Format: ORAL

Rachel O. Cohen1, Deren A.R. Eaton2

1, 2 Department of Ecology, Evolution, and Environmental Biology, Columbia University, New York, USA

In plant communities where generalist pollinators are shared, the likelihood of heterospecific pollen transfer (HPT) increases, potentially affecting female reproductive success. This scenario may drive evolutionary adaptations aimed at either minimizing the occurrence of HPT (pre-pollination) or reducing its impact (post-pollination). While it is commonly assumed that post-pollination mechanisms restrict heterospecific pollen tube growth in the stigma or early in the style, direct measurements of this process are relatively infrequent. When heterospecific pollen tube growth is assessed, studies often focus on intrageneric exchanges, even though plants in diverse communities and may exchange pollen with both close and distant relatives. To explore the impact of HPT in a diverse plant community, I performed experimental crosses involving co-flowering, non-hybridizing plants across a spectrum of genetic distance from an alpine plant community at Rocky Mountain Biological Laboratory (Colorado, USA). Using semi-in vivo pollen tube assays, fluorescence microscopy, and ovary size measurements, I examined pollen tube growth and ovary size change following all-by-all reciprocal crosses with 11 species (representatives from Orobanchaceae, Lamiaceae, Phrymaceae, Linaceae, and Fabaceae). Contrary to expectations that pollen tube growth is more likely among closely related species and rare among distant relatives, I observed consistent pollen tube growth among all species pairs, ranging from conspecifics to those in different plant orders. Generalized linear models revealed no significant effects of genetic distance on the probability of pollen tube growth through the stigma and into the style. These results suggest that in plants with close ecological interactions, reproductive barriers to reproduction may not broadly include suspension of heterospecific pollen tubes within the stigma and style. Instead, pre-pollination barriers, differences in pollen tube growth rates, and post-zygotic barriers likely play more significant roles.