Evolutionary response to herbivory: population differentiation in microsatellite loci, tropane alkaloids and leaf trichome density in Datura stramonium

It is thought that natural selection exerted by herbivores on plants has promoted the evolution of plant traits that function as defence. However, such selective pressures may vary spatially differentiating populations in plant defence phenotypes. Yet, to ascertain the role of natural selection on phenotypic differentiation between populations, it is necessary to discard other evolutionary processes like genetic drift. Evolutionary biologists have designed approaches to determine whether population differentiation has been produced by natural selection in contrast to random processes as a null hypothesis. To accomplish this, we compare the magnitude of differentiation among populations in plant defence against herbivores (selection) and in neutral loci (genetic drift). Our study system is the plant Datura stramonium, whose anti-herbivore defence includes tropane alkaloids and foliar trichomes, and its specialized herbivorous insects. We selected two geographically close natural populations of D. stramonium in Central Mexico and estimated, under controlled conditions, population differentiation at neutral loci (microsatellites) and defence traits (concentration of tropane alkaloids and leaf trichome density). Results indicate very low genetic differentiation at neutral loci between populations but strong and significant phenotypic differentiation in putative defence traits. The average values of tropane alkaloids and leaf trichome density were higher in Ticuman than in Teotihuacan. Twelve out of 21 individual tropane alkaloids were significantly more abundant in plants from Ticuman, and the relative proportion of three of them contrasted markedly. Thus, results point that differentiation between populations of D. stramonium results from natural selection on defence traits.