Olfaction in bark beetles

Bark and ambrosia beetles cause severe economic loss to forestry. Host trees are attacked through the use of odorous signals, such as aggregation pheromones and host volatiles. These odors, along with odors from non-host plants, heterospecific beetles, and fungal symbionts are detected by a divergent family of odorant receptors (ORs), expressed in the insect antennae. However, it is unclear how the evolution of this rapidly evolving receptor family relates to ecological specialization, and how new functions (odor specificities) arise in existing or newly duplicated receptors. In this project, we will undertake a comparative approach to investigate how the ORs have evolved and how they function in three species of bark- and ambrosia beetles (Ips typographus, Dendroctonus ponderosae, and Trypodendron lineatum). These three species differ in their reliance on pheromone communication and in their relationship with fungal symbionts, which in the ambrosia beetle T. lineatum, but not in the bark beetles, has evolved into obligate mutualism. Specifically, we aim to unravel how the differing reliance on pheromones and fungal symbionts in the life of these species relates to the evolution and function of their ORs. We also aim to investigate whether Sensory Drive theory, suggests that habitat choice affects the evolution of communication systems where signals that are easy to detect are favored by selection, can be applied to the evolution of insect ORs. In summary, genomes and antennal transcriptomes will be sequenced and assembled, and the beetle ORs (and other chemosensory genes) will be identified. The phylogenetic relationships of the identified ORs will be
investigated and the function of ORs determined using the heterologous expression in HEK-cells and possibly oocytes. These experiments will be complemented with electrophysiological recordings in species where such data is lacking (i.e., T.lineatum). The longer-term applied aim is to reveal receptors that are key to the survival and reproduction of these pests, and which could be targeted for improved pest control, such as the use of receptor antagonists, or application of ORs in portable biosensors for detection of beetle infestations. This project is different from other beetle OR-projects that are ongoing in the Pheromone group, with the introduction of a novel ambrosia beetle and complementary electrophysiological recordings. The planned genome sequencing and identification of chemosensory genes represent the first from any ambrosia beetle, and the intended electrophysiology with a large array of volatiles from fungal symbionts is likewise the first of its kind in ambrosia beetles.