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 populations, sex ratios indirectly indicate antheridiogen activity. No hermaphroditic gametophytes were identified in any population. Female gametophytes are randomly distributed in each population while male gametophytes tend to cluster. In two of the populations, male sex expression is spatially dependent on females, providing direct evidence of antheridiogen function in situ.

Conclusions: This study provides the first documentation of spatial sex expression in natural gametophyte populations of an antheridiogen-producing tree fern species. The profound impact of antheridiogen on gametophyte sex expression in field settings suggests this system is intimately tied to mating systems, fitness, and genetic diversity in Cyathea multiflora.

Gametophyte populations of the antheridiogen-producing tree fern species Cyathea multiflora were collected at La Selva Biological Station in Costa Rica in January 2019. The sex of each gametophyte was determined, mapped, and spatial statistic approaches were used to examine the distribution of sex in each population.