Tuesday, 22 January 2013

On the hologenome theory; or the link between our microbes and evolution

Bacteria are just great.
We all now know(through science articles and the occasional Yakult ad) that bacteria living within and around us can be beneficial for us.
Studies have been shown that the microbes living happily on our skin, in our blood, in the gut, all affect on our health. As in, if you tamper with them bad things happen. It's symbiosis; both the organism and the microbes benefit.
That's all fine and dandy.
But what if these microbes also affect our evolution?
This was the question posed in a recent article in the New Scientist by Carrie Arnold.
There's two scientists who back this theory. Their names are Richard Jefferson and Eugene Rosenberg.

Basically Jefferson argues that microbes that live within organisms are passed down from generation to generation. But how does this link with evolution?
So let's say there's a group of chipmunks with microbes in their tummies.
Boy Chipmunk decides he likes apples more than nuts, the usual chipmunk diet (Nana Chipmunk is shocked.) So he eats a diet of predominantly apples.
This causes a change in his body as well as the bacteria around and on his body. He may start to smell different, perhaps because the pheromones he secretes are influenced by the microbes on the skin.
Girl Chipmunk Daisy doesn't eat apples, she eats nuts. Boy Chipmunk refuses to mate with her. Then another girl chipmunk, called Susie Chipmunk, who also eats apples, comes along. Boy Chipmunk chooses her as a mate over Daisy Chipmunk.

This is a trend that has been noticed not in chipmunks(unfortunately) but a species of fruit fly called Drosophila. Changing the diet of the fly(from molasses to starch) can change the fly's mating habits. So flies who eat molasses only mate with flies that also eat molasses. Vice-versa with flies that have a starch diet.
So what, you say? Perhaps there's another factor in play here; maybe when flies start to date they find it important to have things in common, such as their diet. Molasses flies can't imagine mating with starch flies; starch flies smell like starch. Ew.
Here's the thing. When all the flies were treated with antibiotics, their mating preference disappeared. They happily mated with the other flies regardless of shared or different diets.
This suggests that the microbes in the flies' stomach or skin had a big impact on their mating preference, and this is where the hologenome theory originates. We can imagine with this knowledge how microbes living on organism affect evolution, and perhaps have an influence on divergent evolution.
You can read more about the study in this article on PubMed.

When I first read about this in the New Scientist, I was a little confused.
Surely bacteria on a fly don't influence the fly's brain cells, so how would they influence its mating choice? And how would another fly know that the fly ate starch and not molasses?
The study on PubMed says that the symbiotic bacteria influences the fly's mating preference by "changing the levels of cuticular hydrocarbon sex pheromones".
So basically because of the fly's changed diet the bacteria on the fly's skin(skin? I'm not sure) release a different pheromone into the air. So molasses flies aren't attracted to starch flies because they release different pheromones.
It's easy to see how this could lead to divergent evolution; the flies on a diet of molasses become better adapted to eating molasses and become a different species from the flies who eat starch, all because the bacteria cause them to mate only with other flies who have a similar diet.

It's interesting to think of it this way; the article in the New Scientist does mention that this theory is almost neo-Lamarckian.
(Lamarck was the guy who proposed a different theory of evolution from Darwin, believing in inheritance of acquired characteristics i.e. giraffes who learn to stretch their neck give birth to a giraffe with a longer neck)
I feel like I need to do a lot more research on this before I fully agree or disagree with the propositions Jefferson and Rosenberg are making about the hologenome theory. But certainly it's interesting to think how evolution might be affected by a huge variety of different things.

If you'd like to hear Jefferson talk about the hologenome theory and its potential uses in biotechnology there is a video you can watch, though it is quite long.
This article also discusses the possibility of symbiotic microbes having an effect on evolution, though it focuses more on the different ways in which microbes affect an organism's phenotype and epigenetics.

If you would like to read about a study in which bacteria in the human gut may have an effect, refer to this slightly difficult article(I may do a post trying to interpret and discuss this study later).

It might also be worthwhile looking at the links to other articles which discuss the hologenome theory on this article page (scroll down to the heading "HighWire Press-hosted articles citing this article")

Again I should mention I am not a biologist, I just read an article in the New Scientist and have done a little light research around this; I apologise for any mistakes made here.

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