Thyme is of the essence: Biochemical polymorphism and multi-species deterrence

Populations of the common thyme (Thymus vulgaris, Labiatae) are characteriz ed by a complex genetic and biochemical polymorphism. Individual plants man ufacture a single dominant monoterpene which gives them a characteristic sm ell and taste, and the genetic determination of these chemotypes involves a series of five loci interacting with each other in epistatic fashion. We t ested the hypothesis that multi-species herbivory and allelopathy are assoc iated with this monoterpene polymorphism. We compared the relative attracti veness and deterrence of various chemotypes by offering either thyme plants or artificial diets laced with monoterpenes to six herbivores: two mollusc s, two insects and two mammals. In addition, we summarize experiments by us and others in which bacteria, fungi and seeds of the grass Brachypodium ph oenicoides, a common competitor of T. vulgaris, were exposed to the individ ual monoterpenes. The results show that the monoterpenes have differential deterrence value against these various species. We conclude that a combinat ion of selective herbivory and allelopathy contributes to the maintenance o f this polymorphism.