Browsing by Author "Du Toit, Nina"

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    Comparative phylogeography and phylogenetic relationships of the four-striped mouse genus, Rhabdomys, and the ectoparasitic sucking louse, Polyplax arvicanthis
    (Stellenbosch : Stellenbosch University, 2013-03) Du Toit, Nina; Matthee, Conrad A.; Jansen van Vuuren, Bettine; Matthee, Sonja; Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology.
    ENGLISH ABSTRACT: Within southern Africa, the widely distributed four-striped mouse Rhabdomys is parasitized by, amongst others, the host-specific ectoparasitic sucking louse, Polyplax arvicanthis. The present study investigated this parasite-host association from a phylogenetic and phylogeographic perspective utilizing mitochondrial and nuclear DNA markers. The findings support the existence of four species within Rhabdomys (three distinct lineages within the previously recognized arid-adapted R. pumilio and the mesic-adapted R. dilectus). These species have distinct geographic distributions across vegetational biomes with two documented areas of sympatry at biome boundaries. Ecological niche modelling supports a strong correlation between regional biomes and the distribution of distinct evolutionary lineages of Rhabdomys. A Bayesian relaxed molecular clock suggests that cladogenesis within the genus coincides with paleoclimatic changes (and the establishment of the biomes) at the Miocene-Pliocene boundary. Strong evidence was also found that the sucking louse P. arvicanthis consists of two genetically divergent lineages, which probably represent distinct species. The two lineages have sympatric distributions throughout most of the sampled range across the various host species and also occasionally occur sympatrically on the same host individual. Further, the absence of clear morphological differences among these parasitic lineages suggests cryptic speciation. Limited phylogeographic congruence was observed among the two P. arvicanthis lineages and the various Rhabdomys species and co-phylogenetic analyses indicated limited co-divergence with several episodes of host-switching, despite the documented host-specificity and several other traits predicted to favour congruence and co-divergence. Also, despite the comparatively smaller effective population sizes and elevated mutational rates found for P. arvicanthis, spatial genetic structure was not more pronounced in the parasite lineages compared to the hosts. These findings may be partly attributed to high vagility and social behaviour of Rhabdomys, which probably promoted parasite dispersal among hosts through frequent inter-host contact. Further, the complex biogeographic history of Rhabdomys, which involved cyclic range contractions and expansions, may have facilitated parasite divergence during periods of host allopatry, and host-switching during periods of host sympatry. Intermittent contact among Rhabdomys lineages could also have prevented adaptation of P. arvicanthis to specific host lineages, thus explaining the lack of host-specificity observed in areas of host sympatry. It is thus evident that the association between Polyplax arvicanthis and Rhabdomys has been shaped by the synergistic effects of parasite traits, biogeography, and host-related factors over evolutionary time.
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    Quantifying the potential of almond production in the Western Cape for future extension and high-density plantings in suitable climatic regions
    (Stellenbosch : Stellenbosch University, 2022-04) Du Toit, Nina; Lotze, Elmi; Hoffmann, Willem H.; Stellenbosch University. Faculty of Agrisciences. Dept. of Horticulture.
