“Evolution of stress-induced mutagenesis in the presence of horizontal gene transfer”

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Yoav Ram and Lilach Hadany (July 2019)

The DOI will be https://dx.doi.org/10.1086/703457

Stress-induced mutagenesis can evolve and increase the adaptation rate even in the presence of horizontal gene transfer

Mutations are usually considered random events. However, over the last 20 years it has become apparent that some microbes generate more mutations under stress conditions such as starvation, DNA damage, and antibiotics. In 2012, Ram and Hadany used mathematical models and computer simulations to show that stress-induced mutation is likely to evolve. Their models focused on microbes that do not exchange genes – that is, asexual microbes, as far as we can think of sex as the exchange of genetic material between unrelated individuals. However, several molecular mechanisms in microbes can lead to a primitive kind of sex called horizontal gene transfer, in which small pieces of DNA are transmitted between cells. For example, some microbes will take up DNA molecules from their environment and incorporate it to their genome in a process called transformation. Theory suggests that horizontal gene transfer will decrease the evolutionary advantage of generating new mutations: why should a cell risk mutating, when most mutations are harmful, if instead it can just pick up an advantageous mutation from another cell?

In new research published in The American Naturalist, Ram and Hadany test if stress-induced mutation can evolve even in the presence of horizontal gene transfer. They find that although horizontal gene transfer reduces the evolutionary advantage of stress-induced mutation, the latter can still be evolutionary advantageous, especially if horizontal gene transfer is also induced by stress. Moreover, they show that stress-induced mutation accelerates adaptation to new environments. Therefore, the authors suggest that mutation and horizontal gene transfer can complement rather than exclude each other, and that mutation is likely to be correlated with stressful conditions even in microbes that undergo horizontal gene transfer.


Abstract

Stress-induced mutagenesis has been observed in multiple species of bacteria and yeast. It has been suggested that in asexual populations, a mutator allele that increases the mutation rate during stress can sweep to fixation with the beneficial mutations it generates. However, even asexual microbes can undergo horizontal gene transfer and rare recombination, which typically interfere with the spread of mutator alleles. Here we examine the effect of horizontal gene transfer on the evolutionary advantage of stress-induced mutator alleles. Our results demonstrate that stress-induced mutator alleles are favored by selection even in the presence of horizontal gene transfer, and more so when the mutator alleles also increase the rate of horizontal gene transfer. We suggest that when regulated by stress, mutation and horizontal gene transfer can be complementary, rather than competing, adaptive strategies, and that stress-induced mutagenesis has important implications for evolutionary biology, ecology, and epidemiology, even in the presence of horizontal gene transfer and rare recombination.