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Abstract
Understanding the maintenance of females in gynodioecious species is a crucial component to elucidating the mechanisms of the evolution of separate sexes. Plant-soil interactions, which mediate plant nutrient relations and could play a significant role in female maintenance, have been understudied in gynodioecious species. For this dissertation, I evaluated the extent to which the belowground factors of arbuscular mycorrhizal fungi (AMF) and soil nutrients explain female maintenance. The sex-differential plasticity (SDP) hypothesis predicts females are more likely to establish in stressful environments, so I investigated whether variation in AMF could explain female establishment. I surveyed the AMF associated with gynodioecious Geranium maculatum and found that sexually dimorphic populations had distinct AMF communities from monomorphic populations. In contrast with predictions from the SDP hypothesis, AMF from dimorphic sites were not more detrimental to growth than AMF from monomorphic sites. I also tested whether AMF and soil nutrients play a role in female compensation, the concept that females will use resources not used for pollen production towards increased seed production. I predicted that females may use these resources also towards increased association with AMF. I found, while females and hermaphrodites did not associate with distinct AMF, females had higher AMF colonization rates than hermaphrodites in the greenhouse. I also found that females allocate leaf nitrogen to higher leaf chlorophyll contents at high soil nitrogen levels than hermaphrodites. I posit that increased carbon acquisition of females in some cases may explain the higher AMF association in the greenhouse, and may be a strategy to increase seed production relative to hermaphrodites. I also found that AMF in monomorphic sites may inhibit the establishment of females; AMF from these sites had the most detrimental effect on female flower number. This work demonstrates that although AMF may not exacerbate stress and maintain females via the SDP hypothesis, AMF may explain why we do not find females in all populations. In addition, female compensation is likely an important mechanism of female maintenance, and is likely mediated by soil nutrients and plant-AMF interactions.