Genome copy number predicts extreme evolutionary rate variation in plant mitochondrial DNA

Rates of molecular evolution span many orders of magnitude across different taxa in nuclear and organellar genomes. Angiosperm mitogenomes display some of the most extreme rate variation. We demonstrate that plant mitogenome substitution rates are negatively associated with mitogenome copy number (the number of mitogenomes per cell). We propose that homologous recombination, the primary repair mechanism in plant organelles, is less effective in low copy number environments, leading to an increase in substitution rates. Fast mitogenome evolution is also associated with larger mitogenomes. These trends were only observed in mitogenomes and not in plastid genomes. Overall, these findings suggest an explanation for extreme rate variation across angiosperm mitogenomes and potentially for other species which use homologous recombination in their organellar genomes.