Plants adapt their defenses to the local pest community
Local pest populations as an evolutionary force
The scientists demonstrate that the main chemical compounds produced by Arabidopsis thaliana in South-western Europe differ from those in North-eastern Europe. This pattern correlates directly with a shift in the composition of the aphid communities. In the second step, the researchers studied experimentally whether different aphid species could directly select for these different chemical compounds under controlled conditions. To this end, they exposed mixed populations of Arabidopsis thaliana to the cabbage and mustard aphid populations typical of North-eastern or South-western Europe. After five plant generations, continuous feeding by the different aphid species led to the selection of different chemical profiles, and these were consistent with the patterns seen in nature. “There is natural variation in chemical defenses which is under genetic control”, explains Züst “and this variation is maintained by geographic variation in the composition of aphid communities”. “Genetic variation is the raw material for evolution”, he continues, “so the maintenance of genetic diversity is essential if populations are to respond to future environmental changes such as climate change or environmental degradation”.
The costs of defense
In the control populations with no aphid feeding, some of the successful genotypes from aphid populations were lost. According to Turnbull, this occurred because defense mechanisms are costly for the plant and often come at the expense of growth: “Genetic diversity was only maintained across the different treatments; within each treatment much of the diversity was lost. In the control populations, this meant the loss of defended genotypes, as here investment in costly defenses brings no benefit to the plant”. Today, the genetic diversity of many plant species is being eroded. For example, agricultural plants are selected for rapid growth and maximum yield at the expense of natural defenses, making the use of pesticides inevitable. In future, the new findings could be used to develop customized seeds that are more resistant to specific local pest communities, thus limiting the use of pesticides.
Tobias Züst, Christian Heichinger, Ueli Grossniklaus, Richard Harrington, Daniel J. Kliebenstein, Lindsay Turnbull. Natural enemies drive geographic variation in plant defenses. Science. October 5, 2012, doi: 10.1126/science.1226397.