The mountain pine beetle carries two fungi from tree to tree in specialized pockets called mycangia (part II). The fungi are never found without the beetle. Likewise, the beetle never occurs without the fungi. The young of the beetle feed on the fungi to gain nutrients not present in their otherwise poor diet of inner bark (phloem) (part III). The melanin in the cell walls of the fungi stain the wood of mountain pine beetle – colonized trees a stunning blue (part I). This close relationship is known as an obligate mutualism – all partners benefit, but they are also highly dependent upon one another for survival.
So, as a bark beetle, why have two fungi? Why not just have one fungus? From research in my lab, it appears that having two fungal partners may have major benefits for the beetle. For one, it’s good to have back up. If you depend upon a partner (in biology, the partner is called a symbiont) for your very survival, and something happens to that symbiont, you’re toast! But if you have two, no sweat, you’re probably still good to go. The second is environmental buffering. If you have only one symbiont, and you require that partner, then you must live within the ecological constraints of that partner. In other words, if your fungal partner can only survive and grow within particular conditions, then you must also live within those conditions. However, if you have two partners, with different ecological boundaries, you may get to live over a broader range of habitat. For example, If you have one partner that likes it cool and one partner that likes it warm, then you can exist over a wider range of conditions (habitats) and also be able to deal with more environmental variability within and among years and locations. In nature, that can be a recipe for success!
You might be thinking there is a little more complexity to the issue of having two partners? You’re quite right - Look for Part V.