Mitochondrial DNA capture and divergence in Pinus provide new insights into the evolution of the genus

The evolution of the mitochondrial (mt) genome is far from being fully understood. Systematic investigations into the modes of inheritance, rates and patterns of recombination, nucleotide substitution, and structural changes in the mt genome are still lacking in many groups of plants. In this study, we sequenced >11kbp mtDNA segments from multiple accessions of 36 pine species to characterize the evolutionary patterns of mtDNA in the genus Pinus. We found extremely low substitution rates and complex repetitive sequences scattered across different genome regions, as well as chimeric structures that were probably generated by multiple intergenomic recombinations. The mtDNA-based phylogeny of the genus differed from that based on chloroplast and nuclear DNA in the placement of several groups of species. Such discordances suggest a series of mtDNA capture events during past range shifts of the pine species and that both vertical and horizontal inheritance are implicated in the evolution of mtDNA in Pinus. MtDNA dating revealed that most extant lineages of the genus originated during Oligocene-Miocene radiation and subgenus Strobus diversified earlier than subgenus Pinus. Our findings illustrate a reticular evolutionary pathway for the mt genome through capture and recombination in the genus Pinus, and provide new insights into the evolution of the genus.