Pollination biology and sexual differentiation ofOsyris alba (Santalaceae) in the Mediterranean region

Osyris alba L. is a widespread dioecious hemiparasitic shrub of S Europe, N Africa, and SW Asia. Male inflorescences are multiflowered whereas each female inflorescence is reduced to a single flower with persistent enlarged bracts. Pollination is a prerequisite for fruit and seed development and wind is unlikely to be an effective means of pollen spread. In southern Italy pollen is transported by small unspecialized flies and beetles. Both male and female flowers produce an indistinguishable sweet odour. Male flowers are produced in large numbers and over a larger period than the females and provide pollen, nectar, and staminal hairs as rewards for pollinators. The presence and function of staminal hairs with tip cells inOsyris alba has been reported for the first time. Female flowers are rewardless, producing neither mature pollen, nectar nor staminal hairs, but possess three modified yellow indehiscent anthers containing no viable pollen which may provide a strong visual feeding stimulus for pollinators. It is suggested that pollinators are attracted by deceit to female flowers by mimicry of the males and the floral mimicry is, therefore, intraspecific and intersexual. The floral characteristics and flowering phenology of male and female plants are consistent with this kind of mimicry. The female flower possesses a tricarpellary ovary with three ovules of which only one develops. The single seed, containing a small embryo and a large, rich endosperm, is borne in a red fleshy bird-dispersed fruit. The reduction in seed number per flower to one highly nutrient-invested seed, together with a reduction of the multiflowered inflorescence to a solitary flower and the sequential production of ripe fruits over an extended fruiting season, suggest that the female function is markedly resource-limited. It is suggested that, although all the reproductive characteristics present inOsyris alba, as well as hemiparasitism, had probably evolved before the end of the tropical Tertiary, they are of adaptive advantage in the nutrient and water-limited environment of the Mediterranean maquis.