A monograph of the genus Polhillia (Genisteae: Fabaceae)

Abstract

Polhillia is a small, but poorly known genus of legumes endemic to the Greater Cape Floristic Region (GCFR) of South Africa. The genus was described three decades ago to accommodate species erroneously placed in Argyrolobium and other Cape legume genera. Polhillia is the third most threatened plant genus in South Africa, hence priority for taxonomic study and current knowledge on distribution patterns are essential for conservation interventions. Eight species were known at the start of this study, all of which are threatened with extinction. Species boundaries were, however, unclear and many Renosterveld fragments, in particular in the Overberg, required further exploration in pursuit of the various Polhillia species. The phylogenetic relationships among species and monophyly of the genus, especially regarding the position of the morphological aberrant P. involucrata, had not been tested. This study was built upon a foundation of widespread field sampling and analysis of herbarium specimens. Having considered morphological, anatomical, palynological and molecular data, we present an updated taxonomy of the genus Polhillia. Morphological characters between species were often obscure and unique characters were uncommon, as such species are recognized based on unique combinations of characters. Pollen and leaf anatomical studies did not provide substantial taxonomically informative characters, although some interspecific differences were noted. Molecular phylogenetic reconstructions confirm Polhillia to be monophyletic and sister to Argyrolobium, and that Argyrolobium may not be monophyletic. The morphologically distinct species, P. involucrata, is strongly supported as sister to the rest of the genus. Internal phylogenetic resolution in Polhillia is weak in many clades, due to the low variability among DNA markers studied, and several polytomies are still present. Based on the morphological and molecular evidence, we place P. involucrata in the new monotypic subgenus Roseopolhillia. Four new species are described (P. fortunata B.du Preez, P. groenewaldii B.du Preez, P. stirtoniana B.du Preez, P. xairuensis B.du Preez), and a previously recognized species (P. canescens C.H.Stirt.) is placed in synonymy under P. connata (Harv.) C.H.Stirt. Red data list assessments are provided for all species to highlight their conservation importance. Maps and compound colour plates are provided for each species.