Mcdadea: A New Genus of Acanthaceae Endemic to the Namib Desert of Southwestern Angola

Acanthaceae represent one of the most ecologically dominant families of plants in the Namib Desert and adjacent portions of Namibia and Angola yet have never been comprehensively treated from a taxonomic perspective in the region. Recent fieldwork in Angola yielded discovery of two populations of plants, morphologically allied to the tribe Ruellieae, that could not be ascribed to any known genus. Morphological study combined with molecular phylogenetic analysis based on ddRAD sequencing that sampled broadly across other lineages of Ruellieae yielded evidence for a new, previously undocumented lineage of Acanthaceae, which we here formally describe under the monotypic Mcdadea. Plants of M. angolensis are characterized as compact, weak-wooded shrubs with dense vegetation, minute corollas, and small, 2-seeded capsules. The species is restricted to limestone mesas and outcroppings in ultra-arid deserts of Namibe Province, southwestern Angola, where plants rely primarily on coastal fog for precipitation. Although highly range-restricted, there are no known threats to this species and it is currently assessed as of Least Concern. Molecular phylogenetic analyses suggest treatment of Mcdadea angolensis within a newly described subtribe of Ruellieae: Mcdadeinae. Additional revisions to subtribal classification are herein presented, including placement of Calacanthus within Ruelliineae, placement of Echinacanthus within Petalidiinae, and description of two additional new subtribes, Dinteracanthinae and Phaulopsinae