Systematics and diversification of the genus Aspalathus (Crotalarieae, Leguminosae)

Madika, Lydia Khumo

Systematics and diversification of the genus Aspalathus (Crotalarieae, Leguminosae)

Thesis / Dissertation

2026

Publisher

University of Cape Town

Department

Department of Biological Sciences

Faculty

Faculty of Science

Abstract

The genus Aspalathus L. (Leguminosae: Crotalarieae) exemplifies many of the traits associated with Cape radiations. With nearly 300 described species, and likely many more yet undescribed, Aspalathus is the second-largest genus within the Cape Floristic Region (CFR) and comprises one of the most taxonomically complex groups in the fynbos. Approximately 98 percent of its species are endemic to the region, and many display striking morphological adaptations to their respective microhabitats, including varied leaf structures, spinescence, flower pigmentation, and fire response strategies. Species of Aspalathus are often adapted to disturbance, especially wildfire, with many exhibiting reseeder life forms—a strategy that includes rapid post-fire germination and growth. Furthermore, like many legumes, they contribute to ecosystem functioning through symbiotic nitrogen fixation, thus playing a critical role in nutrient cycling. Beyond its ecological prominence, the genus has substantial economic relevance through Aspalathus linearis (Burm.f.) R.Dahlgren, the plant used to produce rooibos tea, a globally traded herbal infusion that supports an entire agricultural industry in South Africa. Taxonomically, the genus has posed longstanding challenges. The foundational classification was laid down over 40 years ago by botanist Rolf Dahlgren. Although pioneering, Dahlgren's work preceded the advent of molecular systematics, which now provide much higher resolution tools for inferring evolutionary relationships. Recent decades have seen the discovery and description of over 20 new species of Aspalathus following Dahlgren's treatment. However, efforts to reconstruct the evolutionary tree of Aspalathus using standard DNA barcodes have yielded low-resolution phylogenies, often failing to resolve deeper nodes. These limitations underscore the urgent need for genomic-scale datasets and a reappraisal of existing taxonomic frameworks. This thesis is organized around three core aims: (1) to resolve long-standing phylogenetic uncertainty within Aspalathus, and to test the monophyly of the genus and utility of taxonomic characters, using Sanger sequencing data; (2) to infer backbone topology and infrageneric classification, and to investigate determinants of diversification patterns, using phylogenomic data; and (3) to revise the taxonomy of Aspalathus subgen. Sericea, a morphologically and ecologically distinct subgroup, using wealth of specimens and morphometric analyses. The study is structured into three data chapters aligned with these aims, preceded by a general introduction (Chapter 1) and concluded with a synthesis (Chapter 5) that integrates key findings. Chapter 2 investigates the evolutionary cohesiveness of the genus Aspalathus using Sanger sequencing of four widely used genetic markers: ETS, ITS, matK, and trnS–trnG. A central question was whether Aspalathus, as currently circumscribed, forms a monophyletic group. Phylogenetic analyses, sampling 137 taxa and data analysed using Bayesian methods, produced a polytomy within the Aspalathus clade. Despite the low resolution, two major clades emerged within the polytomy, each showing a closer relationship to the clade containing the two genera Wiborgia Kuntze and Wiborgiella Boatwr. & B.-E.van Wyk, thus indicating that the genus Aspalathus, as traditionally defined, may not be monophyletic. Morphological assessments support this molecular signal: the two clades differ in several key traits, including leaf morphology, presence or absence of petioles, and leaflet shape, reinforcing the hypothesis of a deep evolutionary divergence within the genus. To overcome the limitations of low-resolution Sanger data, Chapter 3 employs the Angiosperms353 target capture sequencing, a cutting-edge phylogenomic method that samples hundreds of genes across the nuclear genome. This broader and more detailed dataset enables the construction of a well-resolved backbone phylogeny for the genus and confirms Aspalathus as being monophyletic. Analyses recover two strongly supported major clades in Aspalathus, designated here as Clade A and Clade B. Clade A includes the majority of species formerly assigned to Sericeae, while Clade B contains the remainder of the genus and is resolved into eight well-supported subclades. These subclades integrate multiple of Dahlgren's traditional groups: Subclade 1 unites Laterales (group 34), Calcicolae (33), Niveae (24), and two species of Pingues (22); Subclade 2 encompass Pingues (22); Subclade 3 links Digitifoliae (31) with the monotypic Vermiculatae; Subclades 4 and 5 together encompass Teretilobae (32), though resolved into two distinct molecular lineages; Subclade 6 is taxonomically complex and polyphyletic, including elements of Callosae (17), Carnosae (18), Cephalanthae (3), Crotalariiformes (4), Gigantes (15), and some Sericeae (1); Subclade 7 comprises the largest radiation, subdivided into Subclade 7A—which includes Pedunculares (6), Borboniae (7), Filicaules (8), Rostratae (9), Rubescentes (10), Adnates (11), Lepthanthae (12), Ternatae (13), Purpureae (14), and two Pingues species (22)—and Subclade 7B, which corresponds to the morphologically cohesive Terminales (29); and Subclade 8 unites Sulphureae (19) with Pachycarpae (21). The depth of divergence and the congruence between molecular and morphological data provide compelling evidence for a taxonomic reclassification. Consequently, this study formally proposes the division of the genus into two subgenera: Aspalathus subgen. Aspalathus and Aspalathus subgen. Sericea. This infrageneric classification confirms both evolutionary history and morphological distinctiveness. Divergence dating indicates that the crown age of Aspalathus is approximately 14.6 million years, with subsequent rapid radiation during the Miocene, coinciding with the expansion and ecological specialization of the fynbos biome. Chapter 4 presents a comprehensive taxonomic revision of Aspalathus subgen. Sericea (Clade A), with the overarching aim of clarifying species and subspecies boundaries, standardizing nomenclature, and providing an updated framework for identification and conservation assessment. The revision is based on extensive herbarium material, verified field observations, and quantitative morphometric analyses, with selected micromorphological traits examined in detail. Data on habit, vegetative and floral morphology, life history traits, and habitat preferences were compiled and analyzed using phenetic and multivariate approaches, allowing for the assessment of inter- and intraspecific variation, updated distribution mapping, and preliminary conservation evaluations. Key findings include the recognition of a morphologically coherent subgenus defined by silvery-sericeous indumentum, trifoliolate leaves with reduced lateral leaflets, and distinctive inflorescence and calyx architecture. The revision refines and expands upon Dahlgren's treatment by integrating quantitative analyses, ecological niche data, lecto-typification of historically ambiguous names, and updated distributional records. Several taxonomic adjustments were made, including the elevation of subspecies to species rank (e.g., A. virgata, A. staurantha, A. lagopus), while historically synonymized taxa such as A. elongata var. densa are highlighted as candidates for potential future reinstatement, pending further material and analysis, and the provisional retention of unresolved species pairs (e.g., A. rotunda–A. fragilis). While Dahlgren's core species groups are broadly supported, this study resolves finer infraspecific limits, identifies previously overlooked narrow endemics, and delivers a modern framework for both systematic research and conservation planning in the Cape Floristic Region. In conclusion, this thesis delivers the most comprehensive and integrative study of Aspalathus to date. By combining traditional morphological analysis with cutting-edge phylogenomic tools, it resolves longstanding ambiguities in the genus's classification and provides a robust evolutionary framework for understanding its diversification. The clear separation of Aspalathus into two major clades, now proposed as subgenera, is supported by strong genetic evidence and morphological coherence. This reclassification not only addresses taxonomic confusion but also enhances our ability to interpret evolutionary processes across the group. Importantly, the timing and context of diversification suggest that the Cape Floristic Region's complex environmental mosaic with its fire-adapted ecosystems, heterogeneous soils, and shifting paleoclimates played a central role in shaping the genus's diversity. Rather than being driven solely by aridification, Aspalathus radiated in response to fine-scale ecological opportunities, from edaphic niches to pollination strategies. Therefore, this work contributes not only to the systematics of an ecologically and economically important genus, but also to a broader understanding of how biodiversity emerges and radiates in global hotspots. It provides a model for integrative, genome-informed taxonomy that can be applied to other complex and species-rich plant groups facing similar classification challenges.

Keywords

Aspalathus

systematics

Reference:

Collections

PhD / Doctoral

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