Abstract
Rhynchosia is the largest genus in the subtribe Cajaninae of the tribe Phaseoleae in the family Fabaceae. In southern Africa, the genus has not been revised in 99 years, and was therefore classified by the South African National Biodiversity Institute as one of the priority genera in need of a taxonomic revision. Consequently, many of the species are poorly known and, thus difficult to identify, while their distribution ranges and correct typification have not been properly established. The last revision of the genus was conducted in 1923 by Baker where 59 species were recognized and arranged into five sections, namely, Arcyphyllum (two spp.), Chrysoscias (four spp.), Cyanospermum (one sp.), Polytropia (two spp.), and Rhynchosia (50 spp.). The four smaller sections (Arcyphyllum, Chrysoscias, Cyanospermum, and Polytropia) have recently been revised, leaving only the type section (Rhynchosia) outstanding. In addition, there is little or no information on the phylogenetic relationships of the genus, and its allied genera, within the subtribe Cajaninae. Therefore, the aim of this study was to provide a comprehensive taxonomic revision for the type section of the genus Rhynchosia in southern Africa, using vegetative and reproductive morphology, and to assess the phylogenetic relationships within the genus, as well as within subtribe Cajaninae, using nuclear (ITS) and plastid (rpl32-trnL) gene regions.
A detailed taxonomic revision of the type section Rhynchosia resulted in the recognition of 47 species, with descriptions and an identification key that will assist in the correct identification of all taxa, as well as nomenclature, distribution, ecology, phenology, and conservation statuses of the species. Two new species were described (R. ngwenyii and R. waterbergensis), Eriosema rogersii and Rhynchosia rogersii are subsumed under R. angulosa as synonyms; R. harmsiana var. harmsiana is reduced to synonymy with R. caribaea var. caribaea, while Rhynchosia harmsiana var. burchellii is elevated to species level (R. burchellii). Rhynchosia argentea (Thunb.) Harv, a homonym of R. argentea Desv. from Angola, is also
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given a new name (Rhynchosia thunbergii) in this study. Correct typification of names is also provided for the species that have not been previously typified, with the rationale behind the choice of the lectotypes. Vegetative and reproductive morphological studies revealed significant variation in the reproductive and vegetative morphological characters between the species in the type section. The characters include leaflet size, type of indumentum on the leaflet surface, stipule shape, type and length of inflorescences, number of flowers, presence, or absence of indumentum on standard petal, presence, or absence of sculpturing on wing petal, as well as the length of the upper calyx lobes are useful in identifying species. The geographical distribution may be helpful to identify the species in the section.
Phylogenetic relationships within southern African Rhynchosia were investigated using molecular data from the nuclear (ITS) and plastid (rpl32-trnL) gene regions, to determine whether Baker’s sectional classification would be upheld, and to unravel the relationships between the genus and allied genera, namely; Eriosema and Bolusafra within the subtribe Cajaninae. Morphological characters were also reconstructed onto the trees to study the pattern of evolution of the characters within the genus.
The phylogenetic study revealed that the genus Rhynchosia is paraphyletic based on molecular data sets from the two DNA markers as the genera Eriosema and Bolusafra are embedded within the genus. Furthermore, the phylogenetic tree obtained did not support Baker’s sectional classification, as none of the sections was recovered as monophyletic in the phylogeny. Ancestral node reconstruction of selected morphological characters revealed that there are significant overlaps between the genera Bolusafra, Eriosema, and Rhynchosia. Hence there is no useful diagnostic apomorphy to separate the genera, which confirm close morphological similarities between them.
This study has provided the first taxonomic revision of the type section Rhynchosia since the last treatment by Baker (1923). Although there is clearly a need for a new infrageneric
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and intergeneric classification, there is a necessity for additional molecular data sets to properly resolve the phylogeny of the genus and allied species with a broader global representation of species.