Cephalanthera exiguarediscovered: new insights in the taxonomy, habitat requirements and breeding system of a rare mycoheterotrophic orchid

Abstract

Five mycoheterotrophic species ofCephalantherahave been proposed from tropical Asia. Until recently, all of them were only known from the type specimens, for which reason it has been difficult to judge if some of them were actually conspecific. The recent discovery of two large populations ofC. exiguain Thailand made it possible to describe the range of variation of this species in much more detail, and we then found it to be well distinguished from the others. A phylogenetic analysis based on nuclear rDNA ITS sequences corroborated the systematic position ofC. exiguaat the generic level. Examination of the previously neglected type ofC. pusillafrom Myanmar lead to the conclusion that this taxon of disputed generic affinity likewise belongs toCephalanthera, and that it is a clearly distinct species. Finally, we accept that alsoC. ericiflorafrom Laos as well asC. gracilisandC. calcaratafrom Yunnan are probably distinct species.Cephalanthera exiguagrows in hill evergreen forest, but we found little floristic similarity between its two localities in Thailand (ISS=22.5%). Ectomycorrhizal trees of the families Fagaceae and Ulmaceae are probably the ultimate carbon source forC. exiguaat both sites, as the mycorrhizal partners of this orchid (basidiomycetes of the family Thelephoraceae) obtain their carbon nutrition through ectomycorrhizal association with forest trees. Pollination experiments demonstrated that the nectarless flowers ofC. exiguaare not spontaneously autogamous. Based on the pollination syndrome and previously published observations on other species of the genus, we suggest thatC. exiguahas an insect‐operated breeding system acting by deceit, and that the pollinators are probably solitary bees.