Air pollution impairs successful mating of flies  

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Air pollution causes about 12% of all deaths worldwide. It is defined as "the combination of outdoor and indoor particulate matter and ozone" (Ritchie & Moser, 2017, 2021), and it affects tens of millions of people. But it also affects plant and animal life on the planet. There are 10 quintillion (a huge number) insects on Earth, but to make it easier to understand, figure that there are 1.4 billion for every person. Insects are important in many roles in the ecosystem, such as plant pollinators, consumers of plants, bacteria, dead organisms, and organic remains, and sources of food for other creatures. What would we do without them? A recent study has shown that air pollution may be killing them off in a unique way.

A team of eight researchers at the Max Planck Institute for Chemical Ecology in Germany exposed common fruit flies to ozone, one component of air pollution, and that caused the chemicals in male flies called sex pheromones to degrade. The result was twofold:

• female flies were no longer attracted to the males for reproduction
• other male flies were attracted to males enough to attempt reproduction with them

There is more than one type of sex pheromone in flies. One that is volatile (evaporates quickly) is the one studied by the researchers. It is produced by bacteria in the gut of male flies and is spread on surfaces or onto female flies when it is excreted. Normally, the specific odor attracts females to the males, and during mating its smell remains on the females, so other males are confused and think they are males and don't mate with them.

Male fly releasing pheromone. Image by Benjamin Weiss (Current Biology)

Volatile chemicals like certain sex pheromones are detected by two structures on the head of the fly: the two stubby antennae and two hairy maxillary palps under that.

Image from How Stuff Works

The antennae act as sensors to detect pheromones at long distances (40-100 cm, or 15-39 inches), but the maxillary palps are needed to assist the antennae when the pheromone is close (<40 cm, or <15 inches). Each of these organs is covered with microscopic thorny projections appropriately called sensilla, that contain neurons which carry the scent signal to a large bundle of cells called the glomerulus for further processing of the type of pheromone sensed.

How flies smell beginning with the antenna (from Neurobiology of Chemical Communication)

Japanese researchers found a way to label specific neurons in flies with a bioluminescent tag that lights up when the fly nears the chemical it senses. Watch this cool video to see it in action.

Video from Current Biology paper, 2018

But when the males were exposed to ozone, the chemical bonds in the pheromone were broken, so it could not act properly to stimulate the flies. Instead, females were not as attracted to males anymore, but other  males were! They displayed unusual mating behaviors as shown below.

Photos courtesy of Benjamin Fabian (Max Planck Society)

They found that seven out of eight species of house flies reacted like that. The one that didn't show this strange behavior is known to excrete a different type of sex pheromone, though. This ozone-generated effect is definitely related to the fly's sense of smell, because one species which doesn't have pheromones does its mating rituals based on visual cues, and it was not affected at all by increased ozone levels.

So, will flies with sex pheromones die off and go extinct, only to be replaced by those that use different or no pheromones? Or will they (more specifically, the bacteria in their gut that makes the pheromone) evolve to produce ozone-resistant pheromones? What about other insects that use pheromones, like ants or bees? Do we have to worry about them in the same way as we do flies? More research is needed, but this study shows at least a potential cause for concern beyond the effects of air pollution on our own bodies.