Evolution from mixed to fixed handedness in mirror-image flowers: insights from adaptive dynamics

Abstract

Mirror-image flowers (enantiostyly) involve a form of sexual asymmetry in which the styles of a flower are deflected either to the left- or right-side, with a pollinating anther orientated in the opposite direction. This curious floral polymorphism, which was known but not studied by Charles Darwin, occurs in at least 10 unrelated angiosperm families and represents a striking example of adaptive convergence in form and function associated with cross-pollination by insects. In several lineages, dimorphic enantiostyly (all flowers on a plant with the same style orientation and populations composed of both left- and right-handed plants) has evolved from monomorphic enantiostyly, in which plants have a mixture of both style orientations. We use a modelling approach based on adaptive dynamics to investigate the emergence of dimorphic enantiostyly in a population of plants with monomorphic enantiostyly under gradual evolution. We show that depending on the balance between inbreeding depression following geitonogamy, pollination efficiency and plant density, dimorphism can evolve from an ancestral monomorphic population. In general, the newly emergent dimorphic population is stable against invasion of a monomorphic mutant. However, our model predicts that under certain ecological conditions e.g., a decline of pollinators, dimorphic enantiostyly may revert to a monomorphic state.