Is floral diversification associated with pollinator divergence? Flower shape, flower colour and pollinator preference in Chilean Mimulus.

Abstract

Adaptation to different pollinators is thought to drive divergence in flower colour and morphology, and may lead to interspecific reproductive isolation. Floral diversity was tested for association with divergent pollinator preferences in a group of four closely related wildflower species: the yellow-flowered Mimulus luteus var. luteus and the red-pigmented M. l. variegatus, M. naiandinus and M. cupreus. Patterns of pollinator visitation were evaluated in natural plant populations in central Chile, including both single-species and mixed-species sites. Floral anthocyanin pigments were identified, and floral morphology and nectar variation were quantified in a common garden experiment using seeds collected from the study sites. Mimulus l. luteus, M. l. variegatus and M. naiandinus are morphologically similar and share a single generalist bumblebee pollinator, Bombus dahlbomii. Mimulus cupreus differs significantly from the first three taxa in corolla shape as well as nectar characteristics, and had far fewer pollinator visits. This system shows limited potential for pollinator-mediated restriction of gene flow as a function of flower colour, and no evidence of transition to a novel pollinator. Mimulus cupreus may experience reduced interspecific gene flow due to a lack of bumblebee visitation, but not because of its red pigmentation: rare yellow morphs are equally undervisited by pollinators. Overall, the results suggest that factors other than pollinator shifts may contribute to the maintenance of floral diversity in these Chilean Mimulus species.