Abstract
Inbreeding in plants typically reduces individual fitness but may also alter ecological interactions. This study examined the effect of inbreeding in the mixed-mating annual Mimulus guttatus on visitation by pollinators (Bombus impatiens) in greenhouse experiments. Previous studies of M. guttatus have shown that inbreeding reduced corolla size, flower number, and pollen quantity and quality. Using controlled crosses, we produced inbred and outbred families from three different M. guttatus populations. We recorded the plant genotypes that bees visited and the number of flowers probed per visit. In our first experiment, bees were 31% more likely to visit outbred plants than those selfed for one generation and 43% more likely to visit outbred plants than those selfed for two generations. Inbreeding had only a small effect on the number of flowers probed once bees arrived at a genotype. These differences were explained partially by differences in mean floral display and mean flower size, but even when these variables were controlled statistically, the effect of inbreeding remained large and significant. In a second experiment we quantified pollen viability from inbred and self plants. Bees were 37–54% more likely to visit outbred plants, depending on the population, even when controlling for floral display size. Pollen viability proved to be as important as floral display in predicting pollinator visitation in one population, but the overall explanatory power of a multiple regression model was weak. Our data suggested that bees use cues in addition to display size, flower size, and pollen reward quality in their discrimination of inbred plants. Discrimination against inbred plants could have effects on plant fitness and thereby reinforce selection for outcrossing. Inbreeding in plant populations could also reduce resource quality for pollinators, potentially resulting in negative effects on pollinator populations.
Highlights
The detrimental effects of inbreeding in naturally outbreeding plant populations have been extensively documented (e.g., [1,2,3])
Robertson et al [15] demonstrated that British bumble bees (Bombus spp.) can, discriminate among M. guttatus genotypes varying in pollen quality and quantity
In examining the response of pollinators to inbreeding in M. guttatus, we asked the following questions: 1) Does inbreeding reduce the probability that a bumble bee will visit a plant? 2) Does inbreeding reduce the number of flowers visited once a bee arrives at a plant? To understand what might be driving pollinator response, we further asked: 3) Do inbreedinginduced changes in display traits account for the differences in visitation? 4) Does pollen viability account for the variation in visitation rates by bumble bees? We found that bumble bees strongly discriminated against inbred plants independent of display traits
Summary
The detrimental effects of inbreeding in naturally outbreeding plant populations have been extensively documented (e.g., [1,2,3]) Most of these studies have focused on physiological, morphological, or individual fitness traits, but recent studies have documented negative effects of inbreeding in plants on their interactions with herbivores (reviewed in [4]) and pathogens [5,6,7]. These studies have demonstrated that inbreeding can have population-level effects but may have community-level effects as well. An elevated level of inbreeding in a plant population might result in a substantially degraded resource for pollinators
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