The role of <i>APETALA1</i> in petal number robustness.

Abstract

Invariant floral forms are important for reproductive success and robust to natural perturbations. Petal number, for example, is invariant in <i>Arabidopsis thaliana</i> flowers. However, petal number varies in the closely related species <i>Cardamine hirsuta</i>, and the genetic basis for this difference between species is unknown. Here we show that divergence in the pleiotropic floral regulator <i>APETALA1</i> (<i>AP1</i>) can account for the species-specific difference in petal number robustness. This large effect of <i>AP1</i> is explained by epistatic interactions: <i>A. thaliana AP1</i> confers robustness by masking the phenotypic expression of quantitative trait loci controlling petal number in <i>C. hirsuta</i>. We show that <i>C. hirsuta AP1</i> fails to complement this function of <i>A. thaliana AP1</i>, conferring variable petal number, and that upstream regulatory regions of <i>AP1</i> contribute to this divergence. Moreover, variable petal number is maintained in <i>C. hirsuta</i> despite sufficient standing genetic variation in natural accessions to produce plants with four-petalled flowers.