A Curious New Species of Xenostegia (Convolvulaceae) from Central Africa, with Remarks on the Phylogeny of the Genus

The polymorphic genus Merremia has been shown to be polyphyletic. In 1980, Xenostegia was separated from the first, based on its longitudinally dehiscing anthers, stigmas with long tapering papillae, and non-spiny, pantoporate pollen, and accommodated two species in the Old World tropics. A recent molecular study and subsequent review of the tribe Merremieae resulted in the distinction of seven genera and corroborated the delimitation of Xenostegia at genus level, with three more species added to it. During fieldwork carried out in 2015 and 2016 in Lomami National Park, Democratic Republic of the Congo, material was collected of a Convolvulaceae species with pinnatifid leaves that was morphologically very close to Xenostegia sapinii, but deviated notably in sepal shape and corolla length. Closer examination revealed additional distinctive characters leading to the conclusion that it represents a distinct taxon new to science. Its pantoporate pollen, longitudinally dehiscing anthers, and stigmas with long papillae seem to confine its placement in Xenostegia. However, its most striking feature is the presence of two free styles, a character otherwise not present in the subfamily Convolvuloideae. The results of our molecular phylogenetic analyses confirmed our hypothesis on our new taxon being a member of Xenostegia. The observed incongruence between the nuclear and plastid-based trees might indicate that the new taxon has a hybrid origin. Based on all evidence, it is recognized at species level, Xenostegia lomamiensis, and it is formally published, along with a full morphological description, illustration, habitat data, and an informal IUCN Red List assessment. A key to the species of Xenostegia in central Africa is provided.