It’s the horror destroyer of fruit flies – a fungus that emits spores that land on the flies and infiltrate their bodies, says Sheena Harvey.
Burrowing into the fly’s gut, the cells of Entomophthora muscae multiply rapidly and feed on the fly’s internal organs. Then they make their way to the brain.
- Cordyceps and Orphiocordyceps: the zombie fungi made famous by The Last of Us
- These brain-hijacking zombie parasites create suicidal rats and pulsating snails – and can even infect humans
This is where the fungus takes control. The common fruit fly (Drosophila melanogaster), found all over the world, is powerless to resist as the fungus directs it to climb the nearest tall object and glue its mouthparts to the top. There, exactly at sunset, it dies, overcome by the fungus infection.
Following death, it continues to hang there, wings raised, with foam-like projections sprouting all over its abdomen containing hundreds of fungal spores. A few hours after this bizarre birthing process is complete, the spores are cannoned out of the foam to float about in the early morning air until they find another hapless fruit fly to infect.
The timing is perfect to ensure the spores stay moist and cool. If they were launched in full day time they might not survive in the light and heat to successfully perpetuate the species. The fungus has no pigment cells and so is vulnerable to UV rays. The spore release also coincides with known peak fly activity, so the unproven theory is that the choice of timing increases the collision opportunities between flies and spores.
How does it maintain perfect timing?
Timing is everything in the natural world to ensure survival. Because of this, every living thing has what science describes as a ‘circadian clock’, taking cues from light levels and temperatures in its environment. In the case of parasites and their hosts, it has long been known that bacteria and viruses exploit the predictability of their victim’s behaviour and movements and hit them at times when they are most vulnerable.
Given the seemingly critical timing of the E muscae fungus taking advantage of the fruit flies, scientists were curious to know if the body clocks of the flies dictated the timing of their death climbs. Was it a change in the light that drove them upwards, and did this accidentally work in the fungus’ favour? Or was there something more deliberate going on?
In a recent study conducted by the Department of Molecular and Cellular Biology at Harvard University in the US, researchers experimented with the fungus and its host flies using different levels and wavelengths of light. They quickly established that light was necessary for fungal infection. But was this light detected by the flies, or the fungus?
The behaviour of fruit flies in the presence of different light conditions is known thanks to previous experiments. The E muscae fungus was analysed to establish its receptiveness to different wavelengths. It was discovered to work well in when white light levels were low, whereas the flies did not. But the fungus was not active in red light, lacking the photoreceptors to detect it, whereas the flies were.
By observing infection rates and zombie fly behaviour under different wavelengths they reached the conclusion that the fungus itself can keep time. It is the puppet master that sets the clock and chooses the moment of the flies’ deaths. It issues commands that dictate the flies’ behaviour based on its own circadian rhythms, not what occur in the fly’s.
The scientists further propose that this kind of timing control exerted by parasites on their hosts is more widespread than just Drosophila melanogaster and E muscae. Especially when the opportunity to spread spores is a relatively short one. As the study paper concludes: “We anticipate that this kind of pathogen-driven circadian timing may be far more common than currently recognized, especially among behavior-modifying parasites with narrow transmission windows.”
