Rocksitter Flies (Lauxaniidae: Cestrotus) are Key-Pollinators of Ceropegia pulchellior—A Threatened and Localised South African Endemic With Foetid-Scented Flowers

Abstract

Short-tongued saprophilous flies are a diverse but under-appreciated group of pollinators, which are particularly important for flowers in the subtribe Stapeliinae (Apocynaceae-Asclepiadoideae-Ceropegieae). This clade of plants is characterised by repeated shifts between tubular kettle-trap and open non-trap flowers and chemical mimicry (of decomposing substrates or dying insects) to attract specific fly pollinators. The biology of most Stapeliinae, particularly those with non-trap flowers, remains poorly studied, hampering our understanding of the mechanisms driving diversification in this plant group. We examined the pollination biology and floral traits of Ceropegia pulchellior, a South African endemic with non-trap flowers confined to Natal Group Sandstone on the Durban escarpment. Observations showed that flowers are visited exclusively by flies which forage on small amounts of nectar in the corona cavities. Flowers were pollinated primarily by lauxaniid flies in the genus Cestrotus, although several muscid fly species also contribute. Pollinator exclusion experiments confirmed that the plants depend on these flies for reproduction. Reproductive success was low (fruit set never exceeding 8% of flowers). Pollen transfer efficiency was relatively high but variable across the flowering period. GC–MS analysis of floral scent revealed that the foetid odour is dominated by aliphatic acids and p-cresol with small amounts of indole, supporting the assumption that C. pulchellior mimics decaying substrates to selectively attract detritus-feeding (saprophagous) flies as pollinators. Analysis of spectral reflectance of flowers indicates that flower colours, when viewed by the fly pollinators, are not chromatically distinct from the habitat background, suggesting that flowers rely on olfactory rather than visual signals to attract pollinators. This study contributes to the growing awareness of the complexity of pollination systems in the Stapeliinae, in terms of the wide diversity of fly taxa involved, the intricate pollen transfer mechanisms, and the unusual floral scent chemistry associated with mimicry of oviposition substrates and food sources.