Generalized pollination, floral scent chemistry, and a possible case of hybridization in the African orchid Disa fragrans

AbstractPollination systems in orchids tend to be specialized as a consequence of restrictive floral morphology and specific advertising signals. Here we document a notable exception: Disa fragrans subsp. fragrans, a taxon from the Drakensberg Mountains of South Africa, which is pollinated by insects belonging to at least four orders (flies, beetles, bees, and moths). Pollinaria of D. fragrans are attached to the feet of these visitors and pollination thus occurs in a rather haphazard fashion. Nevertheless, its pollination success and pollen transfer efficiency are comparable to those of its close relative, Disa sankeyi, which is pollinated by a single genus of wasps. D. fragrans has an exceptionally strong floral scent: volatile emission is 19–86μg per inflorescence per hour, which is up to 100 fold greater than in D. sankeyi. The scent bouquet is comprised of at least 46 compounds, mostly benzenoids and phenylpropanoids, which are known to be general attractants to a wide range of insects. In contrast to D. sankeyi, the flowers of D. fragrans have a high level of spectral purity (chroma) as is typical of many generalist insect-pollinated plants. At a site where D. fragrans co-occurs with D. sankeyi we found a plant with intermediate characteristics that may be a hybrid between the two taxa. The novel case of generalist pollination in D. fragrans documented here serves as an example of how floral advertising traits might evolve during an evolutionary shift from specialized to generalized pollination