Effects of inbreeding on floral volatiles in Mimulus guttatus

Background/Question/Methods

Inbreeding reduces heterozygosity, which allows for the expression of deleterious recessive alleles and decreases the influence of overdominance on fitness. Consequently, many inbred progeny experience a fitness loss relative to outbred progeny. Mimulus guttatus is a primarily bumblebee-pollinated species that exhibits a wide range of outcrossing rates and in which inbreeding depression is often observed.  Previous studies show that bumblebees discriminate against inbred M. guttatus independently of flower size and number. Since plant volatiles play a role in many plant-insect interactions, this suggests that bumblebees may use olfactory cues to discriminate between inbred and outbred M. guttatus. In this study we investigated the effects of inbreeding on floral volatiles in M. guttatus.  

The plants used in this study descend from seed from a M. guttatus population in Napa County, CA. The plants were either fully outbred (F=0) or were inbred for three generations (F=0.875). Plants were germinated in an insect-free growth chamber. Floral volatiles were collected between 10:00 and 18:00 using a push-pull volatile collection system. Volatile samples were eluted and all compounds were quantified using a gas chromatograph fitted with a flame ionization detector. Compounds were tentatively identified using a gas chromatograph-mass spectrometer and the NIST spectral library.

Results/Conclusions

Sixty-two compounds were emitted from all plants, so we used principal components analysis to reduce the dimensionality of this data. The first principal component (PC1) explained 40 percent of the variance in the floral volatile blend, with three compounds showing positive loadings greater than 0.3. PC1 was significantly greater for outbred than inbred plants, even with flower number and corolla width as covariates. There was no significant difference between inbred and outbred plants in total mass or number of compounds emitted. Univariate analyses of individual compounds showed significant differences in emission for six compounds, including the bumblebee pheromone β-farnesene. β-farnesene was emitted in the second-highest amount on average, and was present in 25 of 26 outbred plants, but only in 5 of 10 inbred plants. Thus, β-farnesene played a large role in the floral scent difference between inbred and outbred plants and is a strong candidate for explaining the discrimination of bumblebees against inbred plants. Our next step is to conduct behavioral studies of bumblebee visits to artificial flowers with manipulated scent blends to investigate the role of β-farnesene in bumblebee behavior. These results will provide insight on inbreeding depression of volatile phenotype and its implications in mating-system evolution.