Being rich in nutrients and exposed to pollen vectors, flowers provide ideal habitats for many microbes. Recent studies have revealed variation in flower microbial communities among plant species or individuals within the same population. However, its sexual differences have rarely been investigated. In this study, we analyzed floral bacterial communities of a dioecious plant, Mallotus japonicus, in its natural habitat in Otsu, Japan, using 16S rRNA amplicon sequencing.

We found a strong differentiation in microbial communities between male and female flowers. Female flower samples are overwhelmingly dominated by a single bacteria sequence from the family Sphingomonodaceae. Its abundance peaked at the middle of the flowering. On the other hand, male flowers were dominated by various taxa from the Enterobacteriaceae and Erwiniaceae families, which include various plant pathogens.

We consider that short-lived male flowers are less protected against and frequently colonized by antagonistic microbes, while only limited bacteria can inhabit more defended female flowers, which are retained for several months to develop into fruits. The results suggest that transmissions of antagonistic microbes from a male to a female cause a sexual conflict.