Genetic Basis of Nectar Guide Trichome Variation Between Bumblebee- and Self-pollinated Monkeyflowers (Mimulus): Role of the MIXTA-like Gene GUIDELESS

Nectar guide trichomes play crucial ecological roles in bee-pollinated flowers, as they serve as footholds and guides for foraging bees to access the floral rewards. However, the genetic basis of natural variation in nectar guide trichomes among species remains poorly understood. In this study, we performed genetic analysis of nectar guide trichome variation between two closely related monkeyflower (Mimulus) species, the bumblebee-pollinated Mimulus lewisii and self-pollinated M. parishii. We demonstrate that a MIXTA-like R2R3-MYB gene, GUIDELESS, is a major contributor to the nectar guide trichome length variation between the two species. The short-haired M. parishii carries a recessive allele due to non-synonymous substitutions in a highly conserved motif among MIXTA-like MYB proteins. Furthermore, our results suggest that besides GUIDELESS, additional loci encoding repressors of trichome elongation also contribute to the transition from bumblebee-pollination to selfing. Taken together, these results suggest that during a pollination syndrome switch, changes in seemingly complex traits such as nectar guide trichomes could have a relatively simple genetic basis, involving just a few genes of large effects.