Evolution: mutations are not random

The basic assumption about the origin of mutations, and thus evolution, could be wrong. New mutations in the genome are likely not randomly distributed, a team led by J. Gray Monroe and Detlef Weigel of the Max Planck Institute for Biology in Tübingen reports in Nature. Overview of recent changes in the thale cress genome (Arabidopsis thaliana) they found, according to the working group, that only half as many mutations occur in genes as in the rest of the genome; in the genes that are central to the organism’s function, there were as many as two-thirds fewer new changes. Until now, experts assumed that mutations appear randomly in the genome and that they disappear from important genes only in subsequent generations through natural selection.

The working group examined the genomes of 400 different strains of the plant to detect mutations that had occurred in individual plants and therefore had not yet undergone natural selection. She also cataloged mutations in seeds that had just appeared in the germ line, as well as so-called somatic mutations that were added during the plant’s life. The former are subject to selection only if they render the seed sterile, and somatic mutations are not subject to such restrictions. However, the analysis showed that certain parts of the genome are much more susceptible to mutations than others.

Some mutations are easier to fix

The team then correlated the frequency of mutations in specific regions with the physical and chemical properties of those genomes. These include, for example, epigenetic changes in the DNA itself or in complexes of supporting proteins known as histones, but also the frequency of certain letters in the DNA or how easily accessible a gene segment is from the outside. It turns out that many factors in non-random mutations are already known or at least suspected. Certain factors that promote highly efficient repair of DNA damage occur in very few mutated parts—for example, epigenetic changes in histones.

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