Intra-specific trait variation is receiving renewed interest as a factor affecting the structure of multi-species communities within and across trophic levels. One pervasive form of intra-specific trait variation is sexual dimorphism in animals and plants, which might exert large effects particularly on the communities of host-associated organisms, but the extent of these effects is not well understood. We investigated whether host-associated microbial communities developed differently in the floral nectar of female and male individuals of the dioecious shrubs, Eurya emarginata and E. japonica. We found that nectar-colonizing microbes such as bacteria and fungi were more than twice as prevalent and, overall, more than ten times as abundant in male flowers as in female flowers. Microbial species composition also differed between flower sexes. To examine potential mechanisms behind these differences, we manipulated the frequency of flower visitation by animals and the order of arrival of microbial species to nectar. Animal visitation frequency affected microbial communities more greatly in male flowers, while arrival order affected them more in female flowers. These sex-specific effects appeared attributable to differences in how animals and microbes altered the chemical characteristics of nectar that limited microbial growth. Taken together, our results provide evidence that sexual dimorphism can have large effects on the structure of host-associated communities.

Dryad_Flowering_period_E.japonicaFlowering period of Eurya japonica in 2014.Blooming_days_E.japonica.csvDryad_Inoculation_ExperimentsColony detection in yeast inoculation experiment into flowersInoculation_experiment.zipDryad_E.japonica_field_experimentCFU of nectar microbes (bacteria and fungi) and nectar concentration (Brix) in Eurya japonica flowersE.japonica_field_experiment.zipDryad_E.emarginata_field_experimentCFU of nectar microbes (bacteria and fungi), association between bacteria and fungi, and nectar concentration (Brix) in Eurya emarginata flowersE.emarginata_experiment.zip