RNAi of selected insect genes

Thesis (MSc)--Stellenbosch University, 2016.ENGLISH ABSTRACT: Diuraphis noxia (Kurdjumov, Hemiptera: Aphididae), commonly known as the Russian wheat aphid (RWA), is regarded as one of the most destructive and widely distributed insect species in the world. Nonetheless, the currently available control strategies, including chemical pesticides, biological control agents, and RWA resistant wheat cultivars, are still very limited and rather ineffective. The process of double-stranded RNA (dsRNA)-mediated interference (RNAi) displays high specificity and the prospect of developing into a new specific method for managing agricultural pests. Plants can potentially be genetically engineered to express dsRNA to down-regulate vital gene functions present in pest insects, resulting in the protection of plants. In order to survive and reproduce, aphids require close interaction with their host plants, during which effectors are transported inside the plant to modify host cell processes. Four previously identified RWA salivary secretion proteins were investigated in the present study. However, cloning and sequencing results indicated that only two of the aforementioned proteins – C002 and 14-3-3 epsilon (ɛ) – could be potential protein elicitors in RWA. Thus, these two transcripts were subjected to RNAi experiments via artificial diet feeding and feeding on siRNA injected wheat leaf trials in order to investigate their role in RWA-host interactions and their importance in the survival and reproduction of the RWA. The relative expression levels of C002 and 14-3-3 ɛ at 0h were compared between SAM, the most virulent RWA biotype, and SA1, the least virulent RWA biotype in South Africa, and the results indicated that both transcripts had a higher relative expression in SAM than in SA1. Therefore, suggesting that C002 and 14-3-3 ɛ might play an important part in RWA virulence. From the RT-qPCR results it was evident that successful silencing of both C002 and 14-3-3 ɛ were achieved at 24h after initial siRNA exposure and that the transient silencing effect subsided thereafter. The expression data pertaining to the wheat leaf injection experiments, however, displayed high standard deviations that are not ideal and suggested that the expression of the transcripts differs greatly between the aphids within each group. This is likely due to the custom-made aphid cages and injection procedure of the siRNA into wheat leaves that appears to hinder the accuracy of the results. The fecundity data produced quite inconclusive results due to previously mentioned inadequacies and therefore an accurate and decisive conclusion cannot be drawn as to how the C002 and 14-3-3 ɛ silencing effects the survival and reproduction of the RWA. Both methods used for RNAi – the artificial diet trial and the injection of wheat leaves trial – have their drawback. After considering the RT-qPCR data, it appears as though the artificial diet trial produced more accurate and feasible results. Even so, the injection method establishes a more natural mode of feeding for the aphids and consequently more optimal cages need to be designed and tested to produce precise results.AFRIKAANS OPSOMMING: Russiese koringluis (RWA), is een van die mees vernietigende en wydverspreide insekspesies ter wêreld. Daarteenoor is die huigdige beskikbare beheer-strategieë, wat chemiese plaagdoders, biologiese beheermiddels, en RWA-weerstandige koringkultivars insluit, steeds baie beperk en grootendeels oneffektief. Die proses van dubbelstring-RNA (dsRNA)-gemedieerde inmenging (RNAi) toon hoë spesifisiteit en die vooruitsig om te ontwikkel in „n nuwe spesifieke metode vir die beheer van landboukundige peste. Plante kan potensieël genetiese engineerswese ondergaan om dsRNA uit te druk om die afregulering van noodsaaklike geenfunksies in insekpeste te bewerkstellig en sal lei tot die beskerming van plante. Vir koringluise om te oorleef en voort te plant, benodig hul nabye interaksie met hul gasheerplante waartydens effektore in die plant vervoer word om die gasheerselle se prosesse te verander. Vier voorheen geïndifiseerde RWA-speeksel sekresie-proteiëne is in die huidige studie ondersoek. Die resultate van die klonering en volgorderbepaling het egter getoon dat slegs twee van die proteiëne – C002 en 14-3-3 epsilon (ɛ) - moontlike proteiën-elisitore in die koringluis is. Dus is hierdie twee transkripte blootgestel aan RNAi-eksperimente, deur gebruik te maak van „n kunsmatige dieet benadering asook deur koringblare met siRNA in te spuit om ten einde vas te stel watter rol hul in die RWA-gasheer interaksies vertolk en hul noodsaaklikheid vir die oorlewing en voortplanting van die RWA uit te pluis. Die vlakke van relatiewe uitdrukking van C002 en 14-3-3 ɛ by 0h is vergelyk tussen SAM, die mees virulente RWA-biotipe, en SA1, die mins virulente RWA-biotipe in Suid-Afrika. Die resultate het gedui dat beide transkripte „n hoër relatiewe uitdrukking in SAM relatief tot SA1 het en dus voorspel dat C002 en 14-3-3 ɛ moontlik „n belangrike rol in RWA virulensie vertolk. Dit was duidelik vanuit die RT-qPCR-resultate dat suksesvolle onderdrukking van beide C002 en 14-3-3 ɛ bereik is 24h na aanvanklike blootstelling aan siRNA en dat die tydelike onderdrukking daarna afneem. Die uitdrukkingsdata wat verband hou met die inspuiting van die koringblaar-eksperimente het ongelukkig baie groot standaardafwykkings getoon, wat nie ideaal is nie. Die uitdrukking van die transkripte het dus baie tussen die individuele koringluise binne elke groep gevarieer. Dit is waarskynlik as gevolg van die self-gemaakte koringluishokke en die metode van inspuiting van die siRNA in die koringblare wat die akkuraatheid van die resultate verhinder. Die vrugbaarheidsdata wat versamel is, was redelik onbeduidend as gevolg van die voorafgenoemde tekortkominge en dus kan „n akkurate en beslissende gevolgtrekking rondom die effek van C002 en 14-3-3 ɛ-onderdrukking oor die oorlewing en voortplanting van die RWA nie gemaak word nie. Beide metodes wat ingespan is vir RNAi – die kunsmatige dieet en inspuiting van koringblare benadering – het hul tekortkominge. Na oorweging van die RT-qPCR data, blyk dit dat die kunsmatige dieet metode meer akkurate en haalbare resultate produseer. Daarteenoor vestig die inspuitingmetode „n meer natuurlike manier van voeding vir die koringluise en gevolglik moet meer optimale hokke ontwerp en getoets word wat sal lei to meer noukeuring resultate