Pheromones help flies and other insects identify suitable mates

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Some insects may struggle to find mates of the same species because increasing levels of ozone pollution are breaking down the smells released by potential partners.

Ground-level ozone is a greenhouse gas that is formed when vehicle emissions react with other gases in the air. Levels of the pollutant increase in the summer because sunlight and warmth trigger more of these reactions.

Last year, Markus Knaden at the Max Planck Institute for Chemical Ecology in Germany and his colleagues found that ozone interacts with insect chemical signals called pheromones, which play a key role in mating. They found that male flies became less attractive to females as a result of elevated ozone in the air.

To follow on from that work, the team investigated if this pheromone degradation could affect flies’ ability to distinguish between different species.

The researchers chose to focus on four closely related species of fruit fly: Drosophila melanogaster, Drosophila simulans, Drosophila sechellia and Drosophila mauritiana. Males and females across these species were exposed to high levels of ozone, comparable to conditions on a hot day in a city, for 2 hours. The researchers then gave females the choice to mate with a male of either the same or a different species.

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After exposure to ozone, hybrid offspring were produced around 70 per cent of the time, while the figure was just 20 per cent in a control group that was exposed to ambient air.

The rate of mismatches was highest in D. simulans, which showed no signs of being able to tell species apart after exposure to ozone, even from visual or auditory cues.

“Hybrids are very often sterile,” says Knaden. “So flies invest a lot into their offspring, but the offspring cannot transfer their genes into future generations.”

That means rising levels of surface ozone could exacerbate the devastating decline of insects around the world, he says. “There are more than 1500 insect pheromones that have been chemically described right now, and 90 per cent of them have carbon-carbon double bonds [which can be destroyed by ozone].”

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