Report of mutation biases mirroring selection in Arabidopsis thaliana unlikely to be entirely due to variant calling errors

SUMMARY

It has recently been proposed that the uneven distribution of epigenomic features might facilitate reduced mutation rate in constrained regions of the Arabidopsis thaliana genome, even though previous work had shown that it would be difficult for reduced mutation rates to evolve on a gene-by-gene basis. A solution to Lynch’s equations for the barrier imposed by genetic drift on the evolution of targeted hypomutation can, however, come from epigenomic features that are enriched in certain portions of the genome, for example, coding regions of essential genes, and which simultaneously affect mutation rate. Such theoretical considerations draw on what is known about DNA repair guided by epigenomic features. A recent publication challenged these conclusions, because several mutation data sets that support a lower mutation rate in constrained regions suffered from variant calling errors. Here we show that neither homopolymer errors nor elevated mutation rates at transposable elements are likely to entirely explain reported mutation rate biases. Observed mutation biases are also supported by a meta-analysis of several independent germline mutation data sets, with complementary experimental data providing a mechanistic basis for reduced mutation rate in genes and specifically in essential genes. Finally, models derived from the drift-barrier hypothesis demonstrate that mechanisms linking DNA repair to chromatin marks and other epigenomic features can evolve in response to second-order selection on emergent mutation biases.