    ENGLISH ABSTRACT: Almond (Prunus amygdalus Batsch) has become popular as an alternative crop with South African growers as the need for marginal crops in the Western Cape increases. Growers are also seeking more efficient and sustainable production methods that will reduce inputs to remain globally competitive. High-density plantings proved to be more profitable in commercial crops like peach and nectarine than traditional low-density plantings, but proper canopy management and manipulations are required to maintain the efficiency of these systems. The aim of this study was to assess different spacing and training configurations with respect to maximizing future reproductive potential, for two commercial cultivars. The canopy development and performance of two newly established super- high-density (SHD) (2049 trees.ha -1) training systems in the Robertson region were evaluated for the Soleta cultivar on ‘Garnem’ rootstock. The study focused on the relationship between light interception (LI) and potential yield efficiency, as the orchard was in a vegetative phase during the first two seasons. The Open-Vase (OV) system showed more vigorous growth patterns than Bi-Axis (BA), resulting in a significantly higher stem circumference (in all three seasons), shoot growth and canopy volume (CV) after summer pruning (2020/21) and LI during full bloom (2020/21;2021/22). The OV had a significantly higher trunk cross-sectional area (TCSA), which translated into the significantly higher yield efficiency in the BA system, in 2020/21. The OV had a significantly higher in-shell and kernel weight, -length and -width. Initial results indicated that both systems are suitable for ‘Soleta’ at SHD, but the BA system outperformed the OV system with regard to yield efficiency at the first commercial harvest (three years after establishment). The effect of two alternative rootstocks, ‘Viking’ and ‘Flordaguard’, on the vegetative development of ‘Nonpareil’, was evaluated in the Durbanville region. Rootstock did not influence the stem circumference or the average annual shoot growth during the two consecutive seasons. However, rootstock affected CV, after shoot growth cessation, and LI, after leaf drop. ‘Nonpareil’ trees on ‘Flordaguard’ had a significantly higher CV and LI than trees on ‘Viking’, which indicated that ‘Flordaguard’ resulted in more vigorous growth of the ‘Nonpareil’ scion than ‘Viking’. Almond cultivation at higher densities is a relatively new concept than the historical, traditional systems currently under production. A financial comparison was conducted on the performance of the Soleta (more compact, self-fertilising) and Nonpareil (vigorous, self-infertile) cultivars in the Durbanville, Robertson and Montagu regions, planted in different orchard systems, with varying planting densities and training systems. For each system, the net present value (NPV) and modified internal rate of return (MIRR) were calculated. The NPV at a discount rate of 5% and 10% indicate that the low-density system is preferred for both cultivars. A sensitivity analysis was performed on the year that full production is achieved, as well as on the orchard's life expectancy. With the exception of the MIRR, which increased by 1% for a two-year decrease in the life expectancy of low-density orchard systems, both factors had a negative impact on the NPV and a negative or no impact on the MIRR.
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    A study of Shoprites imported non-foods supply chain
    (Stellenbosch : Stellenbosch University, 2011-12) Du Toit, Nina; Von Leipzig, Konrad; Stellenbosch University. Faculty of Engineering. Dept. of Industrial Engineering.
    ENGLISH ABSTRACT: The Shoprite retail group, based in South Africa, imports over 90% of their non-food goods. Two routes for transporting imports to Johannesburg were studied, specifically, (1) cross-docking the goods through distribution centres in Cape Town and, (2) direct shipments of entire containers via the port of Durban. A literature study comprised of research on supply chain management, logistics and the purchasing supply chain function was conducted. The literature study described various supply chain techniques, such as centralised distribution, and summarised ways of measuring and improving supply chains. An analysis based on the company’s strategies and financial and other data was used to determine the best route. The cross-docking route via Cape Town was chosen as it is aligned with the current company strategies and aids the visual management system used. There is no significant cost saving when using the Cape Town route, eliminating the choice to base the routing decision solely on financial implications. A detailed analysis of Shoprite’s process of importing revealed several problems. The high demurrage cost was studied further and it was shown that a lack of control of the timing of the release of goods from the supplier causes container storage costs to rise. The suggestion of implementing a freight forwarder to monitor the consolidation and timely shipment of goods was made. The promotion of communication between the buyers, suppliers, replenishers and distribution centre management was further identified as a major stumbling block for improved efficiency. If rectified, it would enable the supply chain to be flexible and allow management of the supply chain as a whole, instead of management of individual parts. An important classification was that of the distribution centres as the bottleneck in the supply chain. The supply chain can be improved by managing the flow of imported goods according to the capacity of the distribution centres. The project achieved its aims of identifying a route that should be used to import non-food goods from the East and of assessing the supply chain to provide improvement strategies.