Lack of sex-specific differences in mycorrhizal associations and response to herbivory in the gynodioecious herb, Polemonium foliosissimum

For decades, evolutionary plant biologists have been interested in how females reap a fitness advantage and are thus maintained in gynodioecious plants. Recent attention has focused on the role of multi-species interactions in the maintenance of females, wherein the role of arbuscular mycorrhizal fungi (AMF) is virtually unknown. AMF form symbiotic associations with plants, often resulting in enhanced nutrient acquisition and tolerance to biotic and abiotic stress. Depending on the degree to which females and hermaphrodites rely on their AMF partners, AMF could differentially affect plant tolerance to stress. We conducted a field experiment to examine (1) if females and hermaphrodites of the gynodioecious herb, Polemonium foliosissimum, differentially associate with AMF, and (2) if there was a difference in AMF colonization between the sexes after imposing stress by simulating herbivore damage. First, we compared mycorrhizal colonization between the sexes under ambient (i.e., undamaged) field conditions. Then, for a second set of plants, we simulated herbivory by clipping plants and compared mycorrhizal colonization, leaf phosphorus concentration [P], and fruit and seed production between clipped and control plants. Mycorrhizal colonization did not differ between sexes under ambient field conditions or under the stress imposed by clipping. Clipping did, however, influence both percent root length and vesicle colonization in a plant-size-dependent manner. Overall, colonization and vesicle number increased with plant size for clipped plants, but decreased with size for non-clipped plants. Leaf [P] did not differ between the sexes nor was influenced by clipping, and leaf [P] was not associated with mycorrhizal colonization. Furthermore, neither fruit set nor seed production was associated with mycorrhizal colonization. Our findings suggest that the sexes of P. foliosissimum do not differentially associate with nor differentially garner benefit from mycorrhizal fungi. However, a complete accounting of the costs and benefits of AMF for gynodioecious species must encompass the full life-history of the plant.