Figure 1.

The ponderosa pine–western pine beetle–fungal symbiont interaction. (a) Ponderosa pine is an iconic tree widely distributed across the western USA. The western pine beetle kills ponderosa pine by constructing tunnels and reproducing in the phloem and bark. After successful tree colonization and inoculation of fungal symbionts, developing larvae leave the phloem and tunnel into the nutrient-poor outer bark where they feed heavily on symbiotic fungi that provide critical nutrients to the developing insect [16]. (b) The beetle–fungal symbiosis is maintained via an exoskeletal structure in the female (mycangium; its location highlighted with an ellipse) that harbours glands and excretes unknown substances thought to nurture and promote specificity [16]. After pupation, adult beetles incorporate spores into the mycangium for transport to the next host tree. (c) Shown is a scanning electron microscope image (see the electronic supplemental material, figure S1 for additional images) of symbiotic fungi and spores lining the pupal chamber. (d) Schematic of the complexity of the tree–beetle–fungi interaction. Arrow widths scaled to represent the strength of the interaction.