Pollinator shifts and the loss of style polymorphism in Narcissus papyraceus (Amaryllidaceae).

Darwin proposed that the driving force for the evolution of style polymorphisms is the promotion of cross-pollination between style morphs, through accurate placement of pollen on the pollinator's body. This hypothesis has received much attention, but the effect of different pollinators in the fitness of morphs remains poorly understood. Narcissus papyraceus is a style dimorphic species (long -L- and short -S- styled) with isoplethic (1 : 1) and L-monomorphic populations, mainly visited by long-tongued (LT) nocturnal and short-tongued (ST) diurnal pollinators, respectively. We studied natural female fertility of morphs, and assessed the role of diurnal and nocturnal pollinators. We also quantified female fertility of the morphs in experimental populations with different morph ratio, exposed to predominately long- or short-tongued pollinators. We found that with LT pollinators, both morphs were successfully pollinated in all morph ratio conditions, suggesting that these insects could be involved in maintenance of the polymorphism, although other factors may also play a role. However, with ST pollinators, S-plants displayed less fertility than L-plants, and mating among L-plants was favoured, implying that the polymorphism is lost. These results underscore the role of pollinators on variations in style polymorphism.