Fungus has a thing for zombifying insects, altering their behavior to boost its own potential for spread. Sometimes it’ll also employ mimicry to fool lustlorn bugs into making big, moldy mistakes – but rarely are they known to exploit both strategies. Now, a preprint study has discovered a fungus that does exactly this, sending its host to die on the highest point before altering the chemical signals of the corpse so that it lures in the attention of males. The unwitting necrophilia leaves the male vulnerable to infection as it mates with the dead, allowing for further spread of the fungus.
The paper on bioRxiv, which is yet to be peer-reviewed, looked at the pathogenic fungus Entomophthora muscae and how it influences the house fly Musca domestica. To investigate, they offered up infected and non-infected dead females to male flies to see which, if either, it would attempt to mate with. There appeared to be no significant difference between which carcass the male fly opted for, but the chances of the male mating with both dead females increased if one of the dead was in the late stages of sporulation.
A quick and disgusting segue here: when insects become infected by a pathogenic fungus, their bodies become like factories for fungal spores. This makes them dangerous to other insects, especially any hoping to mate with them, and turns them into salt-shakers of pathogen spread. The same effect is seen in the grim life of cicadas infected with a fungus that crashed the brood cicada party earlier this year.
The next step was to incubate the male flies for 10 days after their unusual blind date to see how their health fared. Here, it became apparent that stage of sporulation was indeed significant, as only 15 percent of males who mated with early-stage corpses got sick, compared to almost three-quarters of those who got it on with late-stage remains.
To establish how this lure to the more mature fungal infection might be detected, the researchers observed the flies’ antennae response to volatiles, chemical signals secreted by flies. Looking at the response to uninfected corpses, infected and spore-producing corpses, and living flies, the results showed that sporulating dead flies were indeed the most enticing.
So, the researchers went one step further and carried out chemical analyses on the secretions of early and late-stage fly corpses to see if there was a specific blend of volatiles that was acting on the living flies. Sure enough, a chemical profile was discovered which the team believes to be evidence for a complex recipe of chemicals released during an infection that benefits the fungus in attracting males and increasing pathogen spread.
“Here, we show that the host-specific and behaviourally manipulating pathogenic fungus, Entomophthora muscae, generates a chemical blend of volatile sesquiterpenes and alters the level of natural host cuticular hydrocarbons in dead infected female house fly (Musca domestica) cadavers,” wrote the authors. “Healthy male house flies respond to the fungal compounds and are enticed into mating with dead female cadavers."
“This is advantageous for the fungus as close proximity between host individuals leads to an increased probability of infection," they explain. "The fungus-emitted volatiles thus represent the evolution of an extended phenotypic trait that exploit male flies’ willingness to mate and benefit the fungus by altering the behavioural phenotype of uninfected healthy male host flies.”