Format: ORAL

Nicholas J. Engle-Wrye1, Ryan A. Folk2

1, 2 Department of Biological Sciences, Mississippi State University, Mississippi State, USA

Little is known about environmental drivers of hybridization, but its phylogenetic distribution across plants is heterogeneous, suggesting that plant traits play an important role. Because animal vectors are responsible for gene flow in most plants, conservatism of biotic niche could explain why some plants are more prone to hybridization, a hypothesis yet to be tested. The taxonomic tribe Heuchereae (Saxifragaceae) is a well-characterized system for pollinator interactions, with studies in Asimitellaria and Lithophragma suggesting the VOCs comprising floral scent are hypervariable at the population level and responsible for pollination selectivity, leading to divergent pollinator assemblages. High levels of pollinator attractant divergence may be responsible for the relatively low levels of hybridization observed in these groups. Heuchera flowers, by contrast, are thought to be pollination generalists and Heuchera species hybridize frequently. Measurement of floral volatiles, yet to be characterized for Heuchera, would offer insight into whether divergence of pollinator attractants can more broadly explain hybridization rate differences across taxa. Given that pollinator-mediated gene flow primarily hinges on VOCs in the relatives of Heuchera, we investigated whether high rates of hybridization observed in Heuchera are associated with low interspecific divergence of VOCs, using as a system the hybrid zone between H. americana and H. richardsonii. We optimized a GC-MS (gas chromatography-mass spectrometry) protocol to characterize VOCs. To identify floral visitation and effective pollinators, we conducted pollination observations at 47 different populations over two years. GC-MS data from ~110 Heuchera representing 21 populations, analyzed with a suite of statistical methods, suggests that VOC compound classes and individual compounds are largely shared within the hybrid complex, while other Heuchera that do not hybridize with these species have distinct species-specific compounds.Pollination observations confirmed shared effective pollinators in the hybrid zone and the overall importance of VOCs in shaping gene flow.