Carrying the seeds of your crop: fungal genetic networks activated during colonization of the ambrosia beetle symbiotic organ

BACKGROUND: Myco-cultivation by ambrosia beetles involves unique symbiotic partnerships with select fungi. These beetles grow and tend their fungal gardens as food for larvae and adults. This close association has led to the evolution of specialized structures, termed mycangia, on or within the beetle to house fungal cells as "seeds" for establishing and maintaining their gardens. However, little is known concerning fungal gene networks involved in mediating the symbiotic colonization of the mycangia of their host beetles. Here, we report on global gene expression changes that occur in the fungus during mycangial colonization of the Xyleborus affinis ambrosia beetle mycangia by its the partner fungus, Harringtonia lauricola, which, while beneficial to the beetle, is the causative agent of the devastating laurel wilt disease in plant hosts. RESULTS: A temporal map, ranging from 1 h to 72 h of H. lauricola gene expression during colonization of X. affinis mycangia was constructed and compared to fungal cells grown in vitro (PDB) and to a previous dataset of H. lauricola isolated from infected plants. These data revealed a rapid adaptation (6-12 h) of fungal cells to the mycangial environment with high expression of genes involved in cell wall remodeling, some of which were shared with H. lauricola-plant infection. Differential expression of genes involved in metabolism, effectors, and pathogen-host interactions (PHI) were noted, providing clues to the nutritional landscape within mycangia as well as discrete mechanisms employed by the fungus to interact with its ambrosia beetle host. Consistent with a dimorphic shift, but also a general suppression of growth, genes involved in hyphal and filamentous growth, and conidiation, showed significantly lower expression in the mycangial environment as compared to in vitro media conditions. Intriguingly, GO terms involved in the RNAi pathway were enriched in upregulated datasets, suggesting a novel role for miRNAs in ambrosia symbioses. CONCLUSIONS: Our data indicate that the transition to the mycangial environment begins almost immediately post-colonization, with gene expression changes noted as early as 1 h, adaptation by 6 h, and maintenance thereafter. Specific metabolic gene networks were identified that suggest nutrient exchange and facilitation of fungal adaptation to the ambrosia beetle mycangia. While aspects of cell wall remodeling appear conserved between fungal colonization of the beetle mycangia and infection of plants, distinct sets of fungal genes involved in nutrient assimilation (transport and metabolism), effector production, suppression of growth, transcription factors, and PHIs are expressed during mutualism. These data open new avenues for functional analysis of genes that define symbiotic associations in context dependent host-microbe interactions that results in disparate, i.e., parasitic vs. mutualistic, outcomes.