Expanding the sex-differential plasticity hypothesis to include the biotic environment: Gynodioecious Geranium maculatum and its arbuscular mycorrhizal partners
For gynodioecious plant species (both female and hermaphroditic plants in some populations), the establishment and success of females holds the key to our understanding of the evolution of separate sexes. One common pattern found in gynodioecious species is that females tend to become established in the “harsher” portion of their distribution. The Sex Differential Plasticity hypothesis suggests that females are more likely to establish in stressful environments, where hermaphrodites may allocate more resources to male function thus allowing females to exceed the threshold relative seed fitness they require to invade. We are exploring treating the available AMF partners in a site as environmental conditions to be considered for plant benefit. If AMF communities are important factors for female establishment, we predict that females may establish in populations with lower quality AMF partners. Specifically, we asked: To what extent does AMF community composition and/or species richness predict the presence of females in a population? We conducted a survey of root tissue from sexually dimorphic and monomorphic populations across the range of G. maculatum. We used a T-RFLP database approach to identify AMF species present on roots and compared species composition and richness between populations with or without females present.
We found that plants within sexually dimorphic populations harbored lower overall AMF species richness in their roots than those of all-hermaphrodite populations. In addition, dimorphic populations had distinct AMF community composition according to MANOVA analyses of AMF operational taxonomic units. These differences were not driven by overall differences between females and hermaphrodites, as they are still evident when females are taken out of the analysis. These findings show that biotic factors could play a role in sexual dimorphism within this species. Future experiments will involve the collection of soil biotic communities from both population types and observations of plant response to these environments to determine whether this is a significant mechanism of female establishment for this species.