Bananas come in a huge array of different types, some say perhaps a thousand or more distinct cultivated varieties, but there’s only one kind found in most grocery stores, the so-called Cavendish. Bananas are produced by growing cuttings from an existing plant, making all Cavendish bananas worldwide genetically uniform. This makes commercial production very vulnerable to drought and diseases.
Fortunately, the banana’s wild relatives, growing in tropical forests in southeast Asia, may offer genetic tools to fight against the threats of disease, pests and climate change.
However, these potential saviors of the world’s most popular fruit are themselves at risk from deforestation and habitat loss, which is why researchers have been collecting and conserving specimens in genebanks. But how do you know when you have enough diversity in your collection?
A study of wild banana diversity was recently featured in a special issue of the journal Crop Science focused on the use of crop wild relatives in crop improvement. The researchers sought to identify and fill gaps in genebanks by collecting a diverse cross-section of wild banana seeds in Papua New Guinea, as part of the Crop Trust’s Crop Wild Relatives Project. Wild bananas, unlike their cultivated cousins, do have seeds.
“We collected 31 samples of eight different species during the mission in 2019, including all banana species that were known to exist on the Papua New Guinea mainland,” says Janet Paofa, a researcher at the Papua New Guinea National Agricultural Research Institute and leader of the trip.
The study found that Papua New Guinea’s wild banana diversity was still not fully covered in genebanks and more samples need to be collected, characterized and evaluated for specific traits and habitats.
“With the Papua New Guinea collection, we are dealing with the broadest sample of wild bananas ever studied,” says Julie Sardos, a scientist at Bioversity International and co-author of the paper. “Both wild and cultivated bananas are not only incredibly diverse in Papua New Guinea, they are also very common and grow in lots of different environments, making this country heaven for banana geneticists like me.”
Papua New Guinea is a particular diversity hotspot for wild banana species that can withstand harsh environmental conditions. The most commonly found banana in the country is Musa acuminata subspecies banksii, the ancestor of most edible bananas and a parent of the Cavendish group.
The study discovered that banksii seeds collected on the mission differed significantly from seeds stored at the International Musa Germplasm Transit Centre (ITC), the world’s largest banana genebank. The ITC is located at the Catholic University of Leuven in Belgium and is managed by Bioversity International, with support from the Crop Trust.
The Papua New Guinea collecting mission also sampled the world’s tallest banana species: Musa ingens, which can reach up to 15 meters in height. There were no samples of this species in the ITC. “I have seen many other wild bananas on collecting trips, but never Musa ingens,” says Paofa. “This trip gave me the opportunity to see this unique wild species and its natural habitat.”
Genebanks conserve bananas in different ways. The seedless cultivated types are conserved in the field (as adult plants ) or by storing tissue cultures in plastic or glass vials or freezing them in liquid nitrogen. However, wild bananas have seeds that can be much more easily stored for long periods, if properly processed.
The researchers also found the banksii samples contained some individuals with an unusual capacity to use water very efficiently, a useful trait for managing drought conditions. The water-thrifty individuals showed a higher proportion of root mass compared with the size of their shoot.
“These plants have a smaller leaf area, so there is less risk of water loss, and a more extended root system, increasing the chance of finding water,” says Sebastien Carpentier, a researcher at Biodiversity International in Leuven, Belgium and the study’s lead author.
The genetic diversity within the banksii populations will make it possible to find the ideal parents to improve drought tolerance in current Cavendish types, according to Carpentier.
Another species found on the mission, Musa balbisiana, is also a good candidate to include in breeding programs because it copes well with unpredictable rainy seasons and higher temperatures, and is also resistant to diseases, such as the banana bunchy top virus (BBTV), adds Carpentier.
“Our current seed bank mainly focuses on resistance to the BBTV virus, which is transmitted through small insects called aphids,” he says. “We found resistance in the balbisiana populations from Papua New Guinea. So, balbisiana offers both BBTV resistance and drought tolerance. Two for the price of one!”
Bananas are just one of many common foods that benefit from the conservation and use of the genetic diversity of their wild cousins. The adaptation of agriculture to climate change through the use of crop wild relatives in plant breeding can help long-term food security for all.
The next blog post in this series will cover research published in the recent special edition of Crop Science on improving resistance to drought, heat and salt stress traits in crops by pre-breeding using their wild relatives.