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Some species of termites are known to cultivate their own crops of fungus within their nests, similar to the way humans maintain farms to feed people. One such species is Odontotermes obesus, which cultivates the fungus Termitomyces. The relationship between these termites and the fungus can be thought of as a sort of symbiotic one. In this case, Termitomyces feeds the termites, and the termites protect the fungus from an invasive "weed-like" fungus called Pseudoxylaria. Pseudoxylaria can quickly overrun Termitomyces if left to its own devices.

A new study, in Science, sheds some light on the methods these insects use to protect their crops, which was previously unclear. The research team investigated these methods through a series of experiments in which Pseudoxylaria was introduced into the termite's crop of Termitomyces combs.

In the first part of the experiment, only a small amount of weed fungus was placed on a comb, and the termites' responses were observed and compared to the response to an uninfected comb. Then, a highly infected comb was introduced next to a healthy comb, and termite responses were observed. Finally, the team attached a healthy comb to an infected comb to find out how the termites responded. Then, the soil boluses, which were used by the termites to cover certain pieces of comb, were analyzed for microbial content and fungistatic properties using sequencing and inhibition assays.

The results were varied, but most of the termites exhibited behaviors indicating targeted actions for reducing the Pseudoxylaria infection. They used a combination of behaviors, including removing, isolating, and encasing Pseudoxylaria with soil boluses to protect their crop.

In the first part of the experiment, the most common response was to remove the Pseudoxylaria tuft and cover it with soil boluses, then clean the comb off. This occurred in 17 out of 57 assay plates, or 29.82% of cases, and was effective in eliminating the infection 94.12% of the time. Other responses included either removing the tuft and cleaning the comb or just removing the tuft and covering it. These were effective in most cases.

The team reported that the least frequent behavior was found in 3 of 57 (5.26%) plates, where Pseudoxylaria tufts were covered in soil boluses while still on the comb, which prevented Pseudoxylaria 33.33% of the time. Meanwhile, no response was seen in 8 of 57 (14.04%) of cases.

The termites also demonstrated removal and encasement strategies with the highly infected comb piece, with some differences. The authors write, "In contrast to the previous assay, termites showed fewer but considerably different behavioral responses toward these highly infected combs by exclusively encasing them with soil boluses, either entirely or partially. Complete encasement was seen in 57 of 101 (56.43%) plates, and it was more efficient (89.47%) in confining the infection compared with partial encasement, which was seen in 32 of 101 plates (31.68%; efficiency = 56.25%)."

When the researchers glued together infected and healthy pieces of comb, the termites were able to quickly identify the infected portion.

"We attached a fresh, uninfected piece of comb to an infected piece using a resin-based adhesive and presented it to the termites. With a few exceptions, termites separated the infected pieces from the fresh comb pieces, accompanied by partial or complete encasement of infected combs, similar to the outcomes of the second experiment," they say.

The use of soil boluses to encase infected pieces of comb was still a bit of a mystery. The team hypothesized that the physical act of encasement might suppress the infection from lack of oxygen or that the termites might employ the use of microbes that help suppress the Pseudoxylaria fungus.

After comparing the soil boluses processed by the termites to soil that had been sterilized, they found that the sterilized soil was not effective in suppressing infection. This led the researchers to conclude that the termites' soil boluses were fungistatic in nature—helping to ward off the Pseudoxylaria.

"Our assays show that termites have evolved an elegant solution of encasing only the weed or weed-infested comb with generic fungistatic soil boluses. This protects the crop from the weed as well as from the fungistatic effects of soil boluses," the authors write.

Insights into fungal inhibition practices in nature help scientists understand and assess other possible avenues for reducing fungal contamination in human food sources, which may lead to greener solutions for human farming challenges.

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