Chemical signal interactions of the bark beetle with fungal symbionts, and host/non-host trees

The symbiosis between the bark beetle (Ips subelongatus) and its fungal symbiont (Endoconidiophora fujiensis) poses a serious threat to larch forests. However, the signaling pathways between these symbiotic partners and their host/non-host trees are not fully understood. Inoculation of the host larch (Larix principis-rupprechtii) with two strains of E. fujiensis induced a rapid and long-term release of monoterpenes. Although the fungi had a level of tolerance to these compounds, many monoterpenes inhibited fungal growth in culture. Moreover, monoterpenes with stronger inhibitory effects on fungal growth exhibited weaker synergistic effects on the attraction of I. subelongatus to aggregation pheromone. Surprisingly, individual isomers of aggregation pheromone components promoted fungal symbiont growth in a culture medium. Non-host volatiles (NHVs) were tested and shown to completely inhibit the growth of fungal symbionts in culture but had no effects on beetle responses to aggregation pheromone, with the exception of (Z)-3-hexen-1-ol. These results reveal convergence and mutualism patterns in the evolution of I. subelongatus and E. fujiensis with respect to host tree volatiles but not in response to NHVs. Ultimately, we put forward a hypothesis that host plants are ecological and evolutionary determinants of bark beetle–fungus symbioses in terms of their complex signaling interactions.