Evolution of Impatiens (Balsaminaceae) in the Albertine Rift – The endemic Impatiens purpureoviolacea complex consists of ten species

Abstract

The Albertine Rift harbours a highly diverse flora with numerous endemic species. An important component of the forest understorey is the herbaceous genus Impatiens. Fieldwork in Burundi, the Democratic Republic of the Congo and Rwanda as well as morphological studies indicated that the Albertine Rift endemic Impatiens purpureoviolacea represents a species complex. We analyzed the hidden diversity of the complex using morphological and molecular data supplemented by herbarium studies. We found that the Impatiens purpureoviolacea complex can be divided into morphologically and phylogenetically well characterized clades containing ten species and a natural hybrid. We describe all of these species, provide a species key and analyze their evolutionary history. Beside Impatiens purpureoviolacea and I. gesneroidea, the already described I. urundiensis is resurrected from synonymy. Two varieties, Impatiens purpureoviolacea var. longicalcarata and I. gesneroidea var. superglabra are raised to species status, and five new species (Impatiens elwiraurzulae, I. lotteri, I. ludewigii, I. lutzmannii, I. versicolor) and a new natural hybrid (I. ×troupinii) are described. Within the mostly insect-pollinated species of the clade, two bird-pollinated species (Impatiens gesneroidea, I. superglabra) evolved independently. The clade split from its sister taxon in the Pliocene and started diversifying during the Pliocene/Pleistocene transition in parallel to an increased mountain uplifting and volcanic activity in the Albertine Rift. It further diversified during the Pleistocene, likely due to the changes in forest cover and connectivity induced by climatic fluctuations.