Phytophagous insects may impose natural selection on plants, which favors resistant plant genotypes and drives the evolutionary diversification of plant species. However, the rapid evolution of plant traits that confer resistance to herbivores (such as aphids) is not for free. As assessed in Science by Züst et al. the plant resistant genotypes are favored when the probability of insect damage is high, but these genotypes pay a cost for resistance that make them disfavored when the probability of herbivore attack is low.
In particular, Züst et al worked on Arabidopsis thaliana that has available unparalleled genetic and molecular resources making this plant an ideal candidate to study the processes of local adaptation to herbivores. As herbivores, the studied the aphid species Brevicoryne brassicae (in the Peter J. Bryant.’s photo) and Lipaphis erysimi as drivers of these patterns as they are both abundant, mobile specialists of Brassicaceae.
Comparing infected and not infected A. thaliana plants, they observed that a rapid adaptation occurred in the selection experiment, as evidenced by a progressive reduction in the effects of aphid feeding on final plant biomass in each of the five observed generations. Interestingly in the absence of aphids, all plants in the selection experiment experienced strong intraspecific competition, and because growth rate is a good predictor of competitive ability, it is hs been not surprising that fast-growing plant genotypes were generally selected, whereas the slowest-growing ones were lost.
As a whole, Züst et al assessed therefore that there is a trade-off between plant resistance to phytophagous insects and the ability to compete with other plants. As wrote by Züst et al in their conclusions: ”Ecological theory has consistently emphasized the role of natural enemies in maintaining diversity both within and among species, but convincing empirical evidence has been lacking. Here, we demonstrate that even functionally similar herbivores such as different species of aphid have the potential to select for specific chemotypes and drive large-scale geographic patterns in plant defense profiles. It therefore seems likely that natural herbivore communities, with their greater variety of feeding styles and specializations, play a major role in shaping and refining the plant defenses observed in natural communities.”
- J.D. Hare (2012). How Insect Herbivores Drive the Evolution of Plants. Science, 338, 50-51.
- T. Züst, C. Heichinger, U. Grossniklaus, R. Harrington, D.J. Kliebenstein, L.A. Turnbull1 (2012). Natural Enemies Drive Geographic Variation in Plant Defenses Science, 338, 116-119.