Autonomous self-pollination and insect visitors in partially and fully mycoheterotrophic species of Cymbidium (Orchidaceae)

Few studies have examined the reproductive ecology of mycoheterotrophic plants, but the existing literature hypothesizes that they adopt a self-pollinating strategy. Although growing evidence indicates that some rewarding mycoheterotrophic plants depend (at least partially) on an insect-mediated pollination system, it remains unclear whether some mycoheterotrophic plants can attract pollinators without nectar or other rewards. Moreover, in a broader evolutionary/ecological context, the question of whether the evolution of mycoheterotrophy induces a shift in pollination pattern is still unknown. Here I present a comparative investigation into the breeding system of two fully mycoheterotrophic orchids, Cymbidium macrorhizon and C. aberrans, and their closest extant relative, the mixotrophic C. lancifolium. Pollination experiments were conducted to determine the breeding system of these plants. In addition, flower visitors that might contribute to pollination were recorded. Flowers at different maturity stages were examined to investigate mechanisms enabling or limiting self-fertilization. While nectarless flowers of C. lancifolium and C. macrorhizon can successfully attract potential pollinator honeybees, all three Cymbidium possess an effective self-pollination system in which the rostellum that physically separates the stigma and pollinia is absent. Because mixotrophic and mycoheterotrophic Cymbidium occupy low-light niches, pollinator foraging would be negatively influenced by low-light intensity. In partial and fully mycoheterotrophic Cymbidium, autogamy would likely be favoured as a reproductive assurance to compensate for pollinator limitation due to their lack of nectar and pollinators' hostile habitat preferences.