The Pacific connection

Anne Vézina Thursday, 22 March 2012

Efforts to conserve Pacific bananas aim to raise awareness on their importance and shed light on their tangled history.

From diploids to triploids

In bananas, the first stirrings of edibility appeared in wild banana plants which, like humans, are diploid, meaning that they have two sets of chromosomes. These plants produced fruits that had more pulp than usual. Since they were still fertile, they continued mating with other banana plants. Evidence suggests that edible bananas went through a series of crosses that have left them increasingly sterile and seedless. Triploid bananas were produced when one of the parents normally passed on a single chromosome set, while the other one contributed both sets, making further sexual reproduction even more unlikely.

Botanists having established that the wild species Musa acuminata was one of the ancestors of bananas, Simmonds surmised that triploid bananas with acuminata-like traits (which he designated by the letters AAA) had been domesticated in Malaysia, where he said the species was at its most variable. It has since been shown that subspecies of acuminata not present in Malaysia (like Musa acuminata ssp. banksii, which is native to the New Guinea region) were also implicated in the domestication of bananas. Since these subspecies occupy distinct geographic regions—such as the Philippines, Indonesia and New Guinea—plants must have been moved around in order to produce the genetically heterogeneous types of edible bananas. More difficult to pin point is where the diploid AAs came in contact with another wild species, Musa balbisiana, eventually giving rise to AB, AAB and ABB cultivars.

Malaoro market in Papua New Guinea
Malaoro market in Papua New Guinea

After his 1954 visit to what is now Papua New Guinea, Norman Simmonds, the author of seminal books on bananas, predicted that the area’s unusually high diversity of diploid bananas (which like their wild relatives have two copies of each gene-bearing chromosome) would soon be replaced by the hardier and more productive bananas that have three copies (see From diploids to triploids). His theory was that, except for the obviously local Fe'i bananas, edible diploids had been introduced from southeast Asia early in the domestication process and owed their survival to the late arrival of triploid bananas.

Collecting missions conducted in 1988-89 did not bear out Simmonds’ prediction. The catalogue of material collected in PNG shows that, more than 30 years after his visit, edible diploids were still widespread, suggesting they were being actively conserved. Not only had they not disappeared, it also turned out that they had not been introduced either. It was eventually demonstrated that bananas had been independently domesticated in the New Guinea area.

Accounts from the other side of the Pacific had mentioned groups of bananas designated Iholena, Maoli and Popoulu after the Hawaiian name of the most important cultivar in each group. Because the ancestors of the people who settled Polynesia are from southeast Asia, Simmonds believed that these bananas were also from that part of the world. Molecular studies have since indicated that these cooking bananas also originated in the New Guinea area.

They were not the only types of bananas introduced by Polynesian settlers. Fe'i bananas were also part of the voyage. For a while, it looked as if these once abundant fruit were irreversibly declining. In the early 2000s, however, reports of high levels of provitamin A carotenoids helped raise interest in their cultivation and conservation. A high-profile Go Local campaign in the Micronesian State of Pohnpei inspired other efforts to reduce dependence on imported foods by promoting and conserving local crops. For example, in the Solomon Islands the Kastom Gaden Association set up four banana collections with support from the Christensen Fund, while in Vanuatu, material collected on eight islands was planted in the Vanuatu Agricultural Research and Technical Centre's field station on the island of Santo.

A field collection for Pacific bananas

In French Polynesia, a field collection set up by the Ministry of Agriculture’s Maurice Wong has been selected as the host for the regional Pacific banana field collection funded by the Global Crop Diversity Trust. The field collection is working in partnership with the regional Pacific collection managed by the Secretariat of the Pacific Commission (SPC) in Fiji, which is conserving material received from member countries. The SPC maintains the plants in its care as tissue cultures and ensures that they are free of viruses before they are sent to French Polynesia. The regional collections are also linked with the international collection managed by Bioversity International at the International Transit Centre (ITC) in Belgium and are part of MusaNet.

The field collection in French Polynesia will focus on Pacific cooking and Fei bananas. Wong is pooling the plants he collected on six islands with material sent by the SPC. The plants will be characterized and evaluated for their agronomic performance. “The collection will also provide an opportunity for identifying duplicates and resolving taxonomic questions that should help target efforts to conserve the most representative diversity”, says Mary Taylor, the SPC Genetic Resources Coordinator. In the case of the cooking bananas, this will be facilitated by comparing the plants’ morphological characters with their genetic profile, which will be analysed by scientists at the Institut Agronomique néo-Calédonien and the French agricultural research centre CIRAD. “We’re hoping something similar will be done for the Fei bananas in the regional collection”, adds Taylor. “It would greatly increase our understanding of this little known group.”

Molecular markers have proved useful in exposing unexpected kinships. For example, they have shown that two important staples cultivated outside the banana’s primary centre of diversity, Plantains and East African highland bananas, also trace part of their ancestry to the New Guinea area. The finding fits observations of PNG edible diploids exhibiting traits that anticipate these bananas.

Classifying these edible diploids will be a complicated undertaking, one of the reasons they are not currently included in the regional Pacific collection. Moreover, many of them are already in collections. The specimens collected during the 1988-89 collecting missions were planted at the PNG National Agricultural Research Institute’s field collection in Laloki and many of them have been duplicated at the ITC. Although they probably represent much of the diversity, it's unlikely that all the edible diploids were collected at the time, according to Jeff Daniells who participated in the missions. Unfortunately, when he went back to PNG in 2002, he noticed that introduced triploid varieties were more common in banana plantings. To stem the trend, he suggests enlisting farmers in what he calls community-based conservation sites. “The farmers”, he says, “could fund their conservation efforts from the sale of fruits and planting material and receive bonuses when they maintain or increase diversity in their plots”. Incentive schemes such as the payment for agrobiodiversity conservation services (PACS) being tested by Bioversity International could be used to support the on-farm conservation of the less marketable cultivars.

Such a set up would help buy time to learn more about these bananas and find out why farmers have kept on growing them ... and maybe prevent Simmonds’ prediction from coming true.

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