Ecology and Evolution of Resistance to Herbivory : Trichome Production in Arabidopsis lyrata

Abstract: In this thesis, I examine variation in occurrence and performance of glabrous and trichome-producing plants of the perennial herb Arabidopsis lyrata. I combine field studies and genetic analysis to (1) examine the function of trichomes as a resistance character in natural populations, (2) compare the magnitude of population differentiation in trichome-production and at putatively neutral marker loci, (3) examine the molecular genetic basis of trichome-production, and (4) quantify the effects of herbivore removal on population growth and relative performance of glabrous and trichome-producing plants. In a survey of 30 populations of A. lyrata in Norway and Sweden, I documented spatiotemporal variation in damage from insect herbivores. With few exceptions, glabrous plants were more damaged by herbivorous insects than trichome-producing plants in polymorphic populations. Damage levels varied substantially among populations and among years. The intensity of herbivory quantified as mean leaf removal to glabrous plants was higher in polymorphic populations than in monomorphic glabrous populations. Within the Swedish range, populations were more strongly differentiated at the locus coding for glabrousness than at eight putatively neutral isozyme loci. This is consistent with the hypothesis that trichome production is subject to divergent selection. A study of the genetic basis of trichome production showed that glabrousness was associated with mutations in an orthologue to GLABROUS1, a regulatory gene known to cause glabrousness in A. thaliana. Comparative data indicate that the genetic basis of glabrousness varies among populations. Experimental removal of insect herbivores in a natural A. lyrata population increased population growth rate and the relative fitness of the glabrous morph. The results suggest that insect herbivory may influence both population dynamics and selection on trichome production in A. lyrata.