Background and aims: Antheridiogen systems are an important and widespread mechanism by which sex expression is controlled and genetic diversity maintained in fern gametophyte populations. However, antheridiogens have rarely been studied outside of the laboratory and little is known about their function in natural populations. Combining predictions based on field and laboratory study, we document the sexual structure of tree fern gametophyte populations and test the effects of antheridiogen in situ. Key results: In all...