Here’s a new detail of how a real-life Cordyceps fungus zombifies still-living animals: Crashing their “blood sugar.”
That’s a recently discovered bit of science related to the zombie-apocalypse video game and TV series “The Last of Us.” The fiction chronicles an imaginary version of Cordyceps that has jumped from insects to humankind. The menace spreads via bites from crazed, ravenous bands of the infected stragglers of our species. Their voracious appetites turn out to be rooted — at least somewhat — in science.
In the real world, humans have named at least 400 hundred species of Cordyceps and, with molecular evidence, now recognize a sister genus (Ophiocordyceps). Overall, these fungi are superb sneaks that can manipulate the behavior of host species they attack. The targets aren’t human, but depending on the fungi species, could be particular kinds of caterpillars, bee larvae, spiders, ants or other arthropods. A few fungi classified as Cordyceps even prey on some other fungi: the lump-forming underground species nicknamed false truffles.
Exploring how a no-hands, no-teeth, no-muscles fungus can manipulate caterpillars, insect pathologist Chengshu Wang and colleagues worked with one of the most famous real-life Cordyceps species, C. militaris. This species infects silkworm caterpillars, which typically eat mulberry tree leaves, says Wang, of the Chinese Academy of Science’s Shanghai Institute of Plant Physiology and Ecology.
A series of feeding experiments shows new details of how a real-life C. militaris turns a silkworm into a food-binging zombie, Wang and colleagues report June 27 in Current Biology. The fungus inside the silkworm has acquired a gene (originally from a virus) for making an enzyme called trehalase. It’s perfect for breaking apart a 12-carbon form of sugar that would normally circulate through the body of a caterpillar nibbling on a leaf. That enzyme messes with the signal of hunger being satisfied, so the caterpillar eats in a frenzy. And that will only make its plight worse.
The enzyme is turning the caterpillar’s normal sugar into a 6-carbon glucose as a breakdown product. That’s a fine feast to nourish a fungus. Cordyceps flourishes while the little caterpillar stays zombie-grade hungry.
A Cordyceps fungus doesn’t immediately kill the caterpillars it infects. Instead, it lets them gobble leaves that nourish the fungus inside to grow bigger and bigger. The caterpillars don’t die until they have wrapped themselves into cocoons and aren’t any more use in feeding the fungus. It then sprouts orange spore-bearing prongs out of the mummy-wrapped cocoon of the, finally dead, insect.
“I steal your own weapon to kill you” is Wang’s take on the thieving fungal overlords.
Startling as the powers of Cordyceps may be, biologists have found other parasites and pathogens that also drive infected creatures into doing favors for their disease. “A fair comparison” with the Wang team’s discovery could be the egt gene discovered in a virus that attacks spongy moths, says Kelli Hoover, an entomologist who does research in chemical ecology at Penn State.
Uninfected spongy moth caterpillars Hoover studied (Lymantria dispar) live cautiously, but that changes if they catch what’s sometimes called treetop disease. Infected moth caterpillars approaching adulthood climb to the exposed tops of trees. Up there, their bodies liquefy and, especially if it rains too, shower infective virus particles onto caterpillars below. Surely there’s some zombie apocalypse potential in that too.
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