Nectar robbers pollinate flowers with sexual organs hidden within corollas in distylous Primula secundiflora (Primulaceae)

Nectar robbers are thought rarely to pollinate flowers, especially those with sexual organs hidden within corollas. In this study, we examined whether robbers pollinate flowers of distylous Primula secundiflora. Distylous plants have two floral morphs. Pin flowers have long styles and short stamens, and thrum flowers have short styles and long stamens. Flowers of P. secundiflora were commonly robbed by bumblebees, and robbing holes were always situated between high and low sexual organs for both floral morphs. We observed that pollen grains of pin flowers were removed while thrum flowers received fresh pollen grains immediately after flowers were robbed. We manipulated flowers so that only nectar robbers could visit them. This resulted in 98 per cent of thrum flowers and 6 per cent of pin flowers setting fruit, and seed number per thrum fruit was also significantly higher than per pin fruit. Our findings suggest that nectar robbers transfer pollen from pin flowers to thrum flowers effectively, and consequently increase male fitness of the pin morph and female fitness of the thrum morph. Such asymmetrical pollen flow caused by nectar robbers may act as an important selective agent in floral fitness and evolution of distyly.