Floral polymorphism and the fitness implications of attracting pollinating and florivorous insects

Background and Aims

Floral polymorphism is frequently attributed to pollinator-mediated selection. Multiple studies, however, have revealed the importance of non-pollinating visitors in floral evolution. Using the polymorphic annual daisy Ursinia calenduliflora, this study investigated the importance of different insect visitors, and their effects on fitness, in the maintenance of floral polymorphism.

Methods

The spatial structure of a discrete floral polymorphism was characterized based on the presence/absence of anthocyanin floret spots in U. calenduliflora. A 3-year observational study was then conducted in polymorphic populations to investigate differences in visitation rates of dominant visitors to floral morphs. Experiments were performed to explore the floral preference of male and female Megapalpus capensis (the dominant insect visitor) and their effectiveness as pollinators. Next, floral damage by antagonistic florivores and the reproductive success of the two floral morphs were surveyed in multiple populations and years.

Key Results

Floral polymorphism in U. calenduliflora was structured spatially, as were insect visitation patterns. Megapalpus capensis males were the dominant visitors and exhibited strong preference for the spotted morph in natural and experimental observations. While this may indicate potential fitness benefits for the spotted morph, female fitness did not differ between floral morphs. However, as M. capensis males are very efficient at exporting U. calenduliflora pollen, their preference may likely increase the reproductive fitness of the spotted morph through male fitness components. The spotted morph, however, also suffered significantly greater costs due to ovule predation by florivores than the spotless morph.

Conclusions

The results suggest that pollinators and florivores may potentially exert antagonistic selection that could contribute to the maintenance of floral polymorphism across the range of U. calenduliflora. The relative strength of selection imposed by each agent is potentially determined by insect community composition and abundance at each site, highlighting the importance of community context in the evolution of floral phenotypes.