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A group of bananas bypassed by modern times offers a glimpse of what the ancestors of the export banana might have looked like. A Bioversity project, supported by the Gatsby Charitable Foundation, has been working with partners to understand the banana diversity of the Comoro Islands and to conserve it before it is too late.

French banana breeder Frederic Bakry pores over a plant portrait photographed by Mohamed Said Mzeouigni. He is searching for visual clues that will allow him to put the plant in its correct place among the known diversity of bananas and plantains. The fruit looks like a plantain but the colour of the male bud is all wrong. The inescapable conclusion is that this plant belongs to a type found only where the photo was taken, the Comoro Islands, a group of islands tucked away off the coast of Africa between Mozambique and Madagascar.

Thanks to the pioneering work done by his colleagues at the French Agricultural Research Centre for International Development (CIRAD), Bakry had known for some time that farmers in the Comoro Islands had been nurturing several unique types of banana. In the 1970s and 1980s French scientists visiting the islands noted the unusually high levels of banana diversity and collected samples for conservation in genebanks. This diversity took on additional significance in 2005, when CIRAD scientists probing the genetic make-up of the Comorian material conserved in the centre's banana collection in Guadeloupe announced that some of the varieties were the closest living relatives of the Cavendish banana that currently dominates the export market and of its predecessor, Gros Michel. It is by no means obvious to the naked eye that these Comorian bananas are of the same stock as the two export bananas, but the discovery of a genetic link has reawakened the dream, thwarted by repeated failures, of one day creating a commercial banana that would taste and perform like Cavendish (or better yet Gros Michel) while also being much more resistant to pests and diseases.

Bakry and his colleague, Christophe Jenny from the Guadeloupe station, were keen to continue documenting this diversity and securing it against threats such as disease, competition from introduced varieties and agri­cultural intensification. They approached Bioversity's Commodities for Livelihoods Programme in Montpellier, France, with a project to survey the islands' Musa diversity. Richard Markham, the programme director, immediately proposed integrating their survey into a Bioversity project on conserving banana biodiversity for Africa, funded by the Gatsby Charitable Foundation.

One of the objectives of this project, which ended in 2007, was to assess the gaps in existing collections in relation to known or postulated centres of diversity. Collecting missions were targeted on Tanzania and Kenya for East African highland bananas, an indigenous group of dessert and cooking bananas, and the Congo basin of the Democratic Republic of Congo for plantains. The Comoro Islands survey presented a perfect opportunity to check on a third group of distinctly African bananas.

Nobody knows when the ancestors of today's African bananas reached the continent or how they arrived there. Bananas originated in the tropical rainforests of Asia. Recent archaeological evidence suggests that cultivated varieties may have arrived on the east coast of Africa as far back as 4000 years ago, although it is impossible to say whether they included the ancestors of today's highland bananas and African plantains. While Africa was becoming a secondary centre of diversity for these two groups, domestication was also proceeding in Asia, where more productive triploid varieties were replacing the ancestral diploid ones (see box, Wild beginnings).

The Comoro Islands, in contrast, became a refuge for diploid bananas derived from Musa acuminata. Bakry and his colleagues speculate that this happened because the islands are small and for most of their history have been off the beaten track of human migration and trade. Nearby Madagascar, for example, is much larger and has more biodiversity—as the theory of island biogeography predicts—but its banana diversity doesn't come close to that of its smaller neighbours (the islands of Pemba and Zanzibar also harbour rare diploids).

Bakry had no difficulty persuading Mohamed Said Mzeouigni, a Master's student at the Université Pierre et Marie Curie in Paris, to survey the banana diversity of the three main Comorian islands. Funding from CIRAD, the French government and Bioversity's Gatsby-funded project allowed Mzeouigni to spend two months in the Comoro Islands. His mission: to describe the plants based on established descriptors and to take photos and leaf samples from every unusual banana plant he was able to find with the help of a local agronomist. (All the information will be included in Bioversity's Musa Germplasm Information System.)

So far, some 48 plants have been tentatively classified into almost as many varieties based on Mzeouigni's photos and notes. Most of the leaf samples were analysed using a technology called flow cytometry, which revealed 9 diploids and 28 triploids (the ploidy of some samples could not be determined because they had deteriorated). The triploids tend to belong to groups that are common elsewhere but Bakry points to a few unique specimens. 'Padji' is a dessert banana that CIRAD scientists have shown is resistant to black leaf streak disease, the fungal disease commonly known as black Sigatoka that is forcing most commercial growers to use an increasingly heavy regime of fungicides. Unfortunately, 'Padji' doesn't have the post-harvest qualities that would allow it to break into the export market.

The diploids are more unusual. With one exception, Bakry reckons that they belong to a rare group of bananas that French scientists call 'mlali'. Some of those varieties, 'Samba' and 'Chicamé' for instance, are unique to the Comoro Islands while 'Mjenga' is also grown in Tanzania. Another, 'Chimwali kananbobwa', is unusual because it is a diploid that looks like a triploid. The great drawback of these diploids, however, is that they are very susceptible to diseases.

Pests and diseases, especially the black weevil, are one of the reasons why banana yields in the Comoro Islands are low, confirms Thierry Lescot, a banana production specialist at CIRAD. He adds that farmers do not use fertilisers and the scarcity of water on the islands precludes irrigation to make up for the lack of rain during the six-month dry season. Comorians traditionally grow bananas on small plots in low-input, mixed cropping systems. Yields have not been keeping up with growth in the islands' population and as a result farmers are under pressure to use pest- and disease-resistant varieties to boost productivity. Improved hybrids, however, are not popular with the islands' farmers or consumers. Modern varieties are mostly bred to solve disease problems, especially those found on large-scale plantations, and often lack the characteristics that would make them appealing to smallholder farmers with a long tradition of eating a large diversity of bananas.

Comorian bananas have become a traded commodity only in the past 40 years. Before that they were grown communally and belonged to everybody. Bakry thinks that the cultural value associated with bananas might have helped the rarer varieties survive to this day, but he is worried about the future. He thinks the islanders should be helped to conserve their unique banana diversity, and suggests setting up a field collection on each of the islands and even establishing a breeding centre that would help Comorians to create their own varieties. Bakry stresses the urgency. About three-quarters of the bananas produced in the archipelago are already the ubiquitous Cavendish. Time is not on the side of the edible diploids and there is still work to be done before scientists have a comprehensive picture of the islands' unique banana diversity.

The conservation of Musa diversity is one of Bioversity's goals, and the Commodities for Livelihoods Programme has developed a strategy for ex situ conservation with the Global Crop Diversity Trust. Sorting out diversity—which includes getting names right and establishing relationships among varieties—is a vital prelude to effective conservation. This is a painstaking activity that requires a strong commitment, if only because we have a tendency to appreciate the importance of diversity only after it has disappeared. Making sure that the Comorian bananas escape that fate is a demanding but important activity.

Further information

n.roux@cgiar.org