Nonrandom genotype distribution among floral hosts contributes to local and regional genetic diversity in the nectar-living yeast Metschnikowia reukaufii

Environmental heterogeneity has been often suggested as a major driving force preserving genetic variation in clonal microorganisms. This study examines this hypothesis for the specialized nectar-dwelling, clonal yeast Metschnikowia reukaufii (Ascomycota, Saccharomycetales). We examined whether M.reukaufii subpopulations associated with flowers of different host plant species, and different individuals of the same host species, differed in genetic characteristics. Amplified fragment length polymorphisms (AFLP) fingerprints of M.reukaufii strains isolated from floral nectar of different host species and individuals sampled at different spatial scales revealed a strong host-mediated component of genetic and genotypic diversity at all scales considered. Genotypes were nonrandomly distributed among flowers of different species and, in the case of the single host species studied in detail (Helleborus foetidus), also among flowers of conspecific individuals coexisting locally. These host-mediated patterns of genetic structuring are compatible with those expected under the diversifying selection hypothesis for the maintenance of local and regional genetic diversity in clonal organisms. It is proposed that a combination of intrafloral selection and biased pollinator-mediated migration may ultimately account for observed host-mediated genetic structuring in populations of M.reukaufii.