Species-specific responses of pine sesquiterpene synthases to sawfly oviposition

Pinus sylvestris (Scots pine) is known to respond to eggs laid by the sawfly Diprion pini on its needles by releasing a blend of terpenoids, including the sesquiterpene (E)-beta-farnesene. These compounds attract a wasp, Closterocerus ruforum, which parasitizes sawfly eggs. D. pini oviposition also enhances the transcription of two sesquiterpene synthases, an (E)-beta-caryophyllene/alpha-humulene synthase (PsTPS1) and a 1(10),5-germacradiene-4-ol synthase (PsTPS2). To gain a better understanding of the function of these sesquiterpenes in promoting insect egg parasitism, we compared the outcome of D. pini oviposition on P. sylvestris with interactions between other pine and sawfly species: Neodiprion sertifer eggs on P. sylvestris, Gilpinia pallida eggs on P. sylvestris, D. pini eggs on Pinus nigra. The first of these attracts the parasitoid C. ruforum, while the latter two do not. As determined by quantitative real-time PCR, both PsTPS1 and PsTPS2 transcripts increased significantly only for those species combinations where the odor of egg-laden pine needles was attractive to C. ruforum. Moreover, enhanced transcription of these genes was found only at those time periods when odor was attractive, i.e. 3days after oviposition. Thus, the PsTPS1 and PsTPS2 genes are good markers for parasitoid attraction. We also characterized a sesquiterpene synthase from P. sylvestris (PsTPS5) which produces (E)-beta-farnesene, the compound previously determined to be responsible for C. ruforum attraction. However, transcript levels of PsTPS5 were not enhanced by oviposition of sawfly species that cause C. ruforum attraction. More research on this experimental system is required to determine the role of oviposition-induced sesquiterpenes in attracting egg parasitoids and the role of sesquiterpene synthases in regulating sesquiterpene formation.