Flies die sooner if they see dead flies

Fruit flies exposed to dead flies of their species age faster, and researchers have identified the precise brain cells that respond to the sight of corpses and trigger this rapid ageing, ultimately leading to an early death.

Christi Gendron and Scott Pletcher at the University of Michigan, Ann Arbor, and their colleagues have been studying how experiences produce physiological changes that affect lifespan. They originally set out to discover if fruit flies (Drosophila melanogaster) respond to other flies being ill. “We didn’t see a response until the infection killed them,” says Pletcher. “That’s how we stumbled on it.”

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Flies exposed to dead flies for 48 hours typically lived around 45 days, compared with over 60 days for flies not exposed to corpses, the team found. By separating the living flies from the dead ones with glass, the team went on to demonstrate that the mere sight of dead flies triggers behavioural changes.

While it might seem surprising that fruit flies are capable of recognising dead members of their own species, lots of animals can do it, including some other insects, says Pletcher.

Now, the team has identified the part of the fruit fly brain involved in perceiving dead flies, using a genetic technique that makes neurons fluoresce when they are activated. The finding was confirmed by showing that inhibiting these neurons blocks the effect on lifespan. “It’s remarkable because it’s just a handful of neurons, like 20 or so,” says Gendron.

The accelerated ageing is due to these neurons affecting signalling related to the hormone insulin. The details aren’t understood yet, but it is known that production of insulin and responses to it affect ageing in many species, including ours.

The team can’t say for sure why flies respond in this way. “Our speculation is that the animals are tuned to perceive dangers in the environment,” says Pletcher. “For us, if we detect danger there is a heightened state of anxiety.”

In other words, the sight of dead flies might make flies stressed and this stress shortens their lives. The flies’ inability to escape from the corpses might further increase this stress, says Gendron.

The team also found that activating a subset of the neurons made flies live longer. The reason for this is unclear, but this subset might have the opposite effect and reduce stress, they suggest.

Gendron and Pletcher hope that by understanding what is happening in fruit flies, they will gain insights that are relevant to mammals and perhaps one day help people such as soldiers and health workers who are often exposed to dead people.