Locations and Developmental Fates of Imaginal Discs and Imaginal Tissues in the Third Instar Larva of Drosophila
What is it and why bother about it?
Drosophila melanogaster is a fruit fly, a tiny insect about 3mm long. It is one of the most useful organisms in biological research, which aid in the discovery of genetics and development 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.
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.
http://www.ceolas.org/fly/intro.html
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