Submitted to: Evolution
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/17/2005
Publication Date: 2/1/2006
Citation: Brunet, J., Sweet, H. 2006. Impact of insect pollinator group and floral display size on outcrossing rate. Evolution. 60:234-246. Interpretive Summary: In many plant species, pollen move from plants to plants via pollinators. The proportion of seeds on a plant fertilized by pollen from other plants is the outcrossing rate. The outcrossing rate affects how genes move between individuals, populations, and between species. Factors that decrease the outcrossing rate may limit gene flow via pollen. Population size, distance between plants, the number of flowers open on a plant, floral morphology, and pollinator type and abundance can influence outcrossing rate of plants. In this study, we examined the influence of these different factors on outcrossing rate in populations of the blue columbine, Aquilegia coerulea. Few plant species have adequate level of variation in these different characteristics making the blue columbine an ideal candidate for this study. We found that floral display and hawkmoth abundance jointly explained 87.6 % of the variation in outcrossing rate and represented the best model to explain outcrossing rate in the blue columbine. Outcrossing rate was larger when fewer flowers were open on a plant and when hawkmoths were more abundant. Our data suggest that manipulating pollinator type and floral display represent potential ways to reduce gene flow in a plant system. Understanding the factors that influence the outcrossing rate helps better estimate the probability of gene flow hence the potential risk associated with transgenes.
Technical Abstract: Species with mixed mating systems (both selfing and outcrossing) often exhibit variation in outcrossing rate among populations. Population size or density, floral display, floral morphology, and pollinator type and abundance can affect outcrossing rates in populations of self-compatible hermaphrodites. We examined variation in outcrossing rate among and within populations of the blue columbine, Aquilegia coerulea and how it relates to differences in population size and density, floral display, herkogamy (distance between anthers and stigmas), and pollinator type and abundance. Mixed mating was common in populations of the blue columbine with outcrossing rates ranging between 0.46 and 0.93. Floral displays, herkogamy levels, and pollinator abundance varied significantly among populations. In single regressions, outcrossing rate significantly decreased with floral display or herkogamy level, and increased with hawkmoth abundance; other factors were not statistically significant. Within a population, outcrossing rates were larger in years of greater hawkmoth abundance. Using multiple regressions, we determined that floral display and hawkmoth abundance was the best model and explained 87.6 % of the variation in outcrossing rate in A. coerulea. Greater floral displays typically increase the level of geitonogamous (among flower) selfing. Hawkmoths did not visit fewer flowers in succession on a plant relative to bumblebees. Hawkmoths preferred female-phase flowers as the first flower visited on a plant while bumblebees preferred male-phase flowers. Visiting a male-phase flower first on a plant tends to increase the level of geitonogamous selfing relative to visiting a female-phase flower first. The majority of the variation in outcrossing rate in the blue columbine can be explained by two ecological factors that impact levels of geitonogamy hence requires little adaptive explanation.