The effects of temperature on fungal symbionts in the mountain pine beetle-fungus multi-partite symbiosis

The mountain pine beetle is an economically and ecologically important insect in western North American forests capable of killing millions of trees during outbreaks. This beetle depends on two fungi, Grosmannia clavigera and Ophiostoma montium, to provide the nutrients required for the beetle to develop and reproduce. Competition between these two fungal associates is expected because they use similar resources. Strong competition should lead to the eventual destabilization of the three-way symbiosis and fixation for the most competitive fungus. However, strong direct competition has not been observed, indicating that some mechanism likely allows the two fungi to coexist in a multi-partite symbiosis with the mountain pine beetle. These fungi exhibit different temperature tolerances, indicating that temperature may play a major role in determining the relative prevalence of the two associates over time as well as the outcome of competition between the two species. This, in turn, may support the long-term stability of the three-way symbiosis with the mountain pine beetle. To investigate the effects of temperature on the fungal symbionts, I collected 88 isolates from three locations in two states (50 G. clavigera and 38 O. montium) and measured their growth rates and sporulation at 5, 10, 15, 21, 25, 30, and 35˚C on artificial media. I also measured the growth rates of, and percent resource capture by, each fungus at 10, 15, 21, and 25˚C in the presence of the other species (inter-specific competition) or in the presence of the same species (intra-specific competition). My results indicate that G. clavigera excels at resource capture at 10˚C, while at 30˚C O. montium dominates. There was no significant effect of geographic origin on growth or sporulation of G. clavigera, supporting the findings of previous studies showing low genetic diversity in this species. In contrast, O. montium isolates from different locations exhibited significant differences in growth rate when grown alone and during competition, indicating population sub-structuring. G. clavigera sporulation was greatest at 30˚C while O. montium sporulated similarly across all temperatures. G. clavigera captured more resources than O. montium at most temperatures, and was able to capture a greater percentage of resources at a greater rate during inter-specific competition than during intra-specific competition at 10 and 15˚C. The reverse was true for O. montium which captured resources better during intra-specific competition, and captured a greater percentage of resources at the lower temperatures during intra-specific competition. These results show that temperature affects growth, sporulation and resource capture by these fungi and thus may influence the stability of the three-way symbioses between the fungi and the host beetle in a variable environment.