Genetic and developmental constraints drive parallelism in flower evolution

Evolution repeatedly gives rise to similar phenotypes, reflecting shared constraints across independent lineages. In flowering plants, transitions to self-fertilization are typically accompanied by reduced flower size, but the mechanisms behind this pattern remain unclear. We show that two independently evolved self-fertilizing species of Capsella both achieved petal reduction by reducing the activity of the growth-promoting gene JAGGED . Because petal development is especially sensitive to JAGGED dosage, modest reductions limit petal growth with minimal effects on other organs, thus minimizing pleiotropy. Both species relied, at least partly, on the same ancestral genetic variants, revealing that preexisting diversity maintained in the ancestral population also facilitated this convergence. These findings illustrate how development constraints and ancestry can channel evolution toward predictable outcomes.