Banana's Genes Unpeeled

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A commercial dessert banana from the seedless Cavendish cultivar in comparison with a typical banana from a wild fertile ancestor such as the one that as been sequenced.
Angélique D'Hont

THE GIST

- Bananas are in peril around the world.

- Knowing the genetic sequence of bananas will help scientists breed more resilient varieties.

- Due to imminent threats, bananas require frequent applications of pesticides.

Bananas are a staple food around the world. But the humble yellow fruit faces pests and diseases that threaten to wipe it out across the globe, from convenience stores in Iowa to rural markets in Uganda.

In an effort to save bananas from imminent demise, scientists have now sequenced the banana genome for the first time, a challenging feat and a major advance in the field.

The accomplishment opens the way for developing better banana crops that are naturally resilient against parasites and other stresses.

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"The banana is very important, especially for tropical and subtropical countries," said Angélique D'Hont, a geneticist at CIRAD, an agricultural research center in Montpelier, France. "Because the future of the banana is in danger, the sequence will help to produce resistant bananas and avoid the utilization of pesticides. It will be much easier now to identify genes which are important."

Bananas were first domesticated 7,000 years ago in Southeast Asia. As people migrated, and crossed their own plants with other species along the way, bananas gradually became seedless, delicious and totally sterile.

Instead of multiplying through sexual reproduction, which mixes up the gene pool, bananas are cultivated through vegetative propagation, which involves simply cutting off a section of one plant to grow on its own. It's the same process used to grow several other major African crops, including cassava, sweet potatoes and yams.

As a result, every single Cavendish banana -- the variety that makes up about half of all bananas eaten around the world -- is an exact clone of every other Cavendish banana.

The shape, color and flavor of these popular fruits are predictable and consistent. But parasites and diseases have adapted to the Cavendish, D'Hont said, making it necessary to use large amounts of pesticides to keep banana crops from collapsing -- up to 50 applications a year in some places.

To decipher the banana's genetic strengths and weaknesses, D'Hont and a large group of colleagues spent two years sequencing a variety of banana called Musa acuminate, which is a simpler relative of the Cavendish.

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Once they put together the sequence, the researchers report today in the journal Nature, they discovered several genes that may be involved in pest resistance.

Among other findings, the researchers identified genes involved in ripening after the application of ethylene, which is often added to green bananas during transport. The sequence also revealed that the banana duplicated its entire genome three times (making an extra copy of every single gene in its genome) -- including once 100 million years ago and once 60 million years ago

Putting together the sequence took so long because, compared to many other crops, the banana genome is extremely complex. Even though all bananas are clones of each other, the original gene forms that came from mother and father plants remain different from each other -- unlike in seeded crops that tend to become inbred, said Simon Chan, a plant biologist at the University of California, Davis.

What's more, bananas have three copies of each chromosome, just like other seedless plants. And for many genes, all three copies are different.

The variety of banana used in the new study had just two of each chromosome, making it simpler than the Cavendish. But by finally deciphering its sequence, scientists will be able to move on to our beloved breakfast fruit and compare the differences.

Knowing the genetic sequence of bananas is a major step toward isolating key genes that will eventually lead to a better banana, Chan said. Future varieties may be able to resist both droughts and diseases, while still tasting good and traveling well.

Those developments are especially important in the developing world, where starchy varieties of bananas supply substantial amounts of calories to the human diet, especially in Uganda and other East African countries. There, Chan said, losing the banana crop would be a humanitarian disaster.

"This is a nice example of how these projects can greatly benefit crops that we might not think about as being important in the developed world but are really, really important for food security and human welfare in developing countries," Chan said.

"It's like a moral imperative for us to work on these plants and sequence the genomes of more of the species that are significant in those parts of the world."