Fruit flies have a taste for alcohol. We already know that they’re to thank for our pleasing beer aromas. And as it turns out, some fruit flies can hold their liquor better than others, thanks to a naturally evolved ethanol resistance. The findings were published in the Journal of Experimental Biology this week.
The cosmopolitan Drosophila melanogaster feed on and breed in decaying fruit, which can contain ethanol concentrations as high as seven percent. Previous studies have shown that those living in temperate regions like Europe are more resistant to ethanol poisoning than those who live in the tropics. Although the physiological basis of this difference is unclear.
So, James Fry from the University of Rochester exposed low levels of ethanol vapor to flies from a tropical African population and flies from a European population using cotton wool soaked in a weak sucrose solution. The tropical flies accumulated up to three times more internal ethanol than the European ones, who can metabolize ethanol faster. When he tested the activity levels of two enzymes that break down alcohol -- alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH) -- both enzymes were more active in the European flies than the African flies.
But there’s more to alcohol tolerance than just destroying the toxin. “You could make something more toxic when you enzymatically change a toxin,” Fry explains to Inside JEB. In the case of alcohol, this means producing acetaldehyde and acetic acid. “Acetaldehyde is very toxic, if you order it from a chemical company, it has a skull and cross bones on it. Acetic acid on the other hand is in vinegar, which is not as toxic.”
Using lineages of flies that were the same except for their third chromosome, Fry showed that faster ethanol elimination can’t fully explain the resistance difference. Relative to African third chromosomes, European third chromosomes offer higher ethanol resistance but without affecting internal ethanol concentrations. Maybe the European chromosome was helping them cope with the byproducts of alcohol detoxification, like acetic acid.
So, Fry added the European chromosome into a population of flies who couldn’t detoxify alcohol. If the European chromosome makes the flies less sensitive to alcohol, then the chromosome should protect them from ethanol accumulation, Inside JEB explains, but if the chromosome helps them cope with the byproducts, then the flies would be as sensitive to alcohol as flies with the African chromosome. Sure enough, the two flies were equally sensitive to alcohol, which means that the higher ethanol tolerance conferred by the European third chromosome is thanks to an increased resistance to the potentially deleterious effects acetic acid. In mice, Science reports, acetic acid appears to cause hangovers.