Browsing by Author "Viljoen, Jan-Adriaan"

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    Biogeography of Isolepis subgenus Fluitantes (C.B. Clarke), Muasya (Cyperaceae): niche conservatism, long distance dispersal and hybridization
    (2010) Viljoen, Jan-Adriaan; Verboom, George Anthony; Cramer, Michael D
    Numerous lineages of the Western Cape of South Africa show affinities with the flora of tropical Africa and with Australasia. Recent work suggests that most migrations between the Western Cape and tropical Africa occur in a northward direction, and that connections between the flora of regions in the Southern Hemisphere are maintained by wind-assisted long-distance dispersal. The Fluitantes clade of lsolepis (Cyperaceae: Cypereae) is distributed throughout these areas and provides a useful study group to assess the general validity of published biogeographical trends. Furthermore, the cooccurrence of several closely related species in the Cape floristic region allows geographical and ecological patterns to be used for inference of speciation processes in the clade. Sequence data of the ITS and atpi-H gene regions were collected for 82 specimens; these were used to construct haplotype networks and phylogenies. By using the Tristan da Cunha endemics in the genus, as well as results from higher-level studies, a dated phylogeny for the Fluitantes clade was constructed and allowed for ancestral character state optimization of distributions by maximum likelihood. Ecological data were extracted from geographic information systems map to test for environmental differentiation in the Cape taxa. The Fluitantes were found to have originated in the Cape 7 million years ago. From there, they spread east and northwards onto the mountains of East Africa and to the islands of the Indian Ocean. Multiple dispersal events to Australia were recorded. Incongruence between the plastid and nuclear gene trees indicate hybridization to have taken place in Australasia, with possible subsequent speciation. Although the multivariate analysis found some ecological differentiation between the three Cape species, there was substantial overlap in all variables, and interpretation of habitat differences was difficult. It is suggested that, instead, differentiation may have taken place at the microhabitat scale, as I. rubicunda occupies low-lying sandy depressions, I. striata occurs at higher altitudes floating in water, and I. Judwigii inhabits the edges of streams and wetlands.
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    Distribution of leaf dissection and sclerophylly along microhabitat gradients in pteridophytes of the Cape peninsula
    (2010) Viljoen, Jan-Adriaan; Verboom, George Anthony; Cramer, Michael D
    Plant species with small leaves and sclerophylls have been reported to occur mainly on dry, low-nutrient soils in situations of high insolation. However; a number of physiological functions have been proposed for the two sets of traits. Ferns are well-suited to the study of leaf structure and its impact in different environments, as they show remarkable variation in leaf dissectedness and sclerophylly, and are able to inhabit a variety of marginal habitats. In this study, ecological and leaf trait data were collected for 17 fern species occurring in Skeleton Gorge, Table Mountain, in the Western Cape. Correlations between the traits and regressions of leaf traits on potential environmental determinants was carried out using both species averages and phylogenetically independent contrasts. The habitat and leaf traits were also subjected to a test of evolutionary trait conservatism. Sclerophyllous plants were found to be have thicker leaves, containing less chlorophyll, but sclerophylly was poorly correlated with leaf dissection. Plants occurring in high-light environments tended to be more sclerophyllous and have more dissected leaves, although these environments also were nutrient-poor. Leaf dissection appears to be primarily a means of dissipating heat by convection, rather than evaporative cooling, but it may also improve nutrient acquisition in low-nutrient soils. Sclerophylly in these ferns was not directly associated with nutrients; instead low sclerophylly seems to be favoured in low-light environments, perhaps because of lower metabolic costs or to reduce self-shading.
