Plants often share pollinators with other species in their community, leading to plant-plant interactions via pollen transfer. Because heterospecific pollen (HP) can have negative effects on conspecific pollen (CP) germination, tube growth and seed production by the recipient plant, much focus has been given to pre-pollination strategies that minimize the receipt of HP. However, community-wide studies demonstrate that many species receive significant amounts of HP, raising the possibility of traits that mitigate costs of HP receipt. We sought to understand the relationship between the fitness cost of HP, and stigma and style traits of the recipient and pollen traits of the donor, using a community wide-crossing design.
We grew 14 plant species (10 recipients, 4 donors) from highly diverse (80 coflowering species) Serpentine seep communities in northern California in a greenhouse. We measured pistil and pollen traits on all species and conducted controlled hand pollinations. Each recipient was hand-pollinated with either conspecific pollen alone or a mix of CP-HP for each donor species. Styles were collected to score pollen tube growth, as well as pollen load and CP:HP ratios. Fruit set and seed set was scored for each pollination.
Effect size (Hedges’ d) was calculated for seed set (seed set of CP-HP mix – CP only/ pooled SD) of recipient-donor pairs and was significantly heterogeneous, with an average effect size reflecting a significant (25%) reduction in seed set from HP receipt. Meta-analysis of effect sizes revealed seed set was significantly affected by the recipient plant species (F7,534 = 97.20, P < 0.0001), the donor plant species (F4,534 = 12.63, P < 0.0001) and their interaction (F28,534 = 13.55, P < 0.0001). Although there were no significant correlations between effect size and recipient style length, flowering overlap, or phylogenetic distance (all P > 0.4), there was a trend for a negative correlation with donor pollen size (R = -0.27, P = 0.13). We are exploring additional traits and their combinations as well as to include a greater number of species in this highly diverse community. This project will greatly contribute to our understanding of how post-pollination mechanisms can contribute to the coexistence of plant species and provide a framework for predicting effects of plant-plant interactions.