Tripartate symbioses with fungi

Ants, fungi, and bacteria

I have to admit that I am fascinated by co-evolution of symbiotic and mutalistic systems. A review by Richard Robinson gives an overview. A great example is the mutalism between ants and fungi where the ants cultivate the fungi for food. There are more layers to the relationship as a fungal parasite (Escovopsis) attacks the cultivated fungi, and a bacteria. Several researchers have studied the coevolution of these studies including Ulrich Mueller and Cameron Currie. Currie and Mueller have published several great studies describing the patterns of coevolution and the nature of the cooperation.

One (PDF link) describes the ancient nature of the relationship due to the congruent phylogenetic trees as the three players are co-speciating in lockstep. The tripartate nature of the relationship here is between the attine genus Apterostigma, fungi from the Lepiotaceae (Agaricaceae, Basidiomycota) clade, and the mycoparasite Escovopsis (Hypocreales, Ascomycota). The attine-fungal relationship is quite ancient and appears to be specific in that ants prefer their specific cultivar.

Another study shows that the ants have specialized structures to maintain a fourth player in the symbiosis, an actinomycete bacteria, Pseudonocardia, that has antifungal properties to kill the invading Escovopsis. To complete the farming metaphor, the ants are thus farming the fungus and spreading the actinomycete as a pesticide. The actinomycete is maintained and grows in specialized structures on the ants’ exoskeleton.

A Grass, a fungus, and a virus.

High temperature exposure of Dichanthelium lanuginosum in the presence of its fungal symbiontA recent paper in Science describes a three-way symbiosis between a grass, its endophytic fungus, and a mycovirus. The plant and the endophyte need each other to grow at temperatures above 38° C. These grasses are found in Yellowstone National Park in high soil temperatures (as high as 65° C). The virus modulates the fungal-plant interactions and is transmitted vertically through fungal conidiospores. The virus is found in high titre in the fungus and the authors demonstrate the necessity of the fungus for the thermotolerance modulation in the fungus and the grass by curing it of the virus showing lack of thermotolerance, and re-introducing it to achieve the wildtype effect.

The mechanism for thermotolerance modulation appears to be still under investigation. There is no usual stress-response induction as the “osmolyte concentration in these plants does not increase” in response to the heat, but the osmolyte levels are higher than plants colonized with virus-free isolates. The authors explore one hypothesis that the endophytes help scavange ROS which are produced at a higher level under stress as part of plant defenses. This would suggest a sort of autoimmune-like cause of death under high stress conditions. However the authors do not find a difference in ROS production in plants with virus+ or virus- endophytes.

Symbioses galore

LichenThis is of course just a few examples of symbioses where fungi participate. Lichens are a symbiosis between a fungus and a photosynthetic cyanobacteria or green algae. I’ll have to write more about the evolution of lichenized fungi later, but they are another facinating example of cooperation between two different species.

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