Short-tongued fly pollination of the vomit-scented Crassula peploides (Crassulaceae) in the southern African Drakensberg Mountains

Abstract

Saprophilous flies are frequent floral visitors, and growing evidence indicates that they play an important role as pollinators, especially at high elevation. Saprophilous fly pollination is often associated with foul-smelling flowers, but this has hitherto only been documented in a relatively limited number of plant families. We studied the pollination and floral traits of Crassula peploides (Crassulaceae), a species with vomit-scented flowers, at two sites in the Drakensberg Mountains of South Africa. Visitor observations and assessment of pollen loads revealed almost exclusive pollination by short-tongued flies (Diptera), primarily of the families Sarcophagidae, Tachinidae, and Muscidae, although pollinator assemblages varied between sites and seasons. Floral scent analysis using gas chromatography coupled with mass spectrometry showed that the odour of C. peploides is dominated by low molecular-weight acids, particularly butanoic and isovaleric acid, key compounds that characterize the smell of vomit and sweaty feet, respectively. Quantification of floral colour in a fly vision model indicated that petal tips, pollen and sepals contrast with petal bases, ovaries and the background gravel substrate, and may thus also function in pollinator attraction. Flies were consistently observed probing floral surfaces with their mouthparts when visiting flowers, and the presence of nectar, albeit in small quantities with low sugar concentration, was confirmed. These results provide the first evidence of functional specialization for pollination by short-tongued flies in Crassulaceae and suggest a role for the foul acid-dominated floral scent in pollinator attraction. Similarities between flowers of C. peploides and other fly-pollinated species in the Drakensberg Mountains suggest the presence of a guild of plants with relatively small, unpleasant-smelling, pale-coloured flowers and an important role for short-tongued flies as pollinators in this high-elevation region.