Autonomous self-pollination and pseudo-fruit set in South African species of Eulophia (Orchidaceae)

Abstract

Autonomous self-pollination is surprisingly common among orchids and is thought to provide reproductive assurance when pollinators are scarce. During investigations of the reproductive biology of the orchid genus Eulophia, consistently high rates of capsule set were observed in Eulophia clavicornis var. clavicornis, E. c. var. inaequalis, E. c. var. nutans and E. tenella. A breeding system experiment showed that E. c. var. nutans is capable of autonomous self-pollination. Emasculated flowers of this taxon did not set fruit, suggesting that agamospermy is unlikely. The likely mechanism of autonomous self-pollination in these taxa was identified as the partial or complete absence of rostellum tissue, allowing contact between pollinia and stigmatic fluid, and thus for pollen tubes to grow from in situ pollinia to the ovules. In some individuals, basal flowers on an inflorescence possess intact rostellae and functional pollinaria, whereas distal flowers lack pollinia. Neither of these two flower classes set capsules. A few individuals of the otherwise outcrossing E. zeyheriana, which normally have well-developed rostellae, show evidence of autonomous self-pollination resulting from interrupted rostellae in these plants. Other outcrossing Eulophia species ( E. speciosa and E. streptopetala) sometimes show high levels of fruit set, seemingly without insect visitation. However, investigations showed that these are pseudo-fruits lacking seeds and are a result of insect parasitism. Therefore, high levels of fruit set alone should not be used to infer autonomous self-pollination in orchids.