,) in different species, and some versions of those genes are not compatible. If a hybrid gets X and Y, the hybrid dies. With three such genes, there are three possible “bad” combinations. But if there is X, Y, and a bacteria (B), there are six possible “bad” combinations, so you get stronger selection pressure against hybridization. There seem to be no “real world” examples of this, though.

Brucker and Bordenstein also note that if hybrids have defective immune systems, they will be more prone both to infections and autoimmune disorders, and these will also provide additional selection pressure against hybrids. While true, these don’t seem to be selection pressures that are specific to bacterial symbionts, which is ostensibly what the paper is about.

Could bacteria be secretly driving evolution under our noses? Hard to say now. Many of the examples here are either theoretical, or are drawn from fairly specific examples, like Wolbachia. Speciation being driven by bacteria endosymbionts or microbiomes may be happening at very low rates, or may be even be only a theoretical possibility.

But I certainly don’t want to underestimate the possibility, or the power of bacteria.

“This is the Age of Bacteria,” Stephen Jay Gould used to say. “It’s always been the Age of Bacteria.”

References

Brucker RM, Bordenstein SR. 2012. Speciation by symbiosis. Trends in Ecology and Evolution: In press. DOI: 10.1016/j.tree.2012.03.011

Photo from Cho et al. (2011), used under a Creative Commons license