A quick and simple introduction to Drosophila melanogaster

What is it and why bother about it?

Drosophila melanogaster is a fruit fly, a little insect about 3mm long, of the kind that accumulates around spoiled fruit. It is also one of the most valuable of organisms in biological research, particularly in genetics and developmental biology. Drosophila has been used as a model organism for research for almost a century, and today, several thousand scientists are working on many different aspects of the fruit fly. Its importance for human health was recognised by the award of the Nobel prize in medicine/physiology to Ed Lewis, Christiane Nusslein-Volhard and Eric Wieschaus in 1995.

Why work with Drosophila?

Part of the reason people work on it is historical - so much is already known about it that it is easy to handle and well-understood - and part of it is practical: it's a small animal, with a short life cycle of just two weeks, and is cheap and easy to keep large numbers. Mutant flies, with defects in any of several thousand genes are available, and the entire genome has recently been sequenced.

Life cycle of Drosophila

The drosophila egg is about half a millimeter long. It takes about one day after fertilisation for the embryo to develop and hatch into a worm-like larva. The larva eats and grows continuously, moulting one day, two days, and four days after hatching (first, second and third instars). After two days as a third instar larva, it moults one more time to form an immobile pupa. Over the next four days, the body is completely remodelled to give the adult winged form, which then hatches from the pupal case and is fertile within about 12 hours. (timing is for 25°C; at 18°, development takes twice as long.)

Research on Drosophila

Drosophila is so popular, it would be almost impossible to list the number of things that are being done with it. Originally, it was mostly used in genetics, for instance to discover that genes were related to proteins and to study the rules of genetic inheritance. More recently, it is used mostly in developmental biology, looking to see how a complex organism arises from a relatively simple fertilised egg. Embryonic development is where most of the attention is concentrated, but there is also a great deal of interest in how various adult structures develop in the pupa, mostly focused on the development of the compound eye, but also on the wings, legs and other organs.

The Drosophila genome

Drosophila has four pairs of chromosomes: the X/Y sex chromosomes and the autosomes 2,3, and 4. The fourth chromosome is quite tiny and rarely heard from. The size of the genome is about 165 million bases and contains and estimated 14,000 genes (by comparison, the human genome has 3,400 million bases and may have about 22,500 genes; yeast has about 5800 genes in 13.5 million base bases). The genome was (almost) completely sequenced in 2000, and analysis of the data is now mostly complete. Several other insect genomes have now been sequenced, including many Drosophila species, and the genomes of mosquito and honey bee, and these are starting to show what is common among all insects, and what distinbuishes them from each other.

Polytene Chromosomes

These are the magic markers that first put Drosophila in the spotlight. As the fly larva grows, it keeps the same number of cells, but needs to make much more gene product. The result is that the cells get much bigger and each chromosome divides hundreds of times, but all the strands stay attached to each other. The result is a massively thick polytene chromosome, which can easily be seen under the microscope.

polytene picture Even better, these chromosomes have a pattern of dark and light bands, like a bar code, which is unique for each section of the chromosome. As a result, by reading the polytene bands, you can see what part of the chromosome you are looking at. Any large deletions, or other rearrangements of part of a chromosome can be identified, and using modern nucleic acid probes, individual cloned genes can be placed on the polytene map.
The standard map of the polytene chromosome divides the genome into 102 numbered bands (1-20 is the X, 21-60 is the second, 61-100 the third and 101-102 the fourth); each of those is divided into six letter bands (A-F) and those are subdivided into up to 13 numbered divisions (the picture above shows band 57). The location of many genes is known to the resolution of a letter band, usually with a guess to the number location (e.g. 42C7-9, 60A1-2). The polytene divisions don't have exactly the same length of sequence in them, but on average, a letter band contains about 300kb of DNA and 15-25 genes.

Finding out more

Many biology and develomental biology textbooks have chapters on Drosophila. There are fewer books devoted specifically to Drosohphila; here are some (links are to the Amazon.com online bookstore):

Descriptive

The Making of a Fly (Peter Lawrence; Blackwell Scientific, 1992).: Probably the easiest simple introduction to Drosophila development and quite readable.
Drosophila (Brian Shorrocks; Ginn & Co, London, 1972): An interesting slim volume looking at the general biology of the fly, with chapters on laboratory and field ecology, simple genetics, behaviour and techniques, from a zoological perspective. Probably out of print, but should be in many academic libraries.
The Development of Drosophila melanogaster (ed. Bate & Martinez-Arias; Cold Spring Harbor Press, 1993): The monster 'blue book', running to over 1,000 pages is a major summary of current knowledge on most developmental aspects of Drosophila; definitely not for the beginner, and utterly overpriced (about US$300, though CSH have recently been selling it at a steep discount). The enclosed picture "Atlas of Drosophila development" is a very useful tool and is available separately.
Biology of Drosophila (ed. M. Demerec; Cold Spring Harbor Press, 1994).: This reprint of the classic 'Demerec' book of 1950 covers a great deal of descriptive biology of the fly, and is available for about USD$40, quite a bargain for an academic book.
The Embryonic Development of Drosophila melanogaster (Campos-Ortega & Hartenstein; Spring-Verlag, 1985): A dense, sometimes hard-to-read but detailed and exhaustive account of embryonic development, the 'green book' as it is called is a standard reference. Here's a review page from the second edition.

Techniques

Drosophila, a Practical Approach (ed. DB Roberts; IRL Press, 1986).: Edited book covering such techniques as lab culture, molecular biology and mutagenesis.
Drosophila, a Laboratory Manual (Michael Ashburner; Cold Spring Harbor Press, 1989): Recipe book for many techniques, ranging from the popular to the obscure.
Drosophila melanogaster: Practical Uses in Cell and Molecular Biology (ed. Goldstein & Fryberg; Academic Press 1994): Excellent coverage of cell biological techniques in Drosophila with both recipies and explanations in 37 specialist chapters, which as usual, vary in their popularity. This is Vol. 44 in the Methods in Cell Biology series.
Fly Pushing : The Theory and Practice of Drosophila Genetics by Ralph Greenspan: Excellent and humorous guide to working with flies in genetic experiments. I wish this was published when I started fly work! Highly reccomended for anyone doing lab work with flies.

Images and Movies

The Lives of a Fly by Mary Tyler is a VHS video which gives a general introduction to fly development.
Drosophila images (through FlyBase).
Animations of Embryogenesis, by morphing of SEM images, by Thom Kauffman & Rudi Turner.

Feedback, please!

This page is intended as a general introduction for newcomers to Drosophila. If you either know a lot or a little about Drosophla and have any suggestion for improvement, or would like to write an article on any area of Drosophila biology for the Virtual Library, please let me know.

http://ceolas.org/VL/fly/intro.html
by Gerard Manning
Last update: Jul 12, 2008
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