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Fruit Fly Culture

August 24, 2015

"Culture" is one of those words with multiple meanings. The word, "mole" means something different to a chemist and a gardener. One meaning of "culture" is used by biologists to express the cultivation of organisms for research purposes. Thus, we have cell culture, tissue culture, microbiological culture, and viral culture. The word can apply, also, to raising the common fruit fly (Drosophila melanogaster) for genetics experiments.

Drosophila melanogasterDrosophila melanogaster, female (left) and male (right).

(Via Wikimedia Commons.)

The other meaning of culture, the one that most people bring to mind upon hearing the word, is the way members of a particular group lead their lives. It's fun to imagine what the title of this article, "Fruit Fly Culture," would mean in that context. Just as our culture has its gourmands, we can imagine a fruit fly food blogger extolling the pleasures of one type of banana over another. I've always preferred Chiquita to Dole.

Fruit flies have been used in genetics experiments for a century, long before the chemical nature of genetics was discovered by James Watson and Francis Crick. A colleague of mine once called YIG (yttrium iron garnet, Y3Fe5O12) the "fruit fly of magnetism," principally because it can be modified in many ways, and it has an arrangement of its atoms and their magnetic directions that makes analysis easy.

Early genetics experiments involved irradiating fruit flies with X-rays to mutate their offspring. They were ideally suited to this task, since they have a generation time of only ten days, and the females lay many eggs per day. Of course, to do such experiments, you need a stock of fruit flies.

The 1989 film, Field of Dreams, is noted for the memorable line, "If you build it, he will come." This is true, also for fruit flies, since any piece of ripe fruit will be quickly colonized by fruit flies. Geneticists took advantage of this by placing banana mush in glass milk bottles to attract some fruit flies, and then sealing them to keep the population intact. In the early days of the field, glass milk bottles of a convenient half-pint (~250 milliliter) size were common.

A one quart US milk bottle, circa 1956A one quart US milk bottle, circa 1956.

Elbridge Amos Stuart, a co-founder of the evaporated milk producer, Carnation, believed that cows pastured in a nice environment ("contented" cows) gave better milk. For many years, Carnation used the slogan, "Carnation Condensed Milk, the milk from contented cows."

(Portion of a photo by Joseph Tylczak, via Wikimedia Commons.)

Just as species go extinct, so do certain products and professions. Milk delivery men ("milkmen") were still common in my childhood, but that profession, with its reusable glass bottles, starting into extinction in the 1960s with the rise of the supermarket. At that point, milk appeared in non-reusable waxed paper and plastic containers.

At the start of the 1980s, there was a shortage of milk bottles for Drosophila culture. Geneticists scrambled to secure supply of their staple item, and they even tried to arrange for a group purchase. This situation is much like a recent problem mathematicians are having with chalk.[2] As reported in gizmodo, mathematicians prefer to use a traditional blackboard for their ruminations using a particular chalk that's been manufactured for 80 years by aJapanese company, Hagoromo Bungu.

This mathemagical chalk is less dusty than others, and it's sturdier, but the company has been sold to a larger office supply company, and the formulation has changed. Although whiteboards have replaced most blackboards, there are advantages to chalk. There's a clear indication when your writing medium is depleted, and nasty chemicals are not required for erasure. Some mathematicians are reported to be hoarding their chalk, but there will be a tipping point sometime in the future.[2]

Eventually, geneticists transitioned to more expensive, but available, items from laboratory supply houses. Since much laboratory "glassware" was transitioning to substitute materials such as polypropylene and polystyrene, polymer fruit fly culture bottles became common. As it turns out, this simple change of material might not be so simple after all.

Because of the atomic nature of matter, materials will develop an electrical charge when they come in contact with other materials. I wrote about this triboelectric effect in a previous article (Triboelectric Generators, July 25, 2012). The tendency for charges to develop is governed by the triboelectric series. While glass and polypropylene might seem to be equivalent as container materials, their tendency to donate or acquire electrons is considerably different, as can be seen in the following figure.

The triboelectric series.
The triboelectric series, drawn using Inkscape from Wikipedia data.)

Scientists and engineers from the University of Southampton (Southampton, UK), Al Baha University (Al Baha, Saudi Arabia), Hokkaido University (Sapporo, Japan), and the Japan Science and Technology Agency (Kawaguchi, Saitama, Japan) have discovered that Drosophila avoid electric fields, and their avoidance is a function of the field strength.[3-4] Chronic exposure to electric fields was found to induce neurochemical changes in their brains and changes in biogenic amine levels.[3]

The root cause of this behavior appears to be physical forces on the wings arising from the charge. This was verified by observing how the wings of stationary flies could be manipulated by an electric field. Experiments on excised wings showed that these forces could lift the wings, and the smaller wings of the males were lifted at lower fields than the larger female wings.[3] As Philip Newland, a professor of neuroscience at the University of Southampton and lead author of the study, explains,
"When a fly was placed underneath a negatively charged electrode, the static field forces caused elevation of the wings toward the electrode, as opposite charges were attracted... Static electric fields are all around us but for a small insect like a fruit fly it appears these fields' electrical charges are significant enough to have an effect on their wing movement and this means they will avoid them if possible."[4]

Such forces on their wings appears to agitate the flies, and this discomfort is shown in the changes to their brain chemistry by increased levels of octopamine, the fruit fly version of human noradrenaline. The increase in this chemical signals stress and aggression. There were also decreased levels of dopamine, so the flies would be more responsive to external stimuli.[4]

Since plastic containers can hold significant charge for long periods, such an effect might cloud the results of some fruit fly studies.[4] As Newland explains,
"Fruit flies are often used as model organisms to understand fundamental problems in biology... 75 per cent of the genes that cause disease in humans are shared by fruit flies, so by studying them we can learn a lot about basic mechanisms."[4]

This effect might be useful as a means of pest control by blocking the ingress of flying insects into homes and greenhouses. It may also have an environmental consequence by modifying the behavior of pollinators, such as bees, near power lines.[4] I wrote about power line electric fields in a recent article (Ionized Air, July 9, 2015).

References:

  1. Field of Dreams, 1989, Phil Alden Robinson, Director, on the Internet Movie Database.
  2. Sarah Zhang, "Why Mathematicians Are Hoarding This Special Type of Japanese Chalk," Gizmodo, June 15, 2015.
  3. Philip L. Newland, Mesfer S. Al Ghamdi, Suleiman Sharkh, Hitoshi Aonuma, and Christopher W. Jackson, "Exposure to static electric fields leads to changes in biogenic amine levels in the brains of Drosophila," Royal Society Proceedings B, vol. 282, no. 1812 (July 29, 2015), DOI: 10.1098/rspb.2015.1198.
  4. Electric fields signal 'no flies zone,' University of Southampton Press Release, July 31, 2015.

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