Microbes are omnipresent and interact in many ways with plants. Floral microbes are particularly of interest, because microbes can disperse together with pollen, and because flowers have potential gateways for antagonistic microbes to inside the plant such as stigmas. It is also likely that microbes have exerted different selective pressures on male and female floral traits.

In this study, we identified bacteria inhabiting female and male flowers of dioecious Mallotus japonicus (Euphorbiaceae) by DNA metabarcoding. We found that a plant pathogen family, Erwiniaceae dominated male flowers. Insect visitors and wind might spread the pathogen between trees. The short lifespan of male flowers (1-2 days) could be a mechanical defense to limit pathogen infection to the plant itself.

In contrast, female-flower samples were dominated by a single bacteria sequence of the genus Sphingomonas. Its abundance increased and then decreased during flowering of ~10 days. Some Sphingomonas species are known to protect their host-plant from diseases. The dominance observed here might be a defensive mechanism against Erwinia infection of females, which have to retain flowers until fruit maturation.

Contrasting floral bacterial communities of male and females strongly suggest the importance of microbes as a driver for the evolution of floral traits.