Floral fragrance and pollination in the yellow monkey flower Mimulus guttatus Fischer ex D.C. (Scrophulariaceae)
A Californian population of the Yellow Monkey
Flower, Mimulus guttatus Fischer ex D.C., contains a floral
fragrance polymorphism such that the plants can be divided
into two classes: fragrant and non-fragrant. This thesis
has examined the effects of the fragrance polymorphism on
the natural pollinators of M. guttatus, with a view to
investigating the role and evolutionary significance of
floral fragrance in apidophilous plants.
Published work by many researchers has suggested
that a fragrance polymorphism in an apidophilous
chasmogamous plant would lead to reduced inter-phenotype
pollinator movements due to the phenomenon of
odour-constancy, and hence to genetic divergence and
possibly sympatric speciation. Alternatively, if the
fragrance phenotypes differed in pollinator attractiveness,
the more attractive phenotype would have a selective
advantage, and would therefore eventually become fixed.
Both of these possible scenarios have been explored in the
course of this project, both by investigation of bee
foraging behaviour on experimental plots containing
fragrant and non-fragrant Mimulus plants, and by
investigation of the fitnesses of the two phenotypes in the
natural population and under glasshouse conditions.
It was found that the two phenotypes did not differ
significantly in terms of measured fitness, and that in the
natural population there was no evidence of reduced
gene-flow between fragrant and non-fragrant plants.
Experimental plot results showed no evidence of
odour-constancy by bees, but revealed that some bumblebee
species discriminated weakly between the two phenotypes.
Pollinators were found to be visiting Mimulus flowers for
pollen, and variation in plant pollen production had
significant effects on within-plant foraging behaviour.
However, bees failed to learn to associate fragrance
phenotype with pollen production, in contrast to previously
published research that has demonstrated that bees readily
learn to associate fragrance phenotype with nectar rewards,
suggesting that pollen-collecting bees may respond
differently to floral cues than nectar-collecting bees.
For a variable such as floral fragrance to have
evolutionary significance it is essential that the
character is heritable, and part of the investigation has
concentrated on investigating the heritability and
penetrance of the character. It was found that inheritance
of fragrance is best explained by a simple additive genetic
model, and that fragrance production is dominant to absence
The possibility that this fragrance polymorphism
has been previously influenced by other pollen vectors and
the population's potential for switching from
bee-pollination to alternative pollen vectors is discussed.