The Evolutionary Dynamics of Gynodioecy inLobelia

Premise of research. Plant mitochondria are expected to show no within-species haplotype diversity. However, sex is determined in some plants by an interaction between mitochondrial cytoplasmic male sterility (CMS) genes and nuclear restorers, resulting in gynodioecy. A question arises as to whether CMS genes are transient or under balancing selection. With balancing selection, rare CMS genes should be favored, potentially resulting in the maintenance of multiple mitochondrial haplotypes. Given that populations with multiple CMS genes are expected to have higher proportions of females, we predicted that the percentage of females in a population would be positively correlated with mitochondrial haplotype diversity.Methodology. We compared sequence diversity in regions of the mitochondrial cob and cox1 genes for populations of multiple species of Lobelia, which includes hermaphroditic and gynodioecious members.Pivotal results. Sequence variation was low, with haplotypes across five North American species varying from the shared most common haplotype of each gene at only one nucleotide each and individuals from a New Zealand species showing either no intraspecific sequence variation (cob) or one single nucleotide variant (cox1). Nevertheless, considering the two best-sampled species, haplotype diversity tended to be higher for both genes in Lobelia siphilitica, a gynodioecious species, than L. cardinalis, a hermaphroditic species. As predicted, we found a positive correlation among populations between the percentage of females and haplotype diversity, both across all species and within L. siphilitica.Conclusions. These results do not support episodic sweeps of CMS genes but are congruent with the hypothesis that balancing selection on these genes contributes to the evolutionary dynamics of gynodioecy in Lobelia.