At first glance vinegar flies and humans might not seem to have much in common, but both have a taste for alcohol. Some ancient humans used alcohol (ethanol) produced by yeast fermentation to sterilise water and James Fry from the University of Rochester explains that vinegar flies probably exploit alcohol's disinfectant properties for protection also. However, not all vinegar flies are equally resilient to alcohol. Flies that live in temperate latitudes, such as Europe, tend to be more alcohol resistant than flies from Africa, but it wasn't clear why temperate flies hold their alcohol better than their tropical cousins. According to Fry there are two possible explanations: one, that temperate flies are better at breaking down the toxin; and another, where temperate flies are simply less sensitive to alcohol. ‘I wanted to look at the physiological basis of the difference,’ says Fry (p. 3996).
First, he exposed flies to simulated rotting fruit (cotton wool soaked in a weak sucrose solution with varying levels of ethanol) over 2 days to find out how much alcohol the flies could safely ingest and how much accumulated in their bodies. Measuring the flies' body alcohol levels, Fry could see that the levels in African flies were 2-3 times higher than those of the European flies; ‘the African flies were breaking it down much less quickly’, says Fry. And when Fry 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, helping the temperate insects to destroy the toxin more quickly.
But Fry explains that there is more to alcohol tolerance than simply destroying the toxin. ‘The general complication is that you could make something more toxic when you enzymatically change a toxin,’ says Fry, and 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,’ says Fry. He also explains that previous genetic studies had shown that African flies that bred with European flies to produce offspring with two African chromosomes and one European chromosome gained much of the alcohol resistance of European flies, despite retaining the weaker African ADH and ALDH enzymes. Was the European chromosome making the flies less sensitive to alcohol or was it making them better at coping with the toxic by-products of detoxification?
Fry obtained a specialised population of flies that could not detoxify alcohol and added the European chromosome. If the European chromosome was making the European flies less sensitive to alcohol, then the chromosome would protect them from alcohol accumulation. However, if the chromosome was somehow helping them to cope with the toxic by-products of alcohol detoxification, the flies would be as sensitive to alcohol as flies that had the African version of the chromosome. Measuring the flies' survival rates, Fry found that both types of fly were equally sensitive to alcohol.
So, the European chromosome helps the flies to be more resistant to alcohol by helping them to manage the toxic by-products of alcohol detoxification. Also, when Fry exposed the Europeanised flies to one of the by-products – acetic acid – they were more resistant to the toxin than the flies with the African chromosome, suggesting that European vinegar flies are better at coping with this alcohol break-down product to help them hold their liquor.