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Monday, August 25, 2003
 

An NSU brief highlights a laboratory in Kampala, Uganda devoted to improving bananas through biotechnology. [the faux green crowd moans].

Ugandans consume more bananas than anyone else - each year they grow and eat 11 million tonnes of East African highland bananas, a savoury version of the yellow fruit that is such big business in the West. Farmed solely for local consumption, the crop is a cornerstone of most meals and is used for brewing beer.

... Because edible bananas do not produce seeds, new groves are planted from cuttings of existing stock. This spreads diseases and pests such as the black sigatoka fungus, root- munching worms, and weevils.

... But the institute's main purpose is to genetically modify bananas. The fruit's sterility hampers breeding to fight off pests and diseases. Crossing disease-resistant varieties with popular crops is usually possible only through gene insertion.

... "In no other crop is there stronger justification for genetic transformation," says Frison. He hopes that the government-funded lab will help to win support for genetic modification of bananas in a country that is as resistant to the technology as Europe. "It could focus a sterile debate on a relevant problem," says Frison.

Bananas seem to be the classic example of the perils of asexual reproduction. Bill Hamilton's Red Queen Hypothesis is based on this simple idea, that sex evolved and became dominant because life forms that shuffle their genes this way provide a moving target for diseases and parasites. The diseases and parasites do it too so it is a continual race. As the Red Queen told Alice in Through The Looking Glass "It takes all the running you can do, to keep in the same place".

So why aren't all life forms sexual? Well, sex is hard. It's awkward, expensive, and requires one to seek out and establish a relationship, however brief, with the proper sort of other. Hamilton demonstrated with computer models that asexual life forms out compete sexual life forms, all things being equal, in simple situations. But, life isn't simple and with the added complexity of parasites Hamilton's models showed sexual life forms were often more successful.

Often more successful, not always more successful. Asexual life forms have defenses too. Parasites might kill some of the species but other families of members can be resistant. The resistant members survive and proliferate. Each family of the asexual species might have slightly different defenses but when a parasite attacks a family lacking a defense for that particular parasite their arsenal of defenses, useless in this instance, is lost to the species.

Sexual life forms do better at keeping large arsenals of defenses, promiscuously sharing their techniques about with one another to the benefit of their weans. However, they are also more likely to retain deleterious mutations, unlike asexual life forms that quickly purge themselves of harmful mutations...by dying out. They harbor them as recessive or dormant genes that pop up from time to time.

So, why are bananas asexual? What's their story? Why do they have those large edible fruits with no seeds? Fruits are usually bait that entice other creatures to eat them and the seeds they contain and so spread the seeds about the landscape, often in dung.

Cultivated bananas, most familiarly the Cavendish variety, like seedless grapes, were domesticated, bred by humans to be seedless. Wild bananas have abundant, sometimes huge, seeds but edible (by humans) bananas are seedless. This may have happened as long as 9,000 years ago in Papua New Guinea though it's hard to be sure and some researchers point to S.E. Asia as the origin. It also seems that bananas didn't make it to Africa until 3,000 years ago though there are about 30 different varieties cultivated there.

There are still abundant stocks of wild bananas, plantains, growing all across the southern hemisphere, from Asia to India. It might be possible to breed resistant varieties of edible bananas from this wild stock, but since domestic varieties are sterile and so can't be crossbred with resistant wild varieties this means starting all over again, a process that would take a very long time.

posted by back40 | 8/25/2003 07:55:00 PM

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