Mutations: When benefits level off

June 8, 2011

Beneficial mutations within a bacterial population accumulate during evolution, but performance tends to reach a plateau. Consequently, theoretical evolutionary models need to take into account a "braking effect" in expected benefits on the survival and the reproduction of organisms. This phenomenon (known as negative epistasis) has, for the first time, been demonstrated experimentally by a French-American collaboration, including a team from CNRS. The results are published in Science on 3 June 2011.

This study was made possible thanks to a unique experiment in the world, conducted in a laboratory of Michigan State University for over twenty years. bacteria are cultivated in the laboratory night and day, 365 days a year, and researchers take samples of the populations at regular intervals in order to analyze their evolution. In the course of this long experiment, it has been demonstrated that some bacteria – the best adapted to the environment – gain the upper hand over the rest of the as generations go by. In other words, natural selection is at work. Conserving bacterial strains through freezing enables researchers to track the memory of this evolution. Better still, they can “revive” at will the ancestral strain and all the strains isolated over the course of the evolution in order to compare bacteria at the end of, for example, 50,000 generations (which, on the human scale, corresponds to nearly two million years). In this way, they are able to quantify the adaptation of bacteria to their environment over time, by evaluating the rate (or “fitness”) of recent strains compared to that of the oldest strains.

The person in charge of this study, Richard Lenski, initiated collaborations with several international laboratories, including Dominique Schneider's team. The use of modern genomics techniques – the analysis of entire genomes as opposed to just several genes as was previously the case – makes it possible to thoroughly characterize the mutations that occur during bacterial evolution and in particular those that have a beneficial effect, responsible for an increase in the selective value of the population. Here, the researchers focused on the interactions between some of these mutations. After having identified the first five combined successively and spontaneously in the , the scientists generated, from the ancestral bacterial strain, 32 mutant strains exhibiting all of the possible combinations of each of these five mutations. They then noted that the benefit linked to the simultaneous presence of five mutations was less than the sum of the individual benefits conferred by each mutation individually. Epistasis thus tends to reduce the benefit conferred by new beneficial mutations, as they appear in increasingly better-adapted individuals, a that explains the slowdown in the adaptive value, observed as continue to adapt.

Thus, beneficial mutations build up during but on the other hand, the performance of the bacterial population tends to level off. Theoretical evolutionary models, which are used to predict results, therefore need to take account of a “braking effect” linked to negative epistasis in the expected benefit on the and reproductive potential of organisms. Moreover, this work demonstrates the existence of networks of interconnected genes and suggests that it may be possible to map them so as to better understand and anticipate their interactions.

Explore further: Slower evolving bacteria win in the end

More information: Negative Epistasis Between Beneficial Mutations in an Evolving Bacterial Population, Aisha I. Khan, Duy M. Dinh, Dominique Schneider, Richard E. Lenski and Tim F. Cooper - Science, 3 June 2011

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1.5 / 5 (2) Jun 08, 2011
Mutation is at the party but it is not the whole ball game in adaptive evolution.
'nuff said.

5 / 5 (1) Jun 16, 2011
Of course it isn't. The key is Natural Selection.

A word to think about.

1 / 5 (2) Jun 17, 2011
Again stupid scientists with stupid conclusions.....
They have 5 mutations, if the mutations happened one after another, like first we have strain without, then strain with 1, then they got strain with 2, and so on and so on, they will have 6 strains combinations, one with 0 mut, 1 with 1, 1, with 2, and so on.And if they happen one after another, there wont be such wearing off effect because the strain with 5 mutation will win over the strain with 4, if the mutation is not beneficial it will be gone, and they wont find it.
Actually when we talk about bacteria we tend to think this way because bacteria dont have sexual reproduction and dont exchange info like we do, but they have a way to exchange true plasmids, so yes there will be 32 combinations indeed, so what they found is that having all the mutations in one place is not good, this is because every mutation represent different strategy, and combining them will lead to nonsense.
1 / 5 (1) Jun 17, 2011
So actually the bacteria started to form new strains having different pack of mutations and this is example for initial stage of divergent evolution, and eventually this strains will diverge and form new species with different strategies for survival.
This breaking effect represent just that not all combination of mutation works.
If every time a new mutation happens and you take the bacteria having it(and it is beneficial), then you separate it from the rest, and wait for another mutation to happen and then you separate the bacteria again, I mean if the mutations happen one after another in bacteria having all the previous one mutations , this breaking point wont come ever.....
not rated yet Jun 21, 2011
Deesky didnt you understand that you do not have the right to rate me, you are stupid rat. I offended you before and you have no balls to stand up for yourself even in the Internet space, so I can imagine what a rat you are in the real life....just pathetic.
If you have another opinion share it, if not just GTFO rat.
Or the only way you can feel good and play smart is just rating 1 in order to hide your stupidity?
5 / 5 (1) Jun 30, 2011
Again stupid scientists with stupid conclusions.....
Deesky didn't you understand that you do not have the right to rate me, you are stupid rat.
Well someone is being stupid. Don't think it was the scientists so you might want to go easy on the hypocrisy.

Deesky does have the right to rank you. It may be poor manners to do so without saying why but he does have the right. And frankly I, for one, get tired of pointing out why I give idiots like Oliver and Marjon ones for almost their posts. They are idiots and deserve them.

However Deesky does seem to rarely give any reason at all for his ranking. In this case I think that first line of yours is reason enough.

5 / 5 (1) Jun 30, 2011
this is because every mutation represent different strategy
No. Mutations are random and there is no strategy involved. Now bacteria CAN produce mutagenic chemicals under stress and in that sense there is a strategy. More stress triggers higher mutation rates.

and combining them will lead to nonsense.
Nonsense. Natural selection prunes combinations that don't work.

form new strains having different pack of mutations and this is example for initial stage of divergent evolution

form new species with different strategies for survival
False. There is no strategy and using that word leads to fuzzy thinking.

I mean if the mutations happen one after another in bacteria having all the previous one mutations , this breaking point wont come ever...
What breaking point? Once a strain fits the conditions then the rate of mutation will usually drop back down.


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