A dominant-negative actin mutation alters corolla tube width and pollinator visitation in Mimulus lewisii

A third of all angiosperm species produce flowers with petals fused into a corolla tube. The various elaborations of corolla tube attributes, such as length, width and curvature, have enabled plants to exploit many specialized pollinator groups. These elaborations often differ dramatically among closely related species, contributing to pollinator shift and pollinator-mediated reproductive isolation and speciation. However, very little is known about the genetic and developmental control of these corolla tube attributes. Here we report the characterization of a semi-dominant mutant in the monkeyflower species Mimulus lewisii, with a substantial decrease in corolla tube width but no change in tube length. This morphological alteration leads to a ˜ 70% decrease in bumblebee visitation rate for the homozygous mutant compared to the wild-type. Through bulk segregant analysis and transgenic experiment, we show that the mutant phenotype is caused by a dominant-negative mutation in an actin gene. This mutation decreases epidermal cell width but not length, and probably also reduces the number of lateral cell divisions. These results suggest a surprising potential role for a 'housekeeping' gene in fine-tuning the development of an ecologically important floral trait.