Ozone pollution increases male-male courtship in common fruit flies: Study

Exposure to high levels of ozone significantly increases male-male courtship among fruit flies, while making the ozone-exposed males less attractive to females, finds a new study. 

As ozone levels rise due to industrial pollution and anthropogenic activities, it can harm a number of species of insects that rely on sex-determining pheromones to find a suitable mate, it added.

Researchers from the Max Planck Institute for Chemical Ecology in Germany found that exposure to ozone leads to deterioration in levels of male pheromone in common fruit flies (Drosophila melanogasterI). 

This pheromone — cis-Vaccenyl Acetate (cVA) — produced by male Drosophila melanogaster governs sex-recognition among insects and also correlates with the male’s attractiveness to a female. 

Ozone exposure disrupts insect sexual communication was published in the journal Nature Communications March 14. 

The cVA has carbon double bonds. The oxidisation of double bonds by increased levels of oxidant pollutants like ozone is responsible for lack of sex discrimination as male pheromones become degraded, a potential challenge for insects communicating with unsaturated pheromones, the researchers wrote. 

The paper read:

Pheromone systems have evolved in preindustrial times with tropospheric ozone values as low as 10 parts per billion (ppb). However, due to the continuous emission of nitrogen oxides (NOx), volatile organic compounds (VOC) and climatic change, the ozone level has already increased to a global yearly average of 40 ppb.

Even short-term exposure to ozone levels of 100 ppb resulted in the degradation of many Drosophilid pheromones and reduced the attractiveness of males to females in 7 of 10 tested species but dramatically increased male-male courtship behaviour, according to the findings. 

This happens due to the lack of sex discrimination when male pheromones become degraded, the researchers explained. 

The scientists acquired wild-type flies (found naturally, not mutated) for study from the Bloomington Drosophila Stock Center, National Drosophila Species Stock Center and Kyoto stock Center. They found that the flies that were exposed to 100 ppb ozone for 2 hours had reduced amounts of cVA. 

Apart from cVA, many other pheromone compounds like (Z)−7-Tricosene (7-T) and (Z)−7- Pentasene (7-P)20, which are known to be involved in reproductive behaviour, were decreased after ozone exposure. 

To study the mating behaviour, they then exposed male flies either for 30 min to ozone ranging from 50-200 ppb or, as a control, to ambient air and afterwards tested their courtship behaviour and mating success with non-exposed females in a no-choice mating assay. 

They found that while there was no difference in males’ behaviour towards females in both groups, ozone-exposed males showed a longer mating latency than male controls. This means, they needed more time to become accepted by the female. 

However, groups of males exposed to 100 ppb of ozone started to court each other intensively and formed a long chain of courting males. 

“When quantifying such male-male courtship behaviour of pairs of ozone-exposed males (30 min at 100 ppb) in a no-choice assay, we indeed found a higher number of trials that resulted in male courtship compared to air-exposed control males,” the researchers said in the study. 

It was found that ozone exposure does not impede the function of pheromone responsive neurons or other levels of signal processing, but rather induces male-male courtship through the degradation of pheromones.

Even the ozone-exposed females had reduced amounts of the described female-specific compounds 7,11-Heptacosadiene and 7,11-Nonacosadiene, the scientists found. 

While pollutants like ozone and nitric oxides degrade floral volatiles, disrupting the chemical communication between plants and their pollinating insects, it can also harm insects in other ways, according to the report. 

“The exposure of flies to rather mild ozone levels that have already been reported for polluted areas degrades their unsaturated pheromones and by that affects the sexual communication in 9 out of 10 tested Drosophila species,” it read. 

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