Spatial distributions of cytoplasmic types and sex expression in Alaskan populations of Silene acaulis

Sex expression in gynodioecious plants is often governed by an interaction between maternally inherited cytoplasmic male sterility (CMS) genes and nuclear male fertility restorer genes. Previous studies have documented an association between females and maternally inherited molecular markers in the cytoplasmic genomes in natural populations, suggesting that these markers are associated with different CMS types. We analyzed the spatial distributions of mitochondrial and chloroplast alleles and the sexual phenotype of individuals within five Alaskan populations of Silene acaulis. Within two populations, mitochondrial alleles, but not sex, were clustered at a scale ≤2 m. We suggest that this pattern is generated by short-distance dispersal of mitochondrial alleles (and CMS) through seed and longer-distance dispersal of nuclear restorers through both pollen and seed. We also investigated whether the association between mitochondrial alleles and sex ratio changed through time (across size classes), as might be expected when either a new CMS type invades a population and female frequency increases or, alternatively, whether females and the mitotypes they carry have been selected against over time. We found no association between sex ratio and mitochondria alleles across size classes. Overall, population sex ratios did not vary among populations within the same mountain range but did differ in populations in the two different mountain ranges that we surveyed. Finally, mitochondrial and chloroplast alleles carried by the same individual were not in complete linkage disequilibrium, suggesting either homoplasy of the cytoplasmic markers we studied or processes generating historical heteroplasmy.