Natural selection drives chemical resistance ofDatura stramonium

Abstract

Plant resistance to herbivores involves physical and chemical plant traits that prevent or diminish damage by herbivores, and hence may promote coevolutionary arm-races between interacting species. AlthoughDatura stramonium’sconcentration of tropane alkaloids is under selection by leaf beetles, it is not known whether chemical defense reduces seed predation by the specialist weevil,Trichobaris soror, and if it is evolving by natural selection. We measured infestation byT. sororas well as the concentration of the plants’ two main tropane alkaloids in 278D. stramoniumplants belonging to 31 populations in central Mexico. We assessed whether the seed predator exerted preferences on the levels of both alkaloids and whether they affect plant fitness. Results show great variation across populations in the concentration of scopolamine and atropine in both leaves and seeds of plants ofD. stramonium, as well as in the intensity of infestation and the proportion of infested fruits byT. soror. The concentration of scopolamine in seeds and leaves are negatively associated across populations. We found that scopolamine concentration increases plant fitness. Our major finding was the detection of a positive relationship between the population average concentrations of scopolamine with the selection differentials of scopolamine. Such spatial variation in the direction and intensity of selection on scopolamine may represent a coevolutionary selective mosaic. Our results support the view that variation in the concentration of scopolamine among-populations ofD. stramoniumin central Mexico is being driven, in part, by selection exerted byT. soror, pointing an adaptive role of tropane alkaloids in this plant species.