Take good care of my fungi - The role of mycetangia in facilitating fungal spore transmission in fungus-farming bark and ambrosia beetles

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The origin of a strict obligate mutualism between ambrosia and bark beetles and their species-specific filamentous fungi was one of the milestones in the evolution of these beetles. These two insect groups currently encompass more than 3,500 species worldwide infesting various types of trees and causing major economic damage to forests and plantations around the globe. The mutualistic fungi play multiple essential roles in the lives of ambrosia and bark beetles, for example, as a source of nutrition within an otherwise nutrient-poor environment and as detoxifiers of tree chemical defenses. Therefore, the protection and transport of these fungal symbionts, is of crucial importance for the survival of their beetle hosts, especially during diapause and dispersal flight. All ambrosia beetles and some bark beetles possess mycetangia, specialized structures for the transmission of mutualistic fungi from one nest or gallery to a new one. For decades, mycetangia have been known to harbor only specific mutualist fungi and no other microbes. However, we know little about what is responsible for this selectivity and whether mycetangia also provide nutrition and regulate the growth of their fungal cargo. This project aims to employ chemical, microbiological and ultrastructural methods to try to answer some of the oldest questions in ambrosia and bark beetle mycetangia research. We will examine the impact of mycetangia on the growth of transmitted fungi by providing essential nutrients. Although mycetangia were described to harbor only mutualistic fungi, our preliminary evidence suggests the presence of bacteria that may fix nitrogen. Such bacteria could significantly increase the low nitrogen content of wood and bark substrates, and thus improve beetle fitness. Finally, we will search for antimicrobial compounds that could be responsible for the long-described selectivity of mycetangia allowing the mutualistic fungus to outcompete ubiquitous antagonistic microbes. By establishing new functions for mycetangia, our work will provide fresh insights into the factors driving the success of ambrosia and bark beetles. Such knowledge could eventually be applied to limit the extensive damage caused by these insects to forests and tree plantations throughout the world.