This killer fungus strikes at sunset. Here’s how

For decades, a grisly mystery has buzzed around one fly-killing fungi. No matter when the fungus infects a fly, it always dies at the same time of day. Death always comes at sunset.

When molecular biologist Carolyn Elya tells people about the phenomenon, they “give me these looks of utter disbelief,” she says. But Entomophthora muscae is indeed a sunset killer, and now, Elya’s team at Harvard has discovered why. The fatal fungi have a built-in kill clock, the researchers suggest June 18 in a preprint on bioRxiv.org.

Entomophthora comes from Greek words meaning “insect” and “destruction.” That’s incredibly apt, Elya says, because the fungi “just completely trash their insect hosts.” E. muscae is famous for turning flies into zombies. Once an infected spore lands on a fly, it pushes its way inside the insect’s body. Nestled amid the fly’s guts, fungal cells multiply and spread to the brain. That’s when things get weird. Like a macabre puppeteer, the fungi forces its host to climb to a high spot and glue its mouthparts there.

Then, as the sun sets on the last day of its life, the fly’s wings lift, and it dies. Infected insects are “effectively a sack of fungus at this point,” Elya says. Cannonlike structures erupt through the bugs’ backs and launch fungal spores into the environment.

No one knew whether the clockworklike time of death was driven by the fly, the fungus or both. Elya’s team ruled out the fly by testing genetically modified insects in the lab. Some had broken body clocks or weren’t able to see certain wavelengths of light signaling time of day. If flies were driving the death timing, these deficits would monkey with their schedule. But no matter the mutant fly tested, they still all died at sunset.

Elya’s team also discovered a set of fungal genes whose activity rose and fell regularly throughout the day. That cycling continued even in total darkness, suggesting that E. muscae keeps track of time using its own internal clock. Scientists have reported such timekeepers in other fungi species, Elya says, but until now, not E. muscae. “Usually, absence of evidence of a clock means that no one has looked,” Elya says.

At this point, Elya can’t say for sure why E. muscae schedules its spore-launching extravaganza in the evening. But she has an idea. That timing could help spores stay cool and moist by avoiding the heat of day. Once the spores find a fly, they could duck inside its body and begin their dirty work, all before the sun comes up.