Friday, January 24, 2014

Drosophila melanogaster

In Chapter 9 of the 4th edition of Intermediate Physics for Medicine and Biology, Russ Hobbie and I discuss how the patch clamp technique combined with genetics methods can be used to answer scientific questions. One example we consider is the potassium channel in the fruit fly.
“Gene splicing combined with patch-clamp recording provided a wealth of information. Regions of the DNA responsible for synthesizing the membrane channel have been identified. One example that has been extensively studied is a potassium channel from the fruit fly, Drosophila melanogaster. The Shaker fruit fly mutant shakes its legs under anesthesia. It was possible to identify exactly the portion of the fly’s DNA responsible for the mutation. When Shaker DNA was placed in other cells that do not normally have potassium channels, they immediately made functioning channels.”
So what is Drosophila melanogaster, and why is it significant? Horace Freeland Judson describes this famous model system in his masterpiece The Eighth Day of Creation: The Makers of the Revolution in Biology. In his Chapter 4, On T. H. Morgan’s Deviation and the Secret of Life, Judson writes
“Thinking of T. H. Morgan, one thinks first, or should, of the common vinegar fly, Drosophila, whose mutants and hybrids and their multitudinous descendants he examined for red eyes and eosin eyes and white eyes, vestigial wings or wild-type, and so on, and which he kept as best he could in hundreds of milk bottles stoppered with cotton wool. With Drosophila, Morgan discovered, for example, the mechanism by which sex is determined, at the instant of the egg’s fertilization, by the pairing of the sex chromosomes, either XX or XY, and the consequent phenomenon of sex-linked inheritance that explains, as we all also know, the appearance of disorders like hemophilia among the male descendants of Queen Victoria. And when Morgan and a student of his, Alfred Henry Sturtevant, perceived that the statistical evidence for linkage of many genes on one chromosome could be extended to map their relative distance one from another along that chromosome, then the hereditary material became palpably a string of beads, a line of points, each controlling a character of the organism.”
In The Wellsprings of Life, Isaac Asimov describes the same experiments.
“What was needed [to understand genetics] was a simpler type of organism [compared to humans]; one that was small and with few needs, so that it might easily be kept in quantity; one that bred frequently and copiously; and one that had cells with but a few chromosomes. An organism which met all these needs ideally was first used in 1906 by the American zoologist Thomas Hunt Morgan. This was the common fruit fly, of which the scientific name is the much more formidable Drosophila melanogaster (‘the black-bellied moisture-lover’). These are tiny things, only about one twenty-fifth of an inch long, and can be kept in bottles with virtually no trouble. They can breed every two weeks, laying numerous eggs each time. Their cells have only eight chromosomes apiece (with four in the gametes).

More genetic experiments have been conducted with Drosophila in the past half-century [Asimov was writing in 1960] than with any other organism, and Morgan received the Nobel prize in medicine and physiology in 1933 for the work he did with the little insect. Enough work was done with other organisms, from germs to mammals, to show that the results obtained from Drosophila studies are quite general, applying to all species.”
If you want to learn more about Drosophila, I suggest the article Drosophila melanogaster: a fly through its history and current use by Stephenson and Metcalfe (Journal of the Royal College of Physicians of Edinburgh, Volume 43, Pages 70-75, 2013). For those who prefer video, here is a great introduction to Drosophila from the Journal of Visualized Experiments, JoVE. Finally, for our 5-year-old readers (or the young at heart), you can purchase a Drosophila melanogaster plush toy here for just ten dollars.

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