US researchers have just completed the DNA sequence of Theobroma cacao, the fruit of which provides the world's chocolate and cocoa. The project – funded by Mars, the chocolate giant – is likely to benefit more than six million chocolate farmers in the tropics, by delivering disease-resistant trees, or tastier fruit, or higher yields per hectare, or all three. That the research was completed on a plant of interest to small farmers in the poorer nations is itself a measure of the progress of genomic science.
Cacao joins more than 180 life forms for which scientists now have the complete genetic sequence. These include rice, wheat and poplar trees; yeast, grapes and the honeybee; chimpanzees, dogs, puffer fish and Norwegian rats; modern humans, the chicken and the laboratory mouse; and a host of microbes, including leprosy, bubonic plague and the malaria parasite. This is a gathering of knowledge that, even 20 years ago, could not have been imagined. Deoxyribonucleic acid, or DNA, was first isolated in a laboratory dish of pneumonia bacteria in 1944. Its story has been unfolded in one human lifetime, first by Francis Crick and James Watson, who described its structure in 1953; then by Frederick Sanger, who in 1975 first discovered a way to read the sequence of the genetic code; and lastly by Alec Jeffreys, who in 1985 identified a way of using repetitive patterns in inherited DNA to pinpoint a murder suspect. But even then, hardly anyone believed that it would be possible to "read" the entire sequence coiled up in the chromosomes of a living cell.
The first living organism to be sequenced, in 1995, was a humble bacterium. The genetic recipes for yeast, a nematode worm and a fruit fly followed, and the human genome was completed in 2000. The heady mix of high-speed computing, sophisticated automation and research enthusiasm soon built up a momentum that proved unstoppable. Scientists are now matching genetic sequences to answer questions about plant and animal evolution, about the life cycles of disease, about human origins, about individual human responses to drug dosage, and about crop resistance to pests and mildews.
The science has already delivered unexpected and humbling answers. Humans, who consider themselves the pinnacle of creation, have only about 30,000 genes. Cacao seems to have 35,000. Wheat DNA is believed to contain 40,000 genes. It is a droll discovery that on a numerical basis, a human seems genetically less complex than a chocolate biscuit. But it was the humans who sequenced wheat and cacao, and not the other way round. So clearly, size isn't everything.
