Molecular genetic approaches, such as marker-based linkage mapping, can help identify whether multiple traits possess common or independent genetic pathways. Such analyses are important to studies of plasticity because they can suggest across-environment genetic correlations that may constrain the evolution of plasticity. In a recent field experiment, recombinant inbred lines of Arabidopsis thaliana (Ler x Col) experienced variable herbivory levels. Herbivorized plants exhibited the classic increase in basal branching that occurs after removal of the apical meristem. This increase in basal branches was associated with enhanced fitness in the herbivorized plants, whereas basal branches had no effect on fitness of non-herbivorized plants. In contrast, fitness of the non-herbivorized plants was strongly influenced by the number of branches on the primary flowering inflorescence. This variable selection across herbivory environments suggests that the observed plasticity is adaptive. Preliminary QTL (quantitative trait loci) and quantitative-genetic analyses show that basal branch number is determined by a different genetic mechanism in herbivorized and non-herbivorized plants; the QTLs for branch number were located on different chromosomes. Although these independent genetic bases are somewhat surprising, the result indicates that plasticity of branch number can evolve freely in response to ambient herbivory conditions. In the non-herbivorized plants, we detected two QTL in common for inflorescence and basal branch number. Thus, these two traits may have a common genetic basis such that selection on inflorescence branches results in a correlated response in basal branch number. These results taken together illustrate that different traits may or may not be correlated in a manner that constrains the evolution of adaptive plasticity.

Key words: adaptive plasticity, Arabidopsis thaliana , evolutionary constraints, herbivory, QTL analyses