The parasitic fungus that encourages houseflies to mate with ‘corpses’

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A fungus that controls houseflies, and then encourages the non-infected ones to mate with dead flies? This is what researchers discovered when they studied a widespread parasitic fungus called Entomophthora muscae. While the fungus’ ability to control the dying housefly was known, this is the first time that researchers observed an ability to impact mating behaviour.

Researchers at the University of Copenhagen and the Swedish University of Agricultural Sciences discovered after the death of the host fly, the fungus secretes compounds that attract other male flies who then try to mate with the corpse. This is what also helps the fungus infect a new host. The research has been published in Nature’s ISME Journal.

In a normal trajectory, once the spores of the fungus land on a fly, it starts consuming the fly’s internal organs for nutrients. After a few days, the fungus takes over the fly’s behaviour and forces it to fly or crawl to a higher position to latch on to something before the fly eventually dies. The fungus also makes the infected fly raise its wings so that its abdomen is exposed, allowing the spores to disperse. Typically, these spores land on another fly as they are dispersed from a higher location. And while this spreading of spores was known, its tendency to control mating behaviour is a new observation.

“We already knew that spores disperse from the cadaver of the infected fly, so that it can land on other flies. But it has also been observed that other flies sometimes come to inspect the dead fly. In some rare instances, it has been observed that some male flies even try to mate with infected female cadavers,” Henrik H. De Fine Licht, associate professor at the University of Copenhagen and corresponding author of the study, told over a video interaction.

“When we held up the cadavers, we could sense a strong smell coming from them, getting even stronger the longer the fly was dead. We then used sonography to find out that yes, the male flies are detecting the smell and responding to it,” De Fine Licht added.

The researchers designed an experiment where a living housefly was kept in a container which was attached to a Y-shaped tube that had two exits. One exit led to a fly corpse that was infected and another led to a corpse that was not infected. The researchers observed that male houseflies held a strong preference for the infected corpse.

“It is a combination of the odours emitted by the fungus and other factors, including whether the cadaver is female, that attracts the flies. With infected female cadavers, we saw a 60 per cent chance that the male fly would try to mate and get infected whereas we only saw a small 15 per cent chance with male cadavers. There was also a higher chance of the fly being attracted the longer the cadaver was dead,” said De Fine Licht.

Houseflies are known to be vectors of a variety of diseases that affect humans, so this fungus could provide an important tool to control their populations. But the fungus itself is difficult to handle, so De Fine Licht believes the volatile compounds that it creates could be replicated to attract and kill houseflies.

Scientists don’t yet know how Entomophthora muscae can take over the behaviour of infected hosts. According to De Fine Licht, the answer to that could open the door towards a better understanding of insect behaviour, maybe even allowing humans to eventually control insects.

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