Sex and the single Banana

It's yellow, it's curved, and it's packed with genetic information that, if decoded, could improve the quality of life of millions of people. Steve Connor explains why scientists the world over are going completely bananas for the next big genome project

You would have thought that such a proudly priapic plant would have no problems. You would imagine that it might come naturally to this most tumescent of fruits; that its phallic appearance would announce it as a true Don Juan of the subtropical scene. You would, of course, be wrong.

In fact, the banana's sex life – or lack of it – is cause for growing concern to farmers and scientists. The reality is that, far from being the joyous product of sexual congress, as most fruits tend to be, the domestic banana that we know and love is an asexual clone, one that results from the sedate, artificial act of vegetative propagation. And no pollinated sex means no annoying seeds, which may be good news for hungry consumers but also means that there's little or no genetic variation – and hence little or no resistance to the banana's many natural enemies.

Devoid of sex, the poor cloned banana is a sitting target for any pest. Finding a way of introducing a little spice – and therefore genetic variety – into the reproductive life of the banana (and its cousin the plantain) is therefore a pressing problem.

That's why a project to do just that has now begun. Announced last week, it involves scientists from 11 countries forming a consortium to decode the banana's genome within the next five years. As with the human genome project, the information will reveal much about the genes that make a banana what it is, and more importantly what it might be with a little extra help. This information – and any resulting advances in genetic modification – will be of profound importance, not just to banana boffins, but to a large proportion of humanity.

The banana is the world's fourth largest staple crop, one on which which the livelihoods of half a billion people depend. But, recently, an evil-sounding beast called the Black Sigatoka fungus has been throwing those livelihoods into jeopardy. Black Sigatoka, along with the weevils, worms and viruses that also routinely attack bananas, is a particularly disturbing menace in the tropics, where the cooking banana and starchy plantain provide up to a quarter of the daily intake of essential calories. Only the banana plantations supplying the lucrative export markets can afford the expensive pesticides and fungicides to defend their crops. For many subsistence farmers, an attack of Black Sigatoka means disaster, and sometimes even starvation.

"Resistant strains are essential for small-holder farmers, who cannot afford the expensive chemicals to begin with," says Emile Frison, director of the International Network for the Improvement of Banana and Plantain (Inibap), the French-based organisation that is helping to run the banana genome project. "When Black Sigatoka strikes, farmers can do little more than watch their plants die. Increased hunger can swiftly follow."

The sweet dessert bananas that all Westerners know are big business, but they only account for about 15 per cent of the 95 million tons of bananas grown annually. The vast bulk of the banana family is made up of the starchy cooking bananas and plantains grown as a staple. But all the minor varieties of cultivated banana are essentially sterile, genetically uniform clones. The banana varieties that do exist have come about not through the normal process of genetic shuffling that occurs during sexual reproduction, but by mutations within a clone that are vegetatively propagated by taking cuttings or "suckers" growing from the base of the plant.

How the banana has got away without sex for so many thousands of years owes much to the hand of man. Although wild bananas do pollinate their flowers – having the botanical equivalent of sex – their fruit is packed full of peppercorn-hard seeds, making them inedible. The soft, yellow flesh of the the edible varieties is the result of a mutation many thousands of years ago that rendered the fruits of these plants sterile. Being sterile, of course, is a major handicap in the wild – which is why the banana would not be where it is today without being propagated and carried there by humans.

There is, in fact, nothing very natural about the banana, which would have remained an obscure plant confined to somewhere in India or Malaysia had it not been for the Stone Age farmer who took a fancy to the fruit of its sterile mutant and propagated the first cutting from one of the suckers. From Asia, prehistoric humans are thought to have taken suckers to Africa, where it quickly spread by further vegetative propagation.

Just how important humans have been to the banana is best illustrated by the story of the Cavendish variety, the one that accounts for about nine out of every 10 bananas sold in British shops.

