Insect host shifts are important because they may be a first step in the evolution of new species and can create new pests of agriculturally important crops. A barrier to many potential insect host shifts are the secondary metabolites or allelochemicals many plants produce in order to defend themselves from plant feeding (herbivorous) pests like aphids. Consequently, insect pests can only be successful in using such plant species as food if they develop mechanisms to overcome sensitivity to the defense compounds that a plant is releasing.
Identifying the initial genetic changes involved in this process has proved elusive. Rothamsted Research scientists, who receive strategic funding from the BBSRC, in collaboration with researchers from the Liverpool School of Tropical Medicine and Bayer CropScience AG in Germany, have characterised novel genetic changes that underlie an insect host shift and the emergence of a new subspecies of crop pest with natural resistant to pesticides. The study is published today in the journal Proceedings of the National Academy of Sciences (PNAS).
The peach potato aphid (Myzus persicae) is a major crop pest, one of the most common greenfly pests within the UK and globally. It can cause substantial crop yield losses through the transmission of up to 120 distinct plant viruses and/or directly through feeding and sucking the sap of plants. A subspecies of peach potato aphid (Myzus percicae nicotianae) has evolved to feed and survive on tobacco plants. Aphids of this subspecies show reduced sensitivity to the secondary metabolite nicotine, which is a potent natural insecticide produced by tobacco. Interestingly, these aphids also show reduced sensitivity to neonicotinoids a class of synthetic insecticides. This study aimed to identify how aphids manage to overcome the toxic effects of the tobacco-produced nicotine and understand how this relates to resistance to neonicotinoids.
Dr Chris Bass of Rothamsted Research, who is funded by a fellowship from the BBSRC, and led the study said: "We are excited that for the first time we have been able to characterise the genetic mutations involved in the initial steps of the host shift of the peach potato aphid to tobacco. We found that a detoxification enzyme called CYP6CY3, which is naturally present in all aphids, is responsible for the metabolism of nicotine to less toxic compounds. However, for this process to occur at significant levels that allow survival of aphids that feed on tobacco plants the gene producing this enzyme needs to be present in many more copies than the normal two copies, up to 100 copies in the most resistant aphids.
"In addition to the gene amplification we have also been able to show that changes in the part of the gene that gives information as to when and where the enzyme should be made (i.e. the regulatory region of CYP6CY3) contribute to the overexpression of the gene. Together these two mechanisms work in concert to produce high levels of the enzyme which breakdowns nicotine and has also pre-adapted tobacco-adapted races to resist man-made insecticides".
Professor Lin Field of Rothamsted Research said: "The findings of this study are very exciting because they provide novel insights into the fundamental evolutionary processes that have driven adaptation in an aphid and similar mechanisms may be employed by other insect species. Additionally, we now have further understanding of the molecular mechanisms that can drive insecticide resistance and this can be utilised when developing pest management strategies."