Developmental and genetic dynamics of ambrosia beetle-fungal partner symbioses

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Ambrosia beetles infest trees, where they dig into the bark to lay their eggs. These beetles are unique in that they also harbor fungi that act as their food source which grows on the tree tissues. Some of these fungi can then infect the tree and in several cases can result in the death of host trees within months of infection. These beetles and their pathogenic fungi occur worldwide and pose a significant threat to forestry and natural ecosystems. However, no genetic models for the study of any ambrosia beetle-fungal partner pairings are available. This proposal will use the Xyloborus beetle-laurel wilt pathogenic fungus (Harringtonia lauricola) as a model. Both the original beetle and the fungus are invasive to the United States and are rapidly spreading throughout the Southeastern part of the country. The beetle carries the fungus to trees, where the fungus is capable of killing otherwise healthy trees. There are over 500 different species of trees that the beetle uses as a host including swampbay, redbay, sassafras, and the agriculturally important crop of avocado, where the only treatment is to cut and burn the trees. In this project, we will study the dynamics and genetics of how the beetle harbors its fungal partner, including how the beetle chooses, promotes the growth of, and carries the fungus from host tree to host tree. These data can provide foundational information that can ultimately be used to help stop the spread of these tree pathogens. In addition, the project will provide opportunities for training and outreach to high school, undergraduate, graduate, and postdoctoral students on fungal biology, disease, symbioses and evolutionary processes. In partnership with programs at University of Florida, the investigator will develop educational material for the high school teachers and the general public on fungal-insect symbioses. Insect-fungal mutualisms are widespread in nature and ambrosia beetle-fungal interactions represent some of the oldest and most successful symbioses known. The fungus serves as the sole source of nutrition for the beetles that burrow into trees to form galleries where they grow and “farm” the fungus for food. These beetles have evolved specialized structures (mycangia) that house, transport and disseminate the fungus. These fungi and beetles have co-adapted to symbiotic lifestyles throughout evolutionary history and include independent evolutions of different types of mycangia that house different fungal symbionts. The fungus, Harringtonia lauricola is a highly destructive tree pathogen affecting avocado and other members of the Lauraceae family and has killed 300 million trees in the US alone. H. lauricola is the mutualistic partner to its original invasive (to the US) beetle carrier (Xyleborus glabratus) and may have now spread to indigenous beetle species. We will investigate the developmental and genetic mechanisms which underlie the relationship between H. lauricola and Xyleborus beetles. The project will use cellular and genetic approaches to test specific hypotheses concerning symbiotic association. We will: (1) probe the cellular dynamics of mycangial colonization, including investigating selection, persistence/maintenance, turnover, and competition in the mycangia using microbiological and cellular (e.g., microscopic) methods, (2) identify genetic mechanisms that mediate fungal symbioses, e.g., what fungal genes/network are important for colonization and fitness within the mycangia, and (3) construct gene expression profiles (i.e., transcriptomics) of the fungus during mycangial colonization to help define the evolutionary processes that has led to fungal-mycangial symbioses.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.