Sunbird pollination of the dioecious root parasite Cytinus sanguineus (Cytinaceae)

Abstract

Cytinus (Cytinaceae) is an enigmatic lineage of parasitic plants distributed in the Mediterranean, Africa and Madagascar, and exhibits variation in floral traits suggestive of multiple pollinator shifts. The flowers of Cytinus sanguineus, a dioecious South African species, are sturdy, bright scarlet, and scentless - a combination of floral characteristics that suggests pollination by nectarivorous birds. However, the emergence of flowers at soil level underneath host shrubs and the absence of perches are not typical features of plants pollinated by sunbirds, which are generally averse to foraging on the ground. We studied the pollination system of C. sanguineus using camera traps, analysis of floral nectar and scent, and spectrophotometry of floral surfaces. We observed that C. sanguineus is pollinated by sunbirds, which readily forage on the ground and transfer pollen on their beaks while drinking copious, diluted nectar from narrowly tubular floral nectaries. High fruit set regardless of spatial separation of male and female flowers indicates efficient pollination. Rodents and insects rarely visit C. sanguineus flowers, likely due to the absence of attractive floral volatiles, and the inaccessibility of nectar for rodents. The pollination system of C. sanguineus is clearly distinguished from those of the rodent-pollinated Cytinus visseri and the ant-pollinated Cytinus hypocistis, where pollinator attraction is achieved through production of specific floral volatiles. Our documentation of bird pollination in Cytinus contributes to understanding of the diversity of pollination systems in this genus.