Nectar-inhabiting microorganisms and the dioecious plant species Silene latifolia

Nectar-inhabiting yeasts are commonly found in many plant species. Even though these microorganisms are abundant, very little is known about how they interact with plants or pollinators. I examined the presence and diversity of these microorganisms in Silene latifolia, a dioecious plant species, and investigated the interactions between plants, pollinators, and yeast. I isolated yeast from S. latifolia and associated species, and identified them using DNA-based phylogenetic analyses and morphology. Twenty-eight species and even more distinct strains were identified. There was evidence of both host specificity and widespread dispersal of yeast species. There were no consistent differences among the sexes of S. latifolia , but males appeared to have a lower diversity and species richness in some years. Eight species were examined with regard to their growth in nectar and sugar solution. Several were capable of growing in high sugar concentrations or water, but far fewer grew in real nectar. More yeast species were capable of growing in female nectar than male nectar, and some yeast species had high positive growth rates in female nectar. Hand-pollination experiments on female flowers inoculated with yeast species (Candida bombi and Metschnikowia reukaufii) indicated no direct effect of these microorganisms on seed production. However, the presence of C. bombi in nectar had a positive effect on seed production of S. latifolia, and there was some indication that M. reukaufii had a negative effect. It is likely these effects were mediated by the effects of yeast on nectar quality or attractiveness, thereby affecting pollinator visitation patterns.