Plant mitochondrial genes are not expected to show variation within a species because of their low rate of sequence evolution. Nevertheless, I present evidence of abundant mitochondrial sequence diversity in the gynodioecious plant Silene acaulis (Caryophyllaceae), which exhibits nuclear-cytoplasmic control of sex determination. Overall we found 22 segregating sites in the cytochrome b gene, organized into nine haplotypes from four populations (Alaska, Colorado, Norway and Finland). Each of the populations had more than one haplotype coexisting, indicating that the polymorphism is local as well as occurring among more distant geographic regions. The proportion of synonymous substitutions among the haplotypes implies an average divergence time of around 16 million years. These results are consistent with the hypothesis that frequency-dependent selection on cytoplasmic male-sterility genes has maintained polymorphism in linked mitochondrial genes.

Key words: Silene acaulis , frequency-dependent selection, gynodioecy, haplotype diversity