Late-acting self-incompatibility and a narrow floral tube as underlying stylar dimorphism in Narcissus (Amaryllidaceae)

Most heterostylous species show self- and intra-morph incompatibility and models established for such taxa have traditionally been applied to the evolution of stylar dimorphism and heterostyly in Narcissus, a genus with late-acting self-incompatibility. The model of Lloyd and Webb (1992a,b) proposed that, in an approach-herkogamous ancestor, stylar dimorphism and heterostyly appeared consecutively as a result of two single mutations selected positively to enhance cross-pollination. Most polymorphic Narcissus are stylar dimorphic with two anther whorls, the lower positioned in the middle of a narrow floral tube, and style lengths that locate the stigmas above or below the low-level anthers. Here, I propose that in an ancestor with open-tubed flowers, late-acting self-incompatibility and variable style length, the narrowing of the floral tube increased self-pollination and ovule discounting in individuals with the stigma at the same height as the low-level anthers, imposing gradual disruptive selection against this phenotype and causing the bimodal distribution of style lengths. This hypothesis stresses the need of avoiding self-interference for the selection of stylar dimorphism, but excludes neither the promotion of cross-pollination as a force for subsequent evolutionary steps towards heterostyly in the genus nor the need of inter-morph pollination for the maintenance of polymorphism.