Comparative analysis of 11 Brassicales mitochondrial genomes and the mitochondrial transcriptome of Brassica oleracea

• We assembled two complete and five draft mitogenomes from Brassicales species.
• We also analyzed the mitochondrial transcriptome from Brassica oleracea.
• The ancestral Brassicaceae mitogenome experienced gene losses and size reduction.
• Two ORFs in B. oleracea have characteristics of cytoplasmic male-sterility factors.
• RNA editing frequency does not correlate with mutation rate or transcript abundance.

To elucidate the evolution of mitochondrial genomic diversity within a single order of angiosperms, we sequenced seven Brassicales genomes and the transcriptome of Brassica oleracea. In the common ancestor of Brassicaceae, several genes of known function were lost and the ccmFN gene was split into two independent genes, which also coincides with a trend of genome reduction towards the smallest sequenced angiosperm genomes of Brassica. For most ORFs of unknown function, the lack of conservation throughout Brassicales and the generally low expression and absence of RNA editing in B. oleracea argue against functionality. However, two chimeric ORFs were expressed and edited in B. oleracea, suggesting a potential role in cytoplasmic male sterility in certain nuclear backgrounds. These results demonstrate how frequent shifts in size, structure, and content of plant mitochondrial genomes can occur over short evolutionary time scales.