The mutational dynamics of the Arabidopsis centromeres

Centromeres are specialized chromosomal regions essential for sister chromatid cohesion and spindle attachment during cell division. Although many centromeres consist of highly variable tandem repeat arrays, the mutational processes driving this variability remain poorly understood. Here, using replicated genome assemblies of Arabidopsis thaliana mutation accumulation (MA) lines, we define the centromeric mutation spectrum. We find that point mutations occur at an almost tenfold higher rates than in chromosome arms, largely driven by non-allelic gene conversion between closely linked repeat units. Large kilobase-scale indels are also frequent and consistently preserve the tandem repeat array by adding or removing only complete repeat units. Analysis of MA lines deficient in the helicase RTEL1 supports the involvement of homology-directed DNA repair in these mutational processes. Moreover, simulations of sequence turnover using the centromere-specific mutation spectrum recapitulate the emergence of homogenized repeat blocks characteristic of natural centromeres. Together, our results show that centromere evolution is driven by a distinct mutational spectrum shaped by homology-directed DNA repair, providing a quantitative framework for how local mutational processes generate and maintain the large-scale architecture of centromeric DNA.