Adaptive differentiation in floral traits in the presence of high gene flow in scarlet gilia (Ipomopsis aggregata).

Abstract

Plant-pollinator interactions are thought to be major drivers of floral trait diversity. However, the relative importance of divergent pollinator-mediated selection vs. neutral processes in floral character evolution has rarely been explored. We tested for adaptive floral trait evolution by comparing differentiation at neutral genetic loci to differentiation at quantitative floral traits in a putative Ipomopsis aggregata hybrid zone. Typical I.aggregata subsp. candida displays slender white tubular flowers that are typical of flowers pollinated by hawkmoths, and subsp. collina displays robust red tubular flowers typical of flowers pollinated by hummingbirds; yet, hybrid flower morphs are abundant across the East Slope of the Colorado Rockies. We estimated genetic differentiation (FST ) for nuclear and chloroplast microsatellite loci and used a half-sib design to calculate quantitative trait divergence (QST ) from collection sites across the morphological hybrid zone. We found little evidence for population structure and estimated mean FST to be 0.032. QST values for several floral traits including corolla tube length and width, colour, and nectar volume were large and significantly greater than mean FST . We performed multivariate comparisons of neutral loci to genetic correlations within and between populations and found a strong signal for divergent selection, suggesting that specific combinations of floral display and reward traits may be the targets of selection. Our results show little support for historical subspecies categories, yet floral traits are more diverged than expected due to drift alone. Non-neutral divergence for multivariate quantitative traits suggests that selection by pollinators is maintaining a correlation between display and reward traits.