Basically, the Weigel lab looked at the mehtylome of Arabidopsis thaliana (mustard weed, the Drosophila of plants) across 30 generations in 10 separate lines. They found that methylation of individual cytosines changed three times more quickly than the rate of DNA mutation. By contrast, wholly methylated regions (for example, at transposable elements), mutated at the same rate as DNA mutation. Additionally, the different lines showed similar methylation patterns.
So what does this mean for evolution? Well, we have to think about methylation that affects functional genes versus methylation that supports architectural integrity. Methylation at transposable elements probably keeps them from jumping, whereas methylation at an individual cytosine might be affecting the functionality of an exonic region. We are still in the infancy of understanding exactly how epigenetic mechanisms impact evolution, but I find the claim that "epigenetics are not important for evolution" to be sadly misinformed. Even when epigenetic marks are not as heritable as DNA marks, they have very important implications for the (non)heritability of traits! And that, my friends, has everything to do with evolution.
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ReplyDeleteI think I mostly just want to play devil's advocate here - but Weigel is an absolute giant in the field of plant evolution, far from misinformed...particularly about evolution of adaptive traits. It seems that he's speaking more towards the role of epigenetics in plant evolution, not across all species and organisms. Of course, pop science websites like SD want to have eye-catching titles, and it certainly succeeded here, but to the detriment of Weigel and Becker - Eh, I could be wrong, but it seems like mainstream science websites and others such as boingboing, may have a tendency to oversimplify and distort scientific findings. I think the essence of what they were trying to convey is right here (and call me out if I'm wrong - I only read the abstract of the article) - "Only when selection wins out over reversion can these epimutations affect evolution," says Hagmann. I don't think they doubt that epigenetics are not important for evolution overall, they're findings simply suggest that due to the high reversion rate of epimutations, it doesn't seem like to support the idea of epigenetic changes having a major role in PLANT (specifically Arabidopsis).
ReplyDeleteI should clarify--the "sadly misinformed" are exactly the pop science articles (SD and boing boing). Weigel actually doesn't ay much about the evolutionary implications, and where he does he is very cautious. It wasn't supposed to be a direct attack against him or his lab, just the pop science journalists who had their take on the study. (And in all fairness, I think the SD article was much better than the boing boing one).
ReplyDeleteThe other point I was making, though, was that there's a general sentiment out there that unless epigenetics can have a transgenerational, lamarckian-like effect, they are unimportant for evolution. My argument is a theoretical one, in that I believe that epigenetics are important for evolution even (and especially) if they are not transgenerationally heritable. It really boils down to early-life effects, when epigenetics can be a mechanism for a certain kind of phenotypic plasticity. When epigenetics render a phenotype permanent across the lifetime, it can make the trait look as if it is genetically (i.e. highly) heritable. But in reality, the trait was only set in early life due to environmental stimuli, and not under direct genetic control. Though this is an oversimplification of the process, consider the following idea: by comparing the relative contributions of genetic polymorphisms and epigenetic marks to the "setting" of a phenotype, we can begin to elucidate just how heritable the trait of interest is. This, in my opinion, is just as important for evolution as the somewhat 'opposite' effect of transgenerational epigenetic effects.