A phylogenetic study of the genus Wiborgia (Crotalarieae, Fabaceae)

Abstract

Wiborgia Thunb. is a legume genus comprising ten species endemic to the Greater Cape Floristic Region of South Africa. Species in the genus are shrubs of 0.5–3.0m in height, with distinct ascending to erect habits and shoots terminating in rigid thorns, and are distinguished from other Crotalarieae by their indehiscent one-seeded fruits. The genus Wiborgia was revised by Dahlgren four decades ago. And to date, phylogenetic relationships within the genus are partially known, through studies that inferred phylogenetic relationships using two DNA markers and included seven out of ten Wiborgia species which provided unresolved species relationships. Therefore, this study aimed to further explore relationships within the genus using five fast-evolving DNA markers and sampling multiple accessions to represent morphological and ecological variability, enabling testing hypotheses on the utility of Dahlgren's key morphological characters and support for infrageneric classification. Phylogenetic relationships in Wiborgia were inferred using multiple molecular markers (ITS, rpl32-trnL, rps16, trnS-trnG, and trnT-trnL) and the data were analysed using model based approaches (Maximum Likelihood, Bayesian Inference), with key morphological characters reconstructed using Maximum Likelihood. Within the Wiborgia clade, two strongly supported clades were observed. In Clade 1, a novel, well-supported sister relationship between the widely-distributed W. mucronata and the narrowly distributed W. tenuifolia was observed, both characterized by thorny glabrous branches. In Clade 2, W. tetraptera was identified to be sister to W. fusca, whilst W. monoptera and W. incurvata formed part of a strongly supported clade. Wiborgia obcordata, the only species in Dahlgren's subgenus Wiborgia, was found to be embedded within subgenus Pterocarpia thus the subgeneric classification of Dahlgren was not supported. Key morphological characters (including inflated/compressed fruit walls, the presence of lateral wings, and the presence of thorns) were identified to derive independently across the phylogeny, and thus unsuitable to delineate subgenera. Our reconstructions have shown that these characters, together with other important characters such as branch pubescence and flower colour, could be used in delineating species. Sister pairs (W. incurvata and W. monoptera; W. tenuifolia and W. mucronata) overlap geographically but in most cases showed differences in floral characters (i.e. colour, morphology).