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    Ecological influences in the biogeography of the Austral sedges
    (2016) Viljoen, Jan-Adriaan; Muasya, A Muthama; Verboom, George Anthony
    The biogeographic history of a species is a result of both stochastic processes such as dispersal and habitat filters that determine where a population with a given set of biological requirements can become established. In this dissertation, I examine the geographical and ecological distribution of the sedge tribe Schoeneae in conjunction with its inferred speciation history in order to determine the pattern of dispersal and the environmental factors that have influenced establishment. The biogeographic reconstruction indicates numerous transoceanic dispersal events consistent with random diffusion from an Australian point of origin, but with a bias towards habitats with vegetation type and moisture regime similar to the ancestral conditions of the given subgroup (open and dry habitats in the majority of cases). The global distribution of the tribe also suggests a preference for low-nutrient soils, which I investigate at the local (microhabitat) scale by contrasting the distributions of the tribes Schoeneae and Cypereae on the Cape Peninsula along soil fertility axes. The relationships between the phenotypic traits of species and their soil nutrient levels are also examined to determine whether the coexistence of the two groups in the Cape can be attributed to differences in nutrient accumulation behaviour or strategy of biomass allocation to roots or structural organs vs. leaves. No robust patterns were observed to identify such adaptations or to distinguish the tribes ecologically, a result that is at least partly due to low statistical power in the data set collected, which constrains the analysis to the use of simple models less able to detect subtle patterns in the ecological history of these sedges.
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    Phylogenetic position of Cyperus clandestinus (Cypereae, Cyperaceae) clarified by morphological and molecular evidence
    (Wiley, 2014) Muthama, Muasya A; Viljoen, Jan-Adriaan; Dludlu, Meshack N; Demissew, Sebsebe
    Extreme morphological reduction and convergent evolution can obscure taxonomic relationships. Th is phenomenon is frequently encountered in Cyperaceae, where characters traditionally used to diagnose genera have been shown to have evolved independently multiple times. Th e Ethiopian high-altitude perennial fi rst described as Cyperus clandestinus was subsequently moved to Ficinia because it has spiral glume arrangement, unlike typical Cyperus species, which have distichous glume arrangement. However, this position has remained uncertain as no nutlets have previously been studied to establish the presence or absence of the gynophore – the synapomorphy for Ficinia. We resolve this 140-year-old puzzle by describing the morphology of the nutlet, which lacks a gynophore, and use DNA sequence data to resolve the taxon within Cyperus. Cyperus clandestinus was found to be closely related to Remirea maritima and Cyperus cyperoides in the C 4 Cyperus clade, whose members predominantly have distichously arranged glumes. Th is provides further evidence for the unreliability of glume arrangement as a character to distinguish between members of the Cyperus and Ficinia clades, whereas gynophore presence is more congruent with molecular data.
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    Radiation and Repeated Transoceanic Dispersal of Schoeneae (Cyperaceae) Through the Southern Hemisphere
    (Botanical Society of America, 2013) Viljoen, Jan-Adriaan; Muasya, A Muthama; Barrett, Russell; Bruhl, Jeremy; Gibbs, Adele; Slingsby, Jasper; Wilson, Karen; Verboom, Anthony
    • Premise of the study: The broad austral distribution of Schoeneae is almost certainly a product of long-distance dispersal. Owing to the inadequacies of existing phylogenetic data and a lack of rigorous biogeographic analysis, relationships within the tribe remain poorly resolved and its pattern of radiation and dispersal uncertain. We employed an expanded sampling of taxa and markers and a rigorous analytic approach to address these limitations. We evaluated the roles of geography and ecology in stimulating the initial radiation of the group and its subsequent dispersal across the southern hemisphere. • Methods: A dated tree was reconstructed using reversible-jump Markov chain Monte Carlo (MCMC) with a polytomy prior and molecular dating, applied to data from two nuclear and three cpDNA regions. Ancestral areas and habitats were inferred using dispersal–extinction–cladogenesis models. • Key results: Schoeneae originated in Australia in the Paleocene. The existence of a “hard” polytomy at the base of the clade reflects the rapid divergence of six principal lineages ca. 50 Ma, within Australia. From this ancestral area, Schoeneae have traversed the austral oceans with remarkable frequency, a total of 29 distinct dispersal events being reported here. Dispersal rates between landmasses are not explicable in terms of the geographical distances separating them. Transoceanic dispersal generally involved habitat stasis. • Conclusions: Although the role of dispersal in explaining global distribution patterns is now widely accepted, the apparent ease with which such dispersal may occur has perhaps been under-appreciated. In Schoeneae, transoceanic dispersal has been remarkably frequent, with ecological opportunity, rather than geography, being most important in dictating dispersal patterns.