Air pollution destroys fruit flies’ ‘love letters’, causing them to accidentally reproduce with the wrong partner

New research shows that air pollution can be particularly harmful to fruit flies. The flies can eventually become so confused that they can no longer distinguish their peers from other types of fruit flies and mistakenly mate with the wrong fly.

Scientists have just discovered a new consequence of air pollution. For example, air pollution appears to affect the pheromones of fruit flies. Pheromones are volatile molecules that individual fruit flies use to convey messages to members of their own species. For example, they use pheromones to attract a mate. The fruit flies can ‘read’ from each other’s pheromones whether the fruit fly they are facing belongs to the same species and is therefore suitable as a partner.

But air pollution – or more precisely: a high ozone concentration – appears to affect these pheromones. This means that fruit flies can no longer determine whether or not another fruit fly is of the same species, so there is a chance that they will accidentally mate with a fruit fly that belongs to a different species. In that case there is hybridization: a cross occurs between two species. Scientist Nanji Jiang contributed to the research. Jiang says: “We know Previous research all that ozone can have a major influence on insects’ choice of mate. During this research we wanted to find out what effect ozone has on the reproductive barriers between different species. This research has shown us that even a small increase in atmospheric ozone can cause fruit flies to hybridize. This is worrying because it could lead to shrinking insect populations because the resulting hybrids are infertile.” The research has been published in the journal Nature Communications.

For the research, the scientists used four different fruit fly species: Drosophila melanogaster, Simulating Drosophila, Drosophila mauritiana in Drosophila sechellia. These four species all use pheromones that are very similar, but mix them in their own way so that they can (normally) be distinguished – and fruit flies can therefore deduce from those pheromones whether they are dealing with a fellow species or not. Many of these pheromones contain double bonds between carbon atoms. But these can be broken by ozone, causing some pheromones to become confusing, the researchers show in their experiment.

For the experiment, specimens of the four different species were first exposed to air with an increased ozone concentration for two hours. This concentration was the same as could be measured during a warm summer day. The scientists then made groups of three fruit flies. In each case, a female and a male of the same species were paired with a male of a different species. After a few hours had passed, the females were collected and allowed to lay eggs. After hatching these eggs, the male genitalia of the offspring were examined to determine which species the female had ultimately mated with. The same experiment was repeated for the study, but with fruit flies that had not first been exposed to an increased ozone concentration. The results of the research clearly show that hybridization occurs more often when fruit flies have been exposed to higher ozone levels.

Unheard song
It is quite surprising, the researchers think. They point out that many fruit fly species do not only rely on pheromones for their reproduction, but also rely on other signals. There are many species that sing their own ‘song’ by vibrating their wings. However, the scientists discovered that these additional tools were of no benefit. Fellow scientist and team member Bill Hansson contributed to the study. Hansson explains: “To our surprise, we discovered that some females were no longer able to recognize the correct male at all – despite the fact that acoustic and visual signals were also present (in addition to the pheromones, ed.).”

The results of the study are significant because many hybrid fruit fly species are not very fertile. It is important to note that there is a difference between hybrid males and hybrid females. Hybrid females are often fertile, while hybrid males are often infertile – or at best considerably less fertile – than ‘normal’ male fruit flies. Hybrid males can therefore play no – or only a very limited – role in the survival of their species and – if there are many of them – ultimately even contribute to its extinction, the researchers warn.

“The family Drosophila consists of more than 1,500 different species,” says researcher Markus Knaden. “We know that there are more than a hundred species that are so similar that hybridization could occur.” There are plenty of opportunities for fruit flies to – by mistake – mate with the wrong species, produce infertile offspring and thus harm their population. And reason enough to take action and combat air pollution, Knaden emphasizes. “We know that insect populations will shrink significantly in the future. We must therefore do everything we can to slow this decline as much as possible.”