Convergence and molecular evolution of floral fragrance after independent transitions to self-fertilization.

Studying the independent evolution of similar traits provides valuable insights into the ecological and genetic factors driving phenotypic evolution. The transition from outcrossing to self-fertilization is common in plant evolution and is often associated with a reduction in floral attractive features such as display size, chemical signals, and pollinator rewards. These changes are believed to result from the reallocation of the resources used for building attractive flowers, as the need to attract pollinators decreases. We investigated the similarities in the evolution of flower fragrance following independent transitions to self-fertilization in Capsella. We identified several compounds that exhibited similar changes in different selfer lineages, such that the flower scent composition reflects mating systems rather than evolutionary history within this genus. We further demonstrate that the repeated loss of β-ocimene emission, one of the compounds most strongly affected by these transitions, was caused by mutations in different genes. In one of the Capsella selfing lineages, the loss of its emission was associated with a mutation altering subcellular localization of the ortholog of TERPENE SYNTHASE 2. This mutation appears to have been fixed early after the transition to selfing through the capture of variants segregating in the ancestral outcrossing population. The large extent of convergence in the independent evolution of flower scent, together with the evolutionary history and molecular consequences of a causal mutation, suggests that the emission of specific volatiles evolved as a response to changes in ecological pressures rather than resource limitation.