Selective contexts favouring asymmetry: a theoretical and empirical study of pollen transfer in mirror-image flowers

Reciprocal herkogamy has evolved multiple times in flowering plants and is thought to enhance cross-pollination and reduce reproductive interference. Mirror-image flowers represent a form of reciprocal herkogamy in which plants have either right- or left-deflected styles (dimorphic enantiostyly) or produce both stylar orientations (monomorphic enantiostyly). The ecological conditions under which these two forms of enantiostyly originate and persist remain poorly understood. Here, we investigate how floral asymmetry affects pollen transfer dynamics and mating outcomes, and under which conditions enantiostyly provides a fitness advantage over straight-styled floral morphologies. We integrate field observations of dimorphic enantiostylous Wachendorfia paniculata with a mathematical model simulating pollen transfer in plants with straight-styled, monomorphic, and dimorphic enantiostylous flowers. The model incorporates pollinator pathways, pollen carryover, and floral display size, and was parameterized using our ecological data. Enantiostylous flowers received more outcrossed pollen than straight-styled flowers, with dimorphism outperforming monomorphism. However, enantiostyly increased stochasticity in pollen transfer due to variation in pollinator pathways. Intrafloral self-pollination was extremely low in enantiostylous flowers and did not differ between monomorphic and dimorphic forms. Dimorphic enantiostylous plants exhibited longer pollen carryover curves and exported more pollen to more mates potentially increasing siring success and mate diversity. Enantiostyly, particularly in dimorphic systems, can provide a selective advantage by improving pollen receipt and export components of fitness through enhanced outcrossing. This mating advantage may be reduced under conditions of pollen limitation or when autogamy in straight-styled flowers is low. Our findings clarify the functional significance of enantiostyly and offer a framework for understanding the evolution of floral asymmetry in angiosperms.