Karst.) and causes widespread damage across Europe and Asia. This insect is endemic to Norway spruce ( Picea abies (L.) H. We sought evidence for signals that may be used to maintain the symbiosis between the Eurasian spruce bark beetle, Ips typographus, and its associated fungi. Additionally, flies recognize and discriminate among different environmental microbes based on their volatile profiles, thereby maintaining a high specificity with beneficial microbes. These fungivorous flies utilize the volatiles produced by yeast to locate and evaluate suitable breeding sites, which maximizes benefits to their offspring. Positive behavior towards signals from mutualists are most likely to evolve if the signals display the partner’s quality, as in the Drosophila-yeast system. The response of insects to microbial signals via specialized sensory pathways represents one mode of evolutionary coadaptation that could maintain interactions with beneficial symbionts. Many cross-kingdom mutualisms are driven by chemicals emitted by one or both partners. Symbiosis could be maintained by chemical cues. However, in this scenario, some harmful microbes, such as Ophiostoma minus (associated with the southern pine bark beetle, Dendroctonus frontalis), may also take advantage of the same conditions and reduce insect fitness. One possibility is that insects create conditions in their galleries in which microbes that are important to them survive, or microbes are particularly adapted to their host tree’s environment. With or without mycangia, it is not clear what factors maintain the interaction between wood-boring insects and their associated fungi. However, certain bark beetles, such as Ips typographus, lack mycangia. Among wood-boring insects, many bark and ambrosia beetle species have morphological adaptations on their body, called mycangia, to carry fungi with them to new host trees and to pass on to offspring. In this environment, symbiont choice may be crucial for insect survival by providing essential nutrients and degrading defenses. The need of wood-boring insects for symbionts may be ascribed to the nature of their substrate, which is generally low in nutrients and high in toxic defense chemicals. Wood-boring insects, like many other organisms, often engage in associations with symbiotic microorganisms to fulfill a range of physiological and ecological functions. These findings indicate that volatile compounds produced by fungi may act as recognition signals for bark beetles to maintain specific microbial communities that might have impact on their fitness. Finally, synthetic blends of fungal volatiles attracted beetles in olfactometer experiments. Testing the activity of these compounds on beetle antennae using single sensillum recordings showed that beetles detect many fungal volatiles and possess olfactory sensory neurons specialized for these compounds. GC-MS measurements revealed that the symbionts emitted VOCs. typographus are attracted to food sources colonized by their fungal symbionts but not to saprophytic fungi and that this attraction is mediated by volatile cues. Behavioral experiments with fungi showed that immature adults of I. Here we tested the hypothesis that volatile organic compounds (VOCs) emitted from fungal symbionts could be cues for bark beetles to recognize and distinguish among members of its microbial community. The spruce bark beetle, Ips typographus, the most destructive forest pest in Europe, has a symbiotic relationship with several fungi that are believed to contribute to its successful invasion of Norway spruce.
However, the chemical signals that maintain these insect−microbe relationships are poorly known compared to those from insect−plant symbioses. Insects have mutualistic symbioses with a variety of microorganisms.
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