Masters Degrees (Microbiology)

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Browsing Masters Degrees (Microbiology) by Author "Asmus, Justin Joseph"

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    The occurrence and physiology of Hyphopichia heimii, associated with beetle frass from Olea europaea subsp. cuspidata
    (Stellenbosch : Stellenbosch University, 2020-03) Asmus, Justin Joseph; Botha, Alfred; Roets, Francois; Stellenbosch University. Faculty of Science. Dept. of Microbiology.
    ENGLISH ABSTRACT: Bark beetles are destructive insect pests that pose a threat to natural forest and orchard ecosystems. It is well known that the ecological success of these insects depends on their symbioses with a range of microorganisms including bacteria and filamentous fungi. Recently, it was demonstrated that unicellular fungi, called yeasts, may also form symbioses with bark beetles. Several studies revealed that bark beetles, such as Dendroctonus frontalis and Dendroctonus ponderosae, are intimately associated with yeasts throughout their life cycles. It was suggested that these unicellular fungi may play vital roles in beetle nutrition, detoxification of host tree chemicals and may possibly influence attack behaviour of these insects. Nevertheless, few studies have attempted to investigate the functional roles of yeasts associated with bark beetles. Studies on the yeast symbionts of beetles could provide a better understanding of the symbioses that exist between trees, bark beetles and their associated microbial symbionts. This study was aimed at investigating the diversity of yeasts, culturable on a benomyl-containing selective medium, that occur in beetle frass originating from wild olive trees growing at several locations in Stellenbosch, South Africa. The yeasts able to grow on the isolation medium represented 28 species, of which 29% were ascomycetous species, while the majority (71%) were basidiomycetous species. The most dominant yeasts appearing frequently on the isolation plates prepared from the frass were the basidiomycetes Colacogloea philyla and an undescribed species, Cryptococcus sp. 3, as well as the rare ascomycetous yeast, Hyphopichia heimii. Using enzyme plate assays, it was found that the basidiomycetous yeast species, as opposed to the representatives of H. heimii, frequently demonstrated wood degrading abilities, by showing hemicellulosic enzymatic activity on the plates. All representatives of H. heimii, C. philyla and Cryptococcus sp. 3 however, were able to assimilate simple carbon sources (e.g. sucrose and cellobiose) that may occur within frass or vascular tissues of the olive trees. It was also found that the representatives of H. heimii and C. philyla were able to produce the plant growth-promoting factor indole acetic acid (IAA), with the highest producer being H. heimii CAB 1593 (3.062 ± 0.250 μg/log CFU) which indicated that these yeasts could be of benefit for their tree hosts. Sterol analysis of the dominant yeasts in the frass indicated that ascomycetes and basidiomycetes produced comparable quantities of ergosterol ranging from 1.365 ± 0.118 to 24.569 ± 0.950 mg/g dry cell weight and were therefore similar with regard to their potential to fulfil the dietary sterol requirements of bark beetles. In addition, it was found that H. heimii was able to convert cis-verbenol to verbenone and may therefore potentially affect beetle behaviour. To conclude, this study has provided tentative evidence of a tripartite symbiosis between the ascomycetous yeast, H. heimii, olive trees and bark beetles.