Biologists are used to hearing stories of microbes manipulating their host. a fungus that turns ants into suicidal zombies, a protozoan that makes rats search for cat urine – but there are few examples of hosts turning tables with their microbes.
My colleagues and I just published an article showing that the beetle buried, Nicrophorus orbicollis, found in eastern North America, alters the smells produced by microbes in its underground nest to thwart competitors who steal the beetle’s hiding place.
A series of unpleasant smells.
I have studied burial beetles for over 30 years, first to understand their parenting behavior and physiology, but more recently their role in the community of scavengers that recycle essential nutrients in the soil.
The scent environment of buried beetles is a disgusting environment for many humans, but one that fascinated me because it is the context in which beetles find their food, advertise their mates, and compete with their rivals.
The volatile chemicals that microbes produce when they grow on a carcass change as the animal breaks down. This changing bouquet of molecules attracts a succession of different insect species.
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The different scent mixtures represent specific stages of decomposition that will indicate which insects specialize in a fresh carcass or the leftovers at the end of decomposition, or something in between. This information can be useful in criminal cases. to determine the postmortem interval.
The center of a buried beetle’s nest is a small, dead animal that a male-female couple moves underground to prepare food for their young.
The microbes that live in a fresh mouse carcass begin to metabolize proteins, emitting sulphurous byproducts that float in the breeze. These scents attract a flying buried beetle in search of a breeding opportunity.
Working with microbiologist Paula Philbrick, I started with field testing to identify the chemicals that burying beetles react to so that we can determine which ones they would like to handle.
We tested two chemicals, dimethyldisulfide and dimethyltrisulfide, which are known to attract scavengers. These chemicals are used by plants that mimic corpses in their own manipulation, fooling flies and beetles in search of carrion by pollinating their putrid flowers.
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However, when we tested these compounds as supplements alongside a fresh mouse carcass, the free-flying burial beetles showed little interest.
Our best guess being misplaced, we were overwhelmed by the idea of randomly testing each of the more than 500 chemicals associated with a rotting carcass.
Tell us what you know
Rather than playing a chemical guessing game, we decided to take another approach, to see if the beetles could show us what was important to them.
Our colleagues Sandra Steiger and Johannes Stökl from the University of Bayreuth used a technique called gas chromatography-mass spectroscopy to compare the volatile molecules emitted by corpses prepared by a pair of N. orbicollis with those emitted by corpses that had not been touched by beetles.
Surprisingly, two sulfur compounds which were not known to be an important signal for any insect, methyl thiocyanate and methyl thioacetate, were reduced more than twenty fold by the work of beetles on the carcass. Why would they do this and how?
Methyl thiocyanate turns out to be a great signal to bury beetles looking for a dead body.
When we returned to the field and placed methyl thiocyanate next to the carcasses, over 90 percent were found burying the beetles on the first night, compared to a 0-20 percent discovery rate. for fresh carcasses without chemical supplement.
Methyl thiocyanate appears to be a heavenly scent for a beetle looking for a recently deceased mouse or rare bird somewhere in the forest that is not claimed by a predator or vertebrate scavenger.
However, once a dead body is discovered, the resident beetles face a problem. The same smells that alerted them could also reveal their scavenging price to competitors.
Burying beetles are great at detecting and responding to information, but do they also control that information?
A disinformation campaign
The transformation of a dead mouse into beetle food is amazing. After burying the corpse, the couple work day and night to remove hair, round the corpse into a ball, and apply anal secretions to the exposed skin, sliding the abdomen inward. a zigzag pattern as it surrounds the case.
Scientists used to believe that the resident burial beetle pair could sterilize the corpse, killing the microbes that release the telltale odors of the corpse hidden beneath the forest floor.
Although the secretions contain antimicrobials, they also contain microbes from the gut of the beetle. The result is a microbial community where microbes are as numerous as in an unprepared carcass, but with fewer microbial species than in the normal mix.
This artificial microbiota emits much less methyl thiocyanate and surprisingly much greater amounts of dimethyl trisulfide, the aforementioned compound associated with the intermediate stages of decomposition, where the larvae of competing fly flies render the carcass worthless to a beetle. enterror.
When we placed dimethyl trisulfide next to a fresh mouse carcass, the flying beetles were unlikely to land, apparently deterred by a scent that indicates the carcass is too decomposed to breed from burial beetles.
A pair of resident beetles makes it difficult for beetle competitors to use scents to find their carcass in two ways: by decreasing chemical attractants and misinforming rivals by increasing chemical deterrents.
When the carcasses prepared for the beetles were removed from the lab and buried in the field, they were much less likely to be discovered than corpses of similar ages that had not been prepared by the breeding beetles. Although resident burying beetles fight to the death if an intruder appears, beetles prefer to avoid the fight altogether.
The complex adaptations of animals with their microbiota are most often associated with gut microbes that aid the host’s digestion or with cultured microbes that provide food.
However, it makes sense that resource specialists, like the burying beetles that constantly encounter an external microbiota, develop similar levels of complexity.
Odors emitted by microbes are essential elements of animal communication, social interactions, sexual selection, predator-prey interactions and plant-fungus symbiosis.
While controlling microbe-derived odors by burying beetles may be one of the best examples, the ubiquity of microbes and their chemicals suggests that similar host manipulations will be common, even though humans have ignored these. adaptations and their importance.
Stephen Trumbo, Professor of Ecology and Evolutionary Biology, University of Connecticut.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
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