SCIENCE: UNDER THE MICROSCOPE: LEWIS WOLPERT TALENT SPOTS FOR THE LABORATORY

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MODEL SYSTEMS matter in biology. The living world is so diverse that if everyone studying, for example, genetics or embryonic development used different animals then comparing results would be very difficult (even though there will undoubtedly be basic similarities). Then there is also the issue of convenience - to work on a system it needs to be kept easily in laboratory conditions. There are exceptions like sea urchin embryos which are lovely for studying development - not only are there millions of eggs in the breeding season but the embryos are transparent, so one can follow the movement of individual cells during growth.

In my own subject there are a number of key model systems. The use of chick eggs goes back to Aristotle, and frogs have been widely used. But neither are good for combining genetics with development because of their long breeding cycles. Today's system is the nematode worm, introduced some 35 years ago by Sydney Brenner. The nematode has a rapid life cycle and a fixed number of cells - just 959. Aquaria of zebrafish now fill many rooms in biology departments for it has become the favoured animal for studying how genes control the development of vertebrates. These two model systems are for development what the fruit fly Drosophila was for genetics. Horses for courses.

The early days of molecular biology relied on E coli and much of cellular genetics now uses yeast. But it is Drosophila that has transformed our understanding of the genetics and development of multicellular organisms.

Drosophila owes its success and fame to US biologist Thomas Hunt Morgan, who won a Nobel Prize in 1933. Morgan was interested in what caused the variations postulated by Darwin in his theory of evolution and so wanted to work on heredity. He experimented on chickens, pigeons and mice, but progress was slow until he found Drosophila. He was fortunate - the best scientists always are - that techniques for keeping and studying the fly were already in progress. It was just the organism he wanted, easy to breed and with a rapid cycle; it was as if God had created Drosophila especially for Morgan.

What, one may wonder, would Morgan make of the most recent use of his wondrous experimental system? "The Fruit Fly: A Model Organism to Study the Genetics of Alcohol Abuse and Addiction" is the title of a paper in the prestigious journal Cell. Most of the targets of alcohol are on the cell membrane in the brain and this results in changes in the activity of certain nerve cells. Individuals who are less sensitive to alcohol at a young age are more prone to alcoholism than those who are more sensitive. There is a strong genetic component to alcoholism but no genes have been identified yet. The fly may provide clues.

Researchers have built an inebriometer, that enables them to measure the flies' ability to cope with alcohol. The flies are put into a long glass column into which alcohol vapour is pumped. As the flies become inebriated they become uncoordinated and roll down the column. The more they are able to resist the intoxicating effects of the alcohol the longer they take to roll down the column. The amounts of alcohol that make them drunk are similar to those in humans, about a 10th of a per cent concentration in the blood. So now they can look for flies which are resistant to the alcohol or particularly sensitive.

They have already identified a gene, called amnesiac, that makes the flies more sensitive; the gene codes for a signalling molecule rather like a hormone. Soon they will find genes that make the flies resistant and will lead to a much better understanding of addiction to alcohol. Morgan would be excited.

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