| Due to climate change, conifer forests are increasingly attacked by bark beetles, which cause serious losses to economies and forest biodiversity. The beetles use their sensitive sense of smell to locate mates and trees to attack. Once a suitable tree is found, the beetles bore in under the bark and release a pheromone, attracting large numbers of conspecifics. The beetles also introduce symbiotic fungi into the tree, which metabolize tree defense compounds, serve as food for the beetles, and accelerate tree death. The proposed project on the destructive spruce bark beetle Ips typographus will contribute to a better understanding of how bark beetles with their fungi can kill highly-defended trees, and how these interactions may be affected by climate change. The results are expected to be crucial for the development of improved pest control, which is urgently needed.
The ecological importance of the beetle-fungi symbiosis is becoming increasingly appreciated. We showed that I. typographus is attracted to the odors of several of its symbionts, and it also discriminates between the smells of different fungi. This together with a large number of sensory neurons on the antennae responding to fungal odors suggest that the symbiosis has played a central role in the evolution of bark beetle olfaction. The responses in these neurons are triggered when odors bind to odorant receptors (ORs). Hence, to understand the sensory basis and evolution of symbiont detection, and then apply this knowledge, we need to study the function of the beetles’ ORs.
The first aim is to identify and functionally characterize the I. typographus ORs that detect fungal odors and reveal their evolutionary origins. ORs that respond to fungal odors will then be silenced to study their importance for the attraction to fungi. The second aim is to investigate whether beetle preferences for fungi vary between populations in relation to local fungal abundance (fungal communities vary geographically), and whether different preferences are driven by variation in OR expression or sequence. The third and directly applied aim is to identify ORs and fungal odors that could be used for improved pest control, including better trap baits and repellents, and the use of OR agonists or antagonists that could disrupt odor communication, and hence beetle reproduction (see also ‘Relevance’ section).
This project will be the first to unravel how symbiosis with microbes has co-evolved with changes in the insect OR gene family. We expect to gain novel insight into the function and divergence of ORs that are used to maintain this across-kingdom interaction. The characterization of a large number (>40) of ORs will greatly improve our understanding of bark beetle olfaction, and position I. typographus as one of the best characterized insects in this regard. The project will also further our understanding of how variation in symbiont communities may drive the evolution of olfactory preferences, possibly resulting in local adaptations. An increased understanding of the beetle-fungi symbiosis together with the identification of key fungal odors and ORs is an important step towards better control of this destructive pest, and hence the preservation of future forests and their biodiversity.
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