Island refuges of banana diversity

Anne Vézina Thursday, 22 May 2014

Bananas are unusual among major crops in that most of the types grown, either for export or local consumption, are farmer-selected varieties rather than improved hybrids produced by breeding programmes. One reason is the difficulty of breeding largely sterile bananas. Phil Rowe once summed up the frustrations of being a banana breeder as having to work with a plant that has no seeds, to get it to produce seeds in order to develop a plant with no seeds. The other is that even when breeders succeed in getting the cultivars that need improvement to take up sex again, the hybrids may be too different to be readily adopted by farmers or consumers.

View from the Indonesian island of Halmahera
View from the Indonesian island of Halmahera

An alternative that is gaining traction among breeders is trying to reconstruct popular cultivars (which happen to be triploid, i.e. have three copies of each gene instead of two as in their wild ancestors) by using in crosses the edible diploid that gave rise to them. The process is explained in a feature on Frederic Bakry, the first breeder to use this strategy. Of course, it can’t work if the diploid ancestors are no longer cultivated in farmers’ fields or haven’t been introduced in a genebank. That’s where the island refuges in the blog's title come in. Islands seem to be the best place to find diploid cultivars.

For example, when CIRAD scientists analysed hundreds of genebank accessions, they found an unsuspected kinship between diploid Mlali bananas collected in the Comoros archipelago at the end of the 1980s, and bananas from the Cavendish and Gros Michel subgroups. The analysis didn't reveal putative diploid parents for another important group of bananas, the Plantains, but during a recent collecting mission to Indonesia, the collecting team reported seeing on the island of Bali a diploid cultivar that looks like a French Plantain.

Ruhuvia Chichi and Broken Heart from the Solomon Islands
Ruhuvia Chichi and Broken Heart from the Solomon Islands

The highest concentration of diploid cultivars is in the Pacific region, especially Papua New Guinea1 , which occupies the eastern half of the island of New Guinea. A 2002 collecting mission in its less well known neighbour, the Indonesian province of Papua (formerly Irian Jaya), predictably uncovered diploid cultivars. The nearby Solomon Islands is also home to odd shaped and colourful edible diploids.

This is not to say that islands haven't played a role in preserving other types of bananas. Some of the cultivars that were domesticated in the Western Pacific, but have since become rare there, can still be found on some of the islands where they have been introduced thousands of years ago. Fe'i bananas, for example, are still cultivated in Tahiti and the Micronesian island of Pohnpei, where they have been rescued from oblivion by the late Lois Englberger.  In Hawaii, another woman, Angela Kepler, has created a safe haven for the archipelago's remaining Iholena and Maoli-Popoulu bananas.

As the UN says about the theme of this year's International Day of Biodiversity: "Islands and their surrounding near-shore marine areas constitute unique ecosystems often comprising many plant and animal species that are endemic — found nowhere else on Earth. The legacy of a unique evolutionary history these ecosystems are irreplaceable treasures".

1 Papua New Guinea is also where the oldest evidence for the domestication of bananas has been excavated, in the depth of Kuk Swamp.