Mitochondrial Genome Diversity and Cytoplasmic Male Sterility in Higher Plants

Characteristic differences exist between the mitochondrial genome organization of fertile and cytoplasmic male-sterile (CMS) lines in a range of plant species. Current evidence suggests that these characteristic mitochondrial genotypes arose by aberrant recombination events, generating chimeric mitochondrial DNA sequences which have subsequently become stabilized, possibly by selective amplification. An investigation of the variation in stoichiometry of the four atpA gene types in maize have suggested evolutionary mechanisms for the generation of mitochondrial genome diversity which are based on amplification of pre-existing, rare recombinant DNA molecules. As with a number of other well-documented examples of genome rearrangement, those involving the atpA gene appear to have no obvious phenotypic significance. However, in a number of cases, recombination events have resulted in either modification of existing mitochondrial genes, leading to the synthesis of a modified polypeptide, e.g. the coxI gene in the 9E sorghum cytoplasm, or the generation of novel open reading frames. In the latter case the unique open reading frame found in the mitochondrial DNA of CMS-T maize plants encodes a 13 kDa polypeptide, previously identified as a CMS-T-specific mitochondrial translation product. Current studies are directed towards establishing a causal link between the 13 kDa polypeptide, mitochondrial enzyme complexes, and the CMS phenotype.