The growing presence of the fruit fly

One of the most important holiday traditions is a full house clean before departure. A less than thorough effort risks not only being judged by any burglars who might happen to pop in but also other uninvited guests. Returning home from a few nights away, I opened the recycling bin to find the remains of a banana and a cloud of shimmering, coppery flies.

Once established, these tiny pests are difficult to evict. Commonly known as the fruit fly, Drosophila melanogaster reproduces extremely quickly. Depending on the ambient temperature they can develop from egg to mature adult in around a week, at which point females can lay 100 eggs in a day.

This remarkable fecundity generally leads to irritation for homeowners trying to eliminate them, but it also makes fruit flies one of the most important organisms for our understanding of genetics.

A particular frustration with fruit flies is that they seem to appear out of nowhere. Often this is because tropical fruit arrives pre-loaded with their eggs, which, if left undisturbed in warm temperatures, will hatch. In premodern times, it was supposed that living organisms could arise spontaneously from nonliving matter.

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This theory, known as abiogenesis or spontaneous generation, suggested that maggots could spontaneously generate from decaying meat, mice could emerge from piles of wheat, or frogs could develop from mud. The appearance of fruit flies on fruit would not have been surprising but instead would have added evidence to the theory.

Throughout medieval Europe, the most read philosopher was Aristotle, and his promotion of abiogenesis meant that it was commonly accepted. Studies of larger animals meant that their reproductive cycles were understood, but invertebrates were still suspected of spontaneously generating. In the late 17th century the Italian scientist Francesco Redi devised a series of experiments to challenge this prevailing belief.

Redi placed meat in open jars, attracting flies. Soon the meat was swarming with maggots, and these hatched into flies. However, when flies were blocked by a jar lid, no maggots appeared. In a similar experiment, a gauze cover allowed air but not flies to enter; the cover was soon crawling with maggots laid by optimistic flies, but the meat remained untouched. Redi therefore demonstrated that flies were necessary for maggots and therefore more flies, and that a lack of air was not preventing spontaneous generation.

The last frontier of abiogenesis was microorganisms. Bad air was often identified as the culprit behind diseases such as malaria, which in Latin literally means bad air. In the 1860s Louis Pasteur conducted experiments with a nutrient-rich broth and swan-necked flasks.

Pasteur heated the flasks to kill any microorganisms already in the broth, then snapped the neck off some. Those with the intact necks did not spoil, while those with no neck did. Germs were not easily able to get through the swan neck and so the broth remained unspoilt.

In 1910, Thomas Hunt Morgan was studying fruit flies at Columbia University when he noticed that one fly among the thousands he had bred had white rather than red eyes. Intrigued, he bred this mutant and noticed that of its offspring only the males had white eyes. Subsequent experiments showed that males were more likely to have white eyes, but females could also inherit the trait.

Females have two x chromosomes, the carriers of genetic material, and males have one x and one y chromosome. If the trait for white eyes was only on the y chromosome, then no females could inherit it. Morgan’s fruit fly studies showed that the trait was on the x chromosome, and suggested that females could only inherit a trait on both x chromosomes.

[ What’s the story with all the fruit flies – and how do you get rid of them? ]

Today, fruit flies remain an essential component of genetic studies. Because their life cycles, genetic make-up, and reproduction are so well understood, they are a model organism. Their easily observable phenotypes and short lifespans allow scientists to trace genetic traits through multiple generations and explore patterns of inheritance.

By exposing fruit flies to various environmental stressors and mutagens, scientists have identified genes responsible for adaptation, illuminating the mechanisms that underpin evolutionary responses to changing environments. The characteristics that make them such a persistent menace have made them essential for understanding how and why life develops. As a historian of evolutionary theory, I’ll salute them. As soon as I get them out of my kitchen.

Stuart Mathieson is a postdoctoral fellow in the School of History and Geography at Dublin City University