Two MYB Proteins in a Self-Organizing Activator-Inhibitor System Produce Spotted Pigmentation Patterns

Many organisms exhibit visually striking spotted or striped pigmentation patterns. Developmental models predict that such spatial patterns can form when a local autocatalytic feedback loop and a long-range inhibitory feedback loop interact. At its simplest, this self-organizing network only requires one self-activating activator that also activates a repressor, which inhibits the activator and diffuses to neighboring cells. However, the molecular activators and inhibitors fully fitting this versatile model remain elusive in pigmentation systems. Here, we characterize an R2R3-MYB activator and an R3-MYB repressor in monkeyflowers (Mimulus). Through experimental perturbation and mathematical modeling, we demonstrate that the properties of these two proteins correspond to an activator-inhibitor pair in a two-component, reaction-diffusion system, explaining the formation of dispersed anthocyanin spots in monkeyflower petals. Notably, disrupting this pattern impacts pollinator visitation. Thus, subtle changes in simple activator-inhibitor systems are likely essential contributors to the evolution of the remarkable diversity of pigmentation patterns in flowers. [Display omitted] •An activator-inhibitor system produces spotted pigment patterning in Mimulus flowers•Expression of the inhibitor, RTO, is promoted by the activator, NEGAN•RTO protein is mobile and inhibits NEGAN function in neighboring cells•Modulating this nectar guide patterning system influences bumblebee visitation Many species exhibit visually striking spotted or striped pigmentation patterns. By studying mutants that alter the red spots that dapple monkeyflower petals, Ding et al. uncover a simple activator-inhibitor system that conforms to long-standing theoretical predictions for how such patterns self-organize during development.