On the adaptive value of cytoplasmic genomes in plants.

Is DNA variation maintained in organelle genomes selectively neutral? The answer to this question has important implications for many aspects of ecology and evolution. While traditionally the answer has been 'yes', recent studies in animals have shown that, on the contrary, mitochondrial DNA polymorphism is frequently adaptive. In plants, however, the neutrality assumption has not been strongly challenged. Here, we begin with a critical evaluation of arguments in favour of this long-held view. We then discuss the latest empirical evidence for the opposing prediction that sequence variation in plant cytoplasmic genomes is frequently adaptive. While outstanding research progress is being made towards understanding this fundamental topic, we highlight the need for studies that combine information ranging from field experiments to physiology to molecular evolutionary biology. Such an interdisciplinary approach provides a means for determining the frequency, drivers and evolutionary significance of adaptive organelle DNA variation.