In evolution, the role of chance reassessed thanks to a “weed”

What is evolution? It is often described as a process combining variation and selection, by which mutations occurring randomly in the genome of an individual influence its ability to ensure offspring in a given environment. Living beings would be the fruit of a genetic lottery passed through the ruthless filter of the selection of the most fit – or at least of the sufficiently fit. Biology textbooks insist on the central role of chance in the process of variation. But perhaps, in the light of work on a modest plant that has become a laboratory model, the lady’s cress (Arabidopsis thaliana), this presentation will have to be refined.

Detlef Weigel, from the Max-Planck Institute for Developmental Biology, in Tübingen (Germany), coordinated an international team showing, in Nature of January 13, that there are biases in the mutations observed in the genome of this fetish plant: their rate of appearance is indeed two times less inside the genes – the so-called “coding” part of the DNA – only near these, and it is lowest for genes encoding proteins essential for cell function and reproduction.

Due to natural selection, which favors the best adapted individuals, the variations in DNA sequences observed in nature provide only very imperfect information on the possible biases of the mutation rate along the genome. Detlef Weigel and his colleagues have therefore developed a strict experimental protocol using a classic genetic tool, mutation accumulation lines. They cultivated in the laboratory 107 lines of Arabis, a self-fertile plant, over 24 generations, each time sowing a single seed (chosen randomly) per line and per generation. In this way, they eliminate all possibility of selection, except against mutations producing non-viable or sterile individuals. At the 25e generation, they sequenced the genome and thus identified all the mutations accumulated in each line during the 24 cycles of propagation.

Not totally random

“The mutations observed in a population those that distinguish your genome from mine, for example are the result of three steps, detail Detlef Weigel. DNA is damaged; the DNA is not properly repaired; selection and other factors determine whether this mutation “survives”. With our protocol, there is so little selection that we see almost all mutations as they arise. » Strikingly, the variations in the mutation rates observed correspond to the genetic variations also observed in the natural populations of’Arabidopsis : more polymorphism in the part of the DNA which confers a form of adaptive plasticity in the face of changes occurring in the environment, and less mutations in the central genes for the plant’s metabolism.

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