Browsing by Author "Bergh, Nicola G"

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    Aspects of the prevention of light damage during drying and rehydration of the desiccation-tolerant grass Eragrostis nindensis
    (1998) Bergh, Nicola G; Farrant, Jill M
    There are two main mechanisms of desiccation-tolerance in angiosperms. Both of these (poikilochlorophylly and homoiochlorophylly) involve adaptations to prevent light-induced damage as the plants dry and rehydrate. The poikiolchlorophyllous grass E. nindensis was investigated to determine physiological responses to light during drying, and mechanisms of tolerance of dehydration. The desiccation-sensitive E. curvula was investigated simultaneously as a control in order to compare responses of tolerant and sensitive relatives. Quantum efficiency of photosystem II was determined using chlorophyll fluorescence parameters and levels of photosynthetic pigments (chlorophylls and carotenoids) and of anthocyanins were measured. Electrolyte leakage of drying and rehydrating leaves was monitored to determine the extent of damage to membranes. Quantum efficiency and photosynthetic pigment contents were reduced in both plant§ dehydrated to <2% RWC; only E. nindensis recovered to initial levels. Both plants accumulated anthocyanins but these reached greater levels in E. nindensis and were found on the entire length of the leaf. On rehydration, E. nindensis lost the anthocyanins as it reconstituted chlorophylls. Neither species showed marked increases in electrolyte leakage but E. curvula did not recover on rewatering.
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    Comparative water relations of indigenous and invasive Australian Proteaceae in fynbos
    (1998) Bergh, Nicola G; Midgley, Jeremy J
    Water-use efficiency (WUE) as reflected in the leaf carbon stable isotope ratio was compared between co-occurring indigenous fynbos proteoids and invasive Australian hakeas H. sericea, H. gibbosa and H. suaveolens. At the driest site, H. suaveolens was slightly more WUE than several co-occurring proteoids; there was no significant difference between hakeas and proteas at the other sites. Transpiration rates of shoots and of whole trees were compared between Hakea sericea and Protea repens growing on Stellenboschberg northeast of Cape Town. Both measurements showed no real difference between the species and it is concluded that differences in water relations are not responsible for the highly competitive growth rates of hakeas in fynbos. It is hypothesised that hakeas may be able to vegetatively outcompete proteoids as a consequence of monopodial architecture and some ability to prevent shade-limitation of photosynthesis. A rough estimate of water loss due to transpiration and interception by H. sericea stands indicates that this species may have a significant effect on catchment water loss relative to open-canopy proteoid fynbos. This effect would be due not to transpiration rates of individual trees but to consistently high densities of mature hakea stands.
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    Erosive processes after tectonic uplift stimulate vicariant and adaptive speciation: evolution in an Afrotemperate-endemic paper daisy genus
    (BioMed Central, 2014-02-13) Bentley, Joanne; Verboom, G A; Bergh, Nicola G
    Background: The role of tectonic uplift in stimulating speciation in South Africa’s only alpine zone, the Drakensberg, has not been explicitly examined. Tectonic processes may influence speciation both through the creation of novel habitats and by physically isolating plant populations. We use the Afrotemperate endemic daisy genus Macowania to explore the timing and mode (geographic versus adaptive) of speciation in this region. Between sister species pairs we expect high morphological divergence where speciation has happened in sympatry (adaptive) while with geographic (vicariant) speciation we may expect to find less morphological divergence and a greater degree of allopatry. A dated molecular phylogenetic hypothesis for Macowania elucidates species’ relationships and is used to address the potential impact of uplift on diversification. Morphological divergence of a small sample of reproductive and vegetative characters, used as a proxy for adaptive divergence, is measured against species’ range distributions to estimate mode of speciation across two subclades in the genus. Results: The Macowania crown age is consistent with the hypothesis of post-uplift diversification, and we find evidence for both vicariant and adaptive speciation between the two subclades within Macowania. Both subclades exhibit strong signals of range allopatry, suggesting that geographic isolation was important in speciation. One subclade, associated with dry, rocky environments at high altitudes, shows very little morphological and ecological differentiation but high range allopatry. The other subclade occupies a greater variety of habitats and exhibits far greater morphological differentiation, but contains species with overlapping distribution ranges. Conclusions: Species in Macowania are likely to have diversified in response to tectonic uplift, and we invoke uplift and uplift-mediated erosion as the main drivers of speciation. The greater relative morphological divergence in sympatric species of Macowania indicates that speciation in the non-sympatric taxa may not have required obvious adaptive differences, implying that simple geographic isolation was the driving force for speciation (‘neutral speciation’).
