Regulatory architecture and standing variation drive parallelism in floral evolution

Evolution repeatedly gives rise to similar phenotypes, suggesting that constraints and biases make some evolutionary trajectories more likely than others, yet the mechanisms underlying this predictability remain poorly understood. Using repeated transitions to self-fertilization in a model Brassicaceae genus, we show that independent reductions in petal size arise from dosage changes at the same pleiotropic growth regulator, JAGGED (JAG). Petal development exhibits particularly high sensitivity to JAG dosage, resulting in organ-specific morphological shifts with minimal effects on other organs and limited pleiotropy. In both selfing species, these changes were drawn from functionally divergent variants whose polymorphism patterns are consistent with long-term maintenance under weak selection. Our results demonstrate how developmental network architecture and the maintenance of functional variation at pleiotropic loci combine to channel evolution toward predictable outcomes.