Doctoral Degrees (Genetics)

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Browsing Doctoral Degrees (Genetics) by Subject "Apples -- Breeding"

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    Defining the QTL for chill requirement during dormancy and dormancy release in apple (Malus x domestica Borkh.).
    (Stellenbosch : Stellenbosch University, 2021-12) Cornelissen, Stephanie; Burger, Johan T.; Rees, Jasper; Stellenbosch University. Faculty of AgriSciences. Dept. of Genetics.
    ENGLISH ABSTRACT: Dormancy is a physiological stage that all deciduous fruit trees experience during winter. It consists of three sub stages, para-, endo- and ecodormancy. During the endodormic stage, apple trees need to be exposed to cold temperatures for their chill requirement to be fulfilled. Most apple varieties have a unique chill requirement, for instance ‘Anna’ requires less than 300 hours. After this requirement is fulfilled, the plant will break dormancy if the environmental conditions are optimal, i.e. the less the chill requirement the earlier the plant will bud. This study looks at the genetic component of chill requirement by firstly determining the region of the apple genome that is primarily associated with budbreak and thereafter the genes within this region and their expression patterns. A mapping population was created by crossing ‘Lady Williams’ with ‘Anna’. ‘Lady Williams’ is a medium to high chill variety and ‘Anna’ is a low chill variety that requires less than 300 chill units. This population was used to generate a genetic map and subsequent QTL analysis, as well as association mapping, were used to determine the region of the apple genome that is associated with budbreak. A major QTL in the first 10Mb distal region of Linkage Group/chromosome 9 was discovered. This region consists of 741 genes, of which 27 were differentially expressed over time when exposed to cold. An RNA time series analysis was performed by periodically extracting RNA from the meristems of cuttings from ‘Anna’, incubated at 4°C for up to 800 hours. The expression patterns of the 27 genes grouped into seven distinct clusters. The major observation that could be made is that there is a difference between the expression patterns before and after the theoretical time of budbreak (300 hours) in ‘Anna’. The 27 genes were differentially expressed before budbreak but there was little to no differential expression after budbreak. Another observation that was made, is that the biggest differences in gene expression were between 0-hours and a 100-hours. This indicated that the plant could have experienced cold shock by transferring it to low temperature incubation and subsequently the onset of endodormancy. Known dormancy-associated DAM genes, were not differentially expressed over time. However, an ortholog of the predicted transcription factor ICE1-like, bHLH, and the gene RING domain ligase 2, that have been shown to be associated with the regulation of ICE1-like, was differentially expressed over time when first introduced to cold. Furthermore, genes regulating the expression of the plant hormones ethylene and auxin were differentially expressed. Other notable genes include ones that encode Tubulin alpha-2 chain and xyloglucan endotransglucosylase/hydrolase 9 proteins, both known to regulate transport through cell wall/membrane, and 5'-adenylyl phosphosulfate reductase 2 that is associated with oxidation-reduction homeostasis. This study serves as a starting point for further investigations of the complex nature of the onset of dormancy, chill accumulation during dormancy and the eventual budbreak, by identifying genes that are associated with dormancy.