The role of in petal number robustness.

Invariant floral forms are important for reproductive success and robust to natural perturbations. Petal number, for example, is invariant in flowers. However, petal number varies in the closely related species , and the genetic basis for this difference between species is unknown. Here we show that divergence in the pleiotropic floral regulator () can account for the species-specific difference in petal number robustness. This large effect of is explained by epistatic interactions: confers robustness by masking the phenotypic expression of quantitative trait loci controlling petal number in . We show that fails to complement this function of , conferring variable petal number, and that upstream regulatory regions of contribute to this divergence. Moreover, variable petal number is maintained in despite sufficient standing genetic variation in natural accessions to produce plants with four-petalled flowers.