The spatial structure of sexual and cytonuclear polymorphism in the gynodioecious Beta vulgaris ssp maritima: I - at a local scale

We have analyzed the spatial distribution of the sex phenotypes and of mito chondrial, chloroplast, and nuclear markers within two gynodioecious popula tions of Beta vulgaris ssp, maritima. Within both populations, sexual pheno type variation is controlled mainly by the cytoplasmic genotype, although i n one study population a joint polymorphism of cytonuclear factors is clear ly involved. In spite of contrasts in the ecology (mainly due to different habitats), a clear common feature in both populations is the highly patchy distribution of cytoplasmic haplotypes, contrasting with the wide distribut ion of nuclear diversity. This high contrast between cytoplasmic vs. nuclea r spatial structure may have important consequences for the maintenance of gynodioecy. It provides opportunities for differential selection since nucl ear restorer alleles are expected to be selected for in the presence of the ir specific cytoplasmic male sterile (CMS) type, but to be neutral (or sele cted against if there is a cost of restoration) in the absence of their CMS type. Selective processes in such a cytonuclear landscape may explain the polymorphism we observed at restorer loci for two CMS types.