Understanding aphids: transcriptomics, molecular evolution and pest control

Abstract

Aphids exhibit fascinating biological features including parthenogenesis, symbiosis, altruism and host-plant preference; all of which would be better understood if genetic tools and molecular biological techniques were applied to them. Aphids are also agricultural pests that vector plant viruses and new approaches to control them are required. This thesis addresses questions motivated by an interest in the biology of aphids and a desire to improve the agricultural impact of aphids. It does so through transcriptomic analyses and RNA interference (RNAi) technology.

I examined the ways in which the transcriptome of aphid changes with host-plant, between tissues, within species and between species. The three-aphid species studied (the green peach aphid: Myzus persicae, the mustard aphid: Lipaphis erysimi, and the cabbage aphid: Brevicoryne brassicae) are all pests of economically important brassica crops (such as cabbage, cauliflower, mustard and canola). These data may provide insights into the way different aphid species deal with plant secondary compounds such as glucosinolates. These data also allowed me to examine the structure, function and evolution of myrosinase enzymes that have allowed some aphid species to develop an anti-predator ‘mustard bomb’.

RNAi has been suggested as a way to specifically target pest that would be more ‘environmentally friendly’ than conventional insecticides. I experimentally assessed the feasibility of orally-delivered RNAi to control aphids and the potential of this technology to be developed as a functional genomic tool. RNAi was fed to aphids via artificial diets at various concentrations and with various delivery agents and via transgenic Arabidopsis thaliana plants that I created that produced dsRNA’s corresponding to aphid genes. These studies lead me to suggest that more work needs to be done to limit the effects of RNase enzymes of the aphid gut digesting orally delivered RNAi and to more carefully characterize factors that may affect within-species variation in RNAi efficacy.