Afrothismiaceae (Dioscoreales), a new fully mycoheterotrophic family endemic to tropical Africa

Abstract

SummaryAfrothismiais a genus of non-photosynthetic mycoheterotrophs from the forests of continental tropical Africa. Multiple phylogenetic inferences using molecular data recover the genus as sister to a clade comprising mycoheterotrophic Thismiaceae and the photosynthetic family Taccaceae, contrary to earlier placements ofAfrothismiaand Thismiaceae within Burmanniaceae. Morphological support for separatingAfrothismiafrom the rest of Thismiaceae has depended on the zygomorphic flowers ofAfrothismia(although some South American species ofThismiaare also zygomorphic) and their clusters of root tubers, each with a terminal rootlet. The number of described species ofAfrothismiahas recently increased substantially, from four to 16, which has provided additional morphological characters that support its distinction from Thismiaceae. Most notably, the ovary inAfrothismiahas a single stalked placenta, and circumscissile fruits from which seeds are exserted by placental elevation (in Thismiaceae, in contrast, there are three placentas, a deliquescing fruit lid, and the seeds are not exserted).Afrothismiastamens are inserted in the lower perianth tube where they are attached to the stigma, and individual flowers are subtended by a single large dorsal bract (in Thismiaceae, stamens are inserted at the mouth of the tube, free of and distant from the stigma, and each flower is subtended by a loose whorl of (2-)3(−4) bracts). Here we formally characterise Afrothismiaceae and review what is known of its development, seed germination, interactions with mycorrhizal Glomeromycota, biogeography, phylogeny and pollination biology. All but one (Afrothismia insignis; Vulnerable) of the 13 species assessed on the IUCN Red List of Threatened Species are either Endangered or Critically Endangered; one species (A. pachyanthaSchltr.) is considered to be extinct.