High fruit sets in a rewardless orchid: a case study of obligate agamospermy in Habenaria

Abstract

Low fruit set and pollination limitation are common characteristics of non-autogamous orchids, especially in rewardless species. The flowers of many Habenaria species are often characterised by long spurs and are mostly pollinated by long-tongued hawkmoths or butterflies. Unlike the flowers of other Habenaria species, the flowers of Habenaria malintana (Blanco) Merr. have very short spurs with no nectar or scent; however, this species is able to maintain high fecundity in south-west China. Breeding system experiments suggested that H. malintana is an obligate agamospermous orchid. Seed set did not need to be triggered by pollen grain deposition on stigmas, and ~100% fruit set was found in different populations and years. In pollen germination experiments, hand-deposited pollen failed to germinate on stigmas. The flowers of H. malintana failed to attract any pollinators, as we did not observe any floral visitors, and no pollinia removal or deposition occurred in both 2013 and 2014 at two study sites. These results strongly suggested that H. malintana has completely abandoned sexual reproduction and has adopted obligate agamospermy to achieve high reproductive output. We suggest that this strategy may have evolved to provide reproductive assurance and reduce the cost of flowers in response to unreliable pollinator service.