Paternal leakage, heteroplasmy, and the evolution of plant mitochondrial genomes

Plant mitochondrial genomes are usually transmitted to the progeny from the maternal parent. However, cases of paternal transmission are known and are perhaps more common than once thought. This review will consider recent evidence, both direct and indirect, of paternal transmission (leakage) of the mitochondrial genome of seed plants, especially in natural populations, and how this can result in offspring that carry a mixture of maternally and paternally derived copies of the genome; a type of heteroplasmy. It will further consider how this heteroplasmy facilitates recombination between genetically distinct partners; a process that can enhance mitochondrial genotypic diversity. This will then form the basis for a discussion of five evolutionary questions that arise from these observations. Questions include how plant mitochondrial genome evolution can be placed on a sexual to asexual continuum, whether cytoplasmic male sterility (CMS) facilitates the evolution of paternal leakage, whether paternal leakage is more likely in populations undergoing admixture, how leakage influences patterns of gene flow, and whether heteroplasmy occurs in natural populations at a frequency greater than predicted by crossing experiments. It is proposed that each of these questions offers fertile ground for future research on a diversity of plant species.