Banana (Musa spp.) is one of the world’s most popular fruits and a mainstay on the family table along with rice, wheat, and maize. Bananas produce few, if any, seeds and are instead vegetatively propagated by taking a part of the plant—an offshoot or a sucker—and sticking it into the ground to grow a genetically identical “copy” of the mother plant. However, the absence of seeds generated by sexual recombination limits the potential of banana to produce genetically diverse offspring that could withstand future environmental and biological threats.
Banana in the East African Great Lakes region where they are a highly valued staple food for over 80 million people. EAHB are so important in Uganda (the second largest producer in the world) that the local name matoke (or matooke) is synonymous with food. An average Ugandan consumes about 0.7 kg of banana daily. Given the importance of EAHB for food security in the Great Lakes Region and in the context of a rapidly changing climate bringing with it extremes of environmental conditions and changes in pest and disease distributions and patterns, a more thorough understanding of the genetic variability— and how to tap it—is crucial to ensure their continued existence and maximize their potential. Presently, EAHB have reached only 9% of their yield potential in Eastern Africa while biotic stresses, such as nematodes, weevils, and diseases such as black sigatoka, and Banana Xanthomonas wilt (BXW), have made a heavy impact on production. IITA conducted a study funded by Irish Aid to advance our understanding of the genetic variability in EAHB in the context of their evolutionary history and determine their potential to adapt to current and future threats. EAHB display quite a range of variation of plant types, grouped in five clone sets: Nfuuka, Musakala, Nakabululu, Nakitembe, and Mbidde. Grouping was based on 73 morphological traits and fruit quality attributes. Simple sequence repeats (SSR)—a type of DNA molecular marker—was used to measure the genetic diversity among the different plant types. About 90 phenotypically diverse cultivars were collected from the Uganda and Kenya germplasm collections which represent the cultivated genepool. These samples were then DNA-fingerprinted using 100 SSR microsatellite markers to investigate their population genetic diversity, to correlate genetic variability with morphological classes, and to determine the evolutionary origins of EAHB from the time they were introduced into Africa.
The findings are surprising. They revealed that EAHB presently cultivated in Africa have minimal genetic variation and are largely genetically uniform, even between and within the Kenyan and Ugandan collections. Basically, the research showed that all EAHB existing today, asexually propagated over multiple generations, can trace their origins from a common ancestor. The Bananas galore. Photo by IITA. 38 39 research also indicated that EAHB have a significantly lower genetic variability than other types of banana such as plantain and the sweet Cavendish banana. The variability found in morphological traits (i.e., flower, fruit, etc.) for EAHB is not reflected at the level of genetic diversity. Consequently, the narrow genetic base means that consumption and associated food security in East Africa are highly vulnerable to environmental and biotic changes and stresses. Simply put, if the climate changes drastically, or if a new banana pest or disease comes up, we may see the end of EAHB in Africa as the crop is not genetically equipped to handle such stresses. Cultivated banana are thought to have arisen from the crossing of two wild banana types in Southeast Asia approximately 7000 years ago. These wild species have two copies of their chromosomes in each cell (called “diploids” in genetic terminology). At some time in the course of eons of intermating, a rare genetic event happened in which an offspring was produced with three copies of their chromosomes in each cell (or “triploids”). Triploid banana—those that we commonly eat these days—are sterile and do not produce fertile seeds. EAHB are one of them. Researchers consider that the triploid-forming hybridization event of EAHB originally occurred in New Guinea and Java. However, the lack of historical records and robust archaeological evidence means that we do not fully understand how and when triploid EAHB arrived in the Great Lakes Region from its center of origin in Asia, although it is thought that the ancestral EAHB first entered Africa about 2500 years ago. Irrespective of where they were first generated, it appears that the current day EAHB all arose from a common ancestral clone. The absence of seeds and the increased fruit size would have made humans prefer this type, thereby giving rise to the current predominance of sterile triploid cultivars. The original ancestral triploid-forming hybridization event would have isolated EAHB reproductively from all other banana, leaving them genetically isolated with minimal genetic variability to deal with environmental changes. This narrow genetic base is maintained through the vegetative reproduction of asexual clones (copies) that are planted as suckers. Over the past 2000 years, this has allowed a rapid expansion in the population size of EAHB in the Great Lakes Region (evident from the molecular data gathered by the research). Through this rapid expansion, somatic DNA-mutations and/or epi-mutations are thought to have accumulated, resulting in the morphologically different variants we see today. Broadening the genetic base of EAHB, while maintaining and improving quality and yield characteristics, is a top research priority for IITA. To some extent this requires accelerated re-domestication of EAHB which involves IITA breeders replicating the crossing events that occurred in Southeast Asia centuries ago with diploid wild banana. In addition, IITA researchers are investigating the use of modern biotechnology tools and applying them to address the adaptability of current EAHB because of their lack of naturally-occurring genetic variation. All these efforts aim to ensure that future generations of Africans will still be able to enjoy an East African Highland Banana. Research by IITA has shown that the most effective and convenient approach, particularly for resource-poor farmers, to reducing losses from CBSD is the use of host-plant resistance or the deployment of less-susceptible cultivars. Historically, much of the breeding work to combat CBSD has focused on tolerance since complete resistance to infection is rare.
In 2015, IITA continued efforts to control, contain, and even push back CBSD on this front. In Tanzania, four IITA-developed varieties tolerant of CBSD and resistant to Cassava Mosaic Disease (CMD, another widespread disease of the crop) were officially released for use by farmers in the country. These were: KBH 2002/363 (Chereko), KBH 2002/066 (Kipusa), KBH 2006/026 (Mkuranga 1), and KBH 2002/482 (Kizimbani).
In addition, IITA is in the advanced stages of evaluating more than 30 highly promising breeding lines in Tanzania. Four of these (UKG 2009/0052, UKG 2009/0128, UKG 2009/0164, and UKG 2009/0181) have performed very well under on-farm conditions and have been proposed for a one-year evaluation under National Performance Trials (NPT)—a final step towards full official release. Once they are released, these varieties will be the first to have dual resistance/tolerance for CBSD and CMD for the Lake Zone of Tanzania, an area where CBSD is so devastating that many farmers have totally abandoned cassava production.
In Uganda, two IITA-developed varieties have been officially released during the year: TZ 130 (NARO-CASS 1) and MM 2006/0130 (NAROCASS 2). Their release is a milestone since these are the first that offer dual resistance/tolerance for CMD and CBSD for the mid-altitude areas of the Great Lakes region.However, IITA continues to pursue the objective of developing a variety that is truly resistant to CBSD. By definition, a truly resistant variety should not be readily infected, even when exposed to large amounts of vector-borne inoculum. If and when infected, such a variety should develop inconspicuous symptoms without adverse effects on growth and yield. It should also support low virus (if any) content and thus be a poor source of infection. Developing a truly CBSD-resistant variety will entail using different modes and new sources of resistance.
To this end, IITA breeders have started to look for and identify such sources of resistance by introducing germplasm from IITA’s Genetic Resources Center in Ibadan, Nigeria. Nine Nigerian cultivars were introduced by tissue culture into Tanzania, where they were evaluated for CBSD resistance in the field for three seasons at Chambezi, a known disease hotspot. Initial findings have