It’s been known for some time that toxins produced by milkweed plants can protect monarch butterlfy caterpillars from predators, such as birds, and from various parasites. Now researchers from the University of Michigan and Emory University have shown that root fungi that form a symbiotic relationship with milkweed plants also play a role in disease transmission.
The symbiotic fungi in question are called arbuscular mycorrhizal fungi. In return for sugars, these fungi provide plants with nutrients and water.
“Mycorrhizal fungi associated with the roots of milkweed plants change the medicinal chemistry of milkweed leaves and therefore the transmission of the monarch parasites,” said Mark Hunter, a professor at the University of Michigan.
“It’s well known that these fungi are important to plants and provide a lot of services, such as helping them cope with different types of stresses,” said Leiling Tao, a postdoctoral researcher at Emory University. “What we didn’t know before was that they also affect host-parasite interaction in animals above the ground.”
In addition to increasing the understanding of disease ecology in general, this could also be important to human health, since about half of new pharmaceuticals are derived from plants.
For the current paper, published in Proceedings of the Royal Society B, the researchers conducted greenhouse experiments on six species of milkweed that produce varying amounts of cardenolides. The plants were grown either with no mycorrhizal fungi, with low levels of it, or with high levels.
Monarch caterpillars were fed leaves from the various milkweed plants and then exposed to the protozoan parasite. The results showed the fungi are associated with both the virulence of the parasite and the ability of the monarchs to resist infection and to survive if infected.
Dosage of the cardenolides is critical, Tao said.
“In some species of milkweed, the presence of the fungi was beneficial for the caterpillars. In some species, it had no effect. And in other milkweed species, the presence of the fungi resulted in more disease for the caterpillars.”
The results also showed that the amount of the nutrient phosphorous associated with the fungi is important to the performance of the caterpillars.
“It’s not just the drug dosage, but also the nutritional environment that determines the overall outcome,” Tao said. “The interactions are really complex. It’s fascinating that even a species that is spatially distant, and from a different ecosystem, can have effects on how another species fights a disease.”
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