Neither floral nor fungal mutualists help to explain the persistence of females in a gynodioecious plant

Background/Question/Methods

Most flowering plants are hermaphrodites, garnering fitness through male and female gametes. Gynodioecy, a dimorphic sex system in which some plants produce hermaphroditic flowers while others are male-sterile and function only as females, puzzled even Darwin. To be evolutionary stable, female plants must compensate for their loss of male function. How they do so remains a mystery for many gynodioecious species. Objectives: Our long-term objectives are to understand how above- and below-ground mutualists and floral enemies contribute to the persistence of females in gynodioecious populations of Sticky Polemonium, Polemonium foliosissimum (Polemoniaceae). Here, we report on two studies to understand how pollination and the presence of arbuscular mycorrhizal fungi affect fitness of females and hermaphrodites. We used hand pollination experiments to examine the degree to which sex-ratio affects pollen limitation in three populations, and we used an artificial clipping experiment to examine if arbuscular mycorrhizal fungi (AMF) differentially affect tolerance to herbivory for females and hermaphrodites in one population.

Results/Conclusions

Data from several years suggest that females have a seed set advantage over their hermaphroditic neighbors, however the differences were not statistically significant due to the high variance among females (e.g., total seed set averaged over three sites for F: 2665 ± 924 vs. H: 1611 ± 194; F_1,55= 1.12, P = 0.39). We hypothesized that plants in sites with high numbers of females would be more pollen limited than those in sites with low numbers of females. Although sites differed in average seed set/fruit (F_2,50 = 11.4, P <0.001), the interaction between site x hand pollination treatment was not significant (F_2,50 = 0.12, P = 0.88). Sticky Polemonium hosts AMF symbionts, however we found no differences between colonization rates of females vs. hermaphrodites. AMF colonization was positively correlated with plant size (F_1,14 =5.2, P=.04) and there was a trend (although not significant) for large hermaphrodites to be more highly colonized than large females (F_{1,12 =} 2.1, p=0.17). Although overall AMF colonization did not differ between clipped and unclipped plants, clipped plants had a significantly higher percent vesicle colonization than unclipped plants (F_1,36 = 4.7, P = 0.04). Our results corroborate those of other studies with gynodioecious plants—i.e., pollen limitation is likely to be more limited by the identity and behavior of pollinators than by sex-ratio, and the results of plant interactions with multiple partners are often subtle and complex.