Episodic and guanine-cytosine-biased bursts of intragenomic and interspecific synonymous divergence in Ajugoideae (Lamiaceae) mitogenomes.

Synonymous substitution rates in plant mitochondrial genomes vary by orders of magnitude among species, whereas synonymous rates among genes within a genome are generally consistent. Exceptionally, genes within the Ajuga reptans (Lamiaceae) mitochondrial genome exhibit unprecedented intragenomic heterogeneity in synonymous sequence divergence, but the biological mechanisms underlying this rate variation remain unclear. We tracked the origin and evolutionary trajectory of mitochondrial rate variations by dense sampling in Ajugoideae and found differences in the timing and magnitude of rate acceleration for particular genes. The most divergent genes accelerated earlier, retained a high rate across Ajugoideae, and are generally devoid of RNA editing, whereas moderately diverged genes accelerated later and retained relatively higher RNA editing frequency. The acceleration of mutation rates correlates with increased guanine-cytosine (GC) content, suggesting a key role for GC-biased gene conversion and/or repair after the breakage of ancestral gene clusters.