Mutualism with aggressive wood-degrading Flavodon ambrosius (Polyporales) facilitates niche expansion and communal social structure in Ambrosiophilus ambrosia beetles

Most wood-boring insects compete with wood decaying basidiomycetes for woody biomass. One clade of ambrosia beetles gained access to rotten wood – an abundant resource unsuitable to most wood-boring insects – by evolving a farming-like mutualism with a white rot polypore. Here we show the mutualist of Ambrosiodmus/Ambrosiophilus, the polypore Flavodon ambrosius, is superior in lignocellulolytic capacity compared to Ascomycota ambrosia fungi and other white rot Basidiomycota. This mutualism facilitated the evolution of large, long-lived, communal colonies with overlapping generations and egg-laying by pre-dispersal progeny females. F. ambrosius resembles other white rot Polyporales in that it causes significant weight loss in wood decay assays and strong polyphenol oxidase reactions, indicative of lignin-modifying enzymes. The symbiosis is asymmetrical: there are many species of Ambrosiodmus and Ambrosiophilus but all use a single known species of Flavodon, which determines the ecological strategy of the entire insect clade.