An important landmark in the history of science was reached on Thursday with the formal publication of the fruit fly genome - the complete set of biochemical instructions to build the insect.

The fly, which is regarded as a pest by farmers, becomes the largest animal so far to have its genetic code deciphered.

The publication is the culmination of almost 100 years of research

Researchers believe the achievement, announced in the journal Science, will have huge implications for human health.

For almost 100 years, the fruit fly (Drosophila melanogaster) has been the premier lab species for learning how the appearance and traits of all living things are encoded in their genes.

It was fruit fly experiments that led to the understanding that genes are carried on long strings in cells called chromosomes.

Scientist like them because their short life cycle of just a few weeks makes it very easy to see how genetic changes move down through different generations.

Medicine of the future

Something like 60% of the genes in the fly can also be found in humans. These include genes for such common human problems as kidney disease, Alzheimer's and cancer.

In fact, 70% of the genes known to cause human malignancy have been found to exist in similar form in the fruit fly. Having access to the entire genetic sequence will open up new areas for study that should lead to new targets for drug therapy in humans.

"Medicine over the coming century is going to be based around understanding the underlying biology of the body and of the cells that make up the body," said Dr Matthew Freeman from the UK's Medical Research Council in Cambridge, where he uses the small insect to study the molecular causes of cancer.

"The fruit fly can lead to that understanding - it will be the absolute and necessary underpinning of the medical research of the future."

The completed genome is the work of many scientists working in several countries. It was a combined academic and industry effort.

The foundation was laid by the Berkeley, European and Canadian Drosophila Genome Projects which produced over a period of years about 25% of the sequence. The rest was obtained within the last 12 months by Celera Genomics, which employed advanced gene-sequencing machines and very powerful computers.

Celera's expertise, which has now been proved on the fruit fly, will result in the genomes of many more organisms being decoded in quick succession - including that of humans.

Computer analysis

All the raw fruit fly data have been deposited at the US National Library of Medicine's GenBank site and is available on the net. Researchers can also access the information on CD-Rom.

The genome is comprised of DNA some 180m bases long. This is bundled up on to four pairs of chromosomes. Computer analysis of the DNA sequence suggests there are 13,601 genes - the discrete regions of the DNA that act as templates to make the fly's proteins.

Short life cycles make genetic studies easier

There are some missing parts of the genome, but these are in areas that are difficult to sequence and are not thought to be important.

"The completed genome is going to make a huge difference," said Dr Freeman. "It means we can get on and ask the real questions which are: what do they do and how do they control cell behaviour and how do they function."

Humans are thought to have about 80,000-100,000 genes.