Eukaryotes Genomes - DROSOPHILA MELANOGASTER

Drosophila melanogaster is a fruit fly and one of the most studied species from the family Drosophilidae.

Drosophila has a long history as a model for genetic study. Research began on the fruitfly over one hundred years ago when biologist, Thomas Hunt Morgan was a studying Drosophila early in the 1900's and discovered a mutant fly with white eyes. Morgan was the first to discover sex-linkage and genetic recombination, which placed the small fly in the forefront of genetic research. Research based on this observation resulted in the discovery of chromosomes and won Morgan the Nobel Prize in 1933. Since then Drosophila research has contributed many times to developmental biology and biochemistry, today, several thousand scientists are working on many different aspects of the fruit fly and 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.

Part of the reason scientists work on Drosophila is historical - (so much is already known about it that it is easy to handle and is well-understood) and part of it is practical:-

Fruit flies are easily obtained from the wild and most biological science companies carry a variety of different mutations.Costs are relatively low and most equipment can be used year after year. They are small and easily handled, they have a short life cycle of just two weeks and they are cheap and easy to keep large numbers. They are fecund; a female may lay hundreds of fertilised eggs during her brief life span. The drosophila egg is about 0.5mm 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 25C; at 18, development takes twice as long.) The resulting large populations make statistical analysis easy and reliable.

Drosophila can easily be anaesthetised and manipulated with very unsophisticated equipment, are sexually dimorphic (males and females are different), making it is quite easy to differentiate the sexes. It is easy to obtain virgin males and females, as virgins are physically distinctive from mature adults. The Drosophila embryo grows outside the body and can easily be studied at every stage of development. The blastoderm stage of the embryo is a syncytium (thousands of nuclei unconfined by cells) so that, for example, macromolecules like DNA injected into the embryo have easy access to all the nuclei. The genome is relatively small for an animal (less than a tenth that of humans and mice). Mutations can be targeted to specific genes.

Originally, Drosophila was mostly used in genetic research, 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.

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.

The Drosophila genome was sequenced in early 2000 by a consortium of over 30 private and public research groups, and was the first complex genome sequenced using a "shot-gun" approach - which involves randomly cloning the DNA, sequencing it and then producing a contiguous sequence using computers.

Researchers have found that the underlying biochemistry of fruit flies and humans is remarkably similar, therefore fruit flies can provide clues to understanding human diseases caused by defective genes. Human tumor-suppressing genes can be seen in flies easier than in mouse data pointing out that experiments can be done using fly genes that would be impractical (or unthinkable) using human subjects. Especially useful is the identification of networks of other genes that interact with known disease genes, and their associated metabolic pathways. The implications for medicine are immediate. A recent transgenic fly, for example, is proving invaluable in the study of the pathology of the complex human disease, Parkinson's disease. To this end researchers are continuing to refine the D. melanogaster sequence already produced.

References:

http://www.ceolas.org/fly/intro.html
http://www.sanger.ac.uk/Projects/D_melanogaster/
http://www.ensembl.org/Drosophila_melanogaster/
www.nature.com/genomics/papers/
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/D/Drosophila.html
http://biology.arizona.edu/sciconn/lessons2/Geiger/intro.htm