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Doctor of Philosophy




Grbic, Vojislava


The two-spotted spider mite Tetranychus urticae Koch is a key agricultural pest that causes significant yield losses in a wide range of economically important crops. Rapid development of resistance to several classes of pesticides in T. urticae necessitates introduction of alternative management strategies to control this pest. Indole glucosinolates (IGs) are secondary metabolites found in Brassicaceae plants (including Arabidopsis thaliana) that have been shown to be effective against T. urticae and could be potential candidates to control spider mites. However, a laboratory population selected on IG-containing Arabidopsis was able to evolve adaptation to this plant. The overall objective of this thesis was to identify the mechanism of adaptation of two-spotted spider mites to Arabidopsis and IGs. Similar expression of marker genes and levels of plant defense-related metabolites after feeding of IG-adapted and non-adapted adult spider mites indicated that plant suppression is not the strategy used by spider mites to adapt to Arabidopsis and IGs. On the other hand, higher activity of P450 monooxygenases in IG-adapted mites and the negative effect of inhibitors of these detoxification enzymes on fecundity of adapted spider mites suggested that spider mites use detoxification to overcome the effect of IGs. HPLC-mediated detection of conjugates of IG breakdown products in adapted compared to non-adapted mites supports the involvement of detoxification in the adaptation of T. urticae to IGs. In addition, RNA-seq analysis showed induction of detoxification enzyme genes upon mite feeding on IGs. Upregulation of genes associated with growth, development and fecundity in adapted spider mites suggests that T. urticae neutralizes the negative effect of IGs. Genes that were differentially upregulated in adapted compared to non-adapted spider mites likely capture gene sets associated with the adaptation to IGs, suggesting that these genes can be further used in manipulation of T. urticae to avoid development of adaptation or to succumb it.

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