The story starts in southern China, in 1826, when Charles Telfair, a plant collector and resident of Mauritius, took a fancy to some banana plants he had spied on his travels. Three years later, he sent a pair of them to a friend in England. On this friend's death, they were sold to the Duke of Devonshire, who grew them successfully in his glasshouses at Chatsworth House. The variety was formally named in 1836 after the Duke of Devonshire's family name, Cavendish.

From Chatsworth, horticulturalists spread the Cavendish variety far and wide, always by vegetative propagation of the suckers. John Williams, a missionary, took suckers from Chatsworth to Samoa, the Friendly Islands and Fiji. It is supposed to have reach Hawaii via Tahiti in 1855, at about the same time that it was brought to Australia and Papua New Guinea. Spanish missionaries are also thought to have taken the other varieties from the Canary Islands (where they may have been introduced by French missionaries who had been to China), to the Caribbean, and to Central and South America.

The banana is, after all, an ideal food. At least one imaginative creationist has seriously suggested that its near-perfect design is evidence of God's existence. It is ergonomically shaped to fit the human hand, with a non-slip surface. It has an outward indicator of ripeness – green, yellow and black. Its disposable wrapper has a tab at one end for removal and perforated edges for easy peeling. Add the fact that the banana has a pointed end and curved shape for easy entry into the mouth, and who could argue that it was indeed an act of divine inspiration?

More seriously, however, the banana represents the fine line between life, misery and death for millions of people. "You have to understand how fundamentally important the banana is to many parts of the world," says James Ferguson, a reknowned historian of the Caribbean. "They are an absolute lifeline for poor communities across the world. You can grow a plant outside your house and it's a reliable source of carbohydrates. They are especially good in hurricane-prone countries because they can be grown from nothing to bear fruit in just nine months."

The advent of mass refrigeration in the early 20th century meant it became economically viable to export the fruit from the "banana republics" of the tropics to the US and, later on, Europe. The banana became a symbol of post-war prosperity, being a much sought-after fruit during and after rationing. Bananas were also the first thing East Germans wanted to buy after the Wall came down in 1989.

These days, European countries alone munch their way through about 2.5m tons of bananas each year. About 7 per cent of this trade comes from the Caribbean, thanks to special trading relations that date back to colonial times. The remaining trade is largely with Latin America, where US mulinational interests are able to afford chemical fertilisers and pesticides that help produce bigger, cheaper, but less environmentally sound bananas.

The dessert banana, which accounts for less than 15 per cent of global production, has recently been the focus of a bitter trade war between the US and Europe. American interests in Latin America wanted the EU to loosen its relationship with the Caribbean. An all-out banana war was only averted this year when the EU promised to change its quota system, which favoured the Caribbean, by 2006.

But talk of banana wars mean little to the millions of people who see the banana and plantain as an essential part of their everyday diet rather than an after-dinner treat. "More than a popular snack, bananas are a staple food that many African families eat for every meal," says Dr Frison of Inibap. Rich in vitamins A, C and B6, bananas also contain high levels of calcium, potassium, and phosphorus.

For the past 30 years, however, Black Sigatoka, has been undermining this rich source of sustenance. The fungus has now spread to almost every banana-growing region in the world and typically reduces yield by between 30 and 50 per cent.

Commercial varieties of bananas rely extensively on repeated spraying, sometimes drenching the crop up to 50 times a year. This is about 10 times greater than the average amount of agrochemicals used on intensively grown crops in industrialised countries. Unravelling the genes on each of the banana's 11 chromosomes might reveal a genetic solution to the problem of disease, Dr Frison says: "If we can devise resistant banana varieties, we could possibly do away with fungicides and pesticides all together."

Tapping the genetic variety of the wild, sexually active varieties of the plant may help to maintain the "top banana" status of the fruit. It is a long way from the time when Alexander the Great was said to have been the first European to discover the banana, when he witnessed Indian sages eat a yellow, crescent-shape fruit suspiciously resembling the modern-day wonder. Improving on nature, however, is what the banana has relied on for it phenomenonal success and ubiquity.

Where would we be without the banana? And, equally, where would the banana be without us?