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    The evolution of annuality in association with a shift to more arid environments in the daisy genera Ifloga and Tricogyne
    (2007) Trisos, Christopher; Bergh, Nicola G; Verboom, George Anthony
    An annual life history is often viewed as a model adaptation to arid environments. Annuality is predicted to have evolved in response to low adult survival and high seedling survival. In this study I evaluated the idea that increases in aridity should be associated with the evolution of an annual life history. I also investigated the correlated evolution of annuality and growth form. Ancestral character states for life history characters and climate variables were mapped onto a molecular phylogeny (obtained using plastid trnL-trnF and psbA-trnH and nuclear ETS sequences) of the genera Trichogyne and Ifloga (Asteraceae). Bayesian methods were used for phylogeny inference and maximum likelihood methods for ancestral state reconstructions. Only two phylogenetically independent contrasts were obtained and so the association between changes from annuality to perenniality and increases in aridity along branches of the tree were recorded and evaluated using Fisher's exact test. In order to account for ancestral character state reconstruction uncertainty, four different possible scenarios suggested by the maximum likelihood methods for the evolution of annuality were examined. This is the first molecular phylogeny of the group. Bayesian analysis of the sequence data places the Trichogyne+ Ifloga clade within the Gnaphileae. The genus Ifloga is shown to be paraphyletic. Trichogyne ambigua, as currently described, is polyphyletic and may contain two species. The origin of the Trichogyne+ Ifloga clade is within southern Africa and a northwards migration via the arid corridor is suggested to explain the disjunct distribution of the two Northern Hemisphere species. There is an association between the duration of the moisture growing season and the evolution of annuality. This is consistent with the idea that annuality is favoured by long drought periods making perennation difficult. The evolution of annuality was correlated with a non-woody, tufted, growth form. Amphi-basicarpy was discovered for T. polycnemoides, making it only the second known example of this reproductive strategy within Asteraceae.
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    The evolution of the Afrotemperate-endemic genus Macowania (Asteraceae) in the Drakensberg region of South Africa
    (2010) Bentley, Joanne; Verboom, George Anthony; Bergh, Nicola G
    The cosmopolitan Asteraceae tribe Gnaphalieae, or paper daisies or everlastings, form a significant component of both the dry and cool temperate floras of southern Africa. Within this tribe exists a small Afrotemperate genus, Macowania, endemic to the grassland biome of South Africa and occurring almost exclusively within the Drakensberg region, apart from two disjunct species in North Africa. The age, relationships and geographic origin of Macowania is investigated in order to provide insight into the factors affecting speciation, especially uplift events, on this small Afrotemperate genus. A well-supported phylogenetic hypothesis based on both nuclear and chloroplast genes suggests that Macowania is sister to a clade corresponding to the Relhania clade s.s., and that these are in turn sister to a clade containing the genera Athrixia and Pentatrichia. Macowania is monophyletic only with the inclusion of the enigmatic monotypic genus Arrowsmithia, resulting in the future synonymy of Macowania with Arrowsmithia. The anomalous species M. pinifolia, previously part of the genus Athrixia, is placed in a polytomy with the Relhania s.s. clade and the remaining species of Macowania and Arrowsmithia. DNA sequence data could not be obtained for several Macowania species, including the taxa from North Africa. The placement of these species within Macowania is confirmed by means of a parsimony analysis of morphological characters against a molecular backbone constraint tree. One species, M. tenuifolia, is well-supported in two different placements within Macowania by chloroplast and nuclear DNA sequence data. The best position of this species is inferred by incongruence decomposition analysis and morphological affinities. Bayesian relaxed clock methods and ancestral area reconstruction using maximum likelihood and squared change parsimony estimate the age and ancestral area of the genus, and determine the timing and route of colonisation of the Drakensberg. Diversification within Macowania is consistent in timing with the uplift events during the Miocene and Pliocene that resulted in significant vertical movement in eastern South Africa, suggesting that colonisation of the high-elevation Drakensberg grassland by Macowania was promoted by uplift. The topographic heterogeneity and increased river action resulting from the uplift may also have promoted evolution into new habitats and potentially mediated the movement of the ancestor of Macowania into the Drakensberg region via riparian habitats.
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    Topography as a determinant of range extent and overlap : a species level phylogenetic reconstruction and geographical range analysis of Syncarpha (Asteraceae)
    (2011) Haiden, Sarah; Verboom, George Anthony; Bergh, Nicola G
    Understanding what determines species' geographic range extents has several implications for questions in ecology, evolution and conservation biology. The Cape Floristic Region of South Africa is noted for its remarkably high geographic species turnover, often attributed to the exceptional environmental heterogeneity of the region. The complex and highly dissected topography of the CFR provides a model environment in which to investigate the relationship between altitude and species range extent, as well as explore the role of topography in speciation and current range overlap. I examined these questions in the context of Syncarpha, a genus within the Asteraceae tribe Gnaphalieae (paper daisies). A Bayesian analysis of combined plastid and nuclear genes provided the robust, dated phylogenetic hypothesis required to assess the monophyly of the genus, as well as reconstruct the signal of geographic speciation within the lineage. The phylogeny recovered Syncarpha as polyphyletic, comprising two clades with good support, placing the small CFR-endemic genus Edmondia as sister to the larger Syncarpha clade. Using realised range extent estimates and modelled potential distributions of Syncarpha and Edmondia species, this study confirms the importance of topography as a factor constraining species' distributions, and thereby enhancing the scope for their allopatric isolation. The relationship between altitude and realised range extent was found to be unimodal, with ranges being restricted at both high- and low-altitudes, and more extensive at intermediate altitudes. Range filling (the ratio between realised and potential range extent) was also lower in high- and low-altitude taxa compared to mid-altitude taxa. Dispersal limitation, owing to the insular nature of montane habitats, seems the most likely mechanism to restrict the ranges of high-altitude taxa, whereas edaphic factors are more likely responsible for the restricted ranges of low-altitude taxa. Furthermore, age-range correlations confirm the role of altitude in maintaining a stronger signal of allopatry among recently diverged clades, where montane clades present lower levels of range overlap than those at intermediate altitudes. Thus, the role of topography in limiting dispersal, and hence constraining species distributions, has consequences for understanding the historical diversification of a lineage, as well as implications for management practices in light of climate change-induced range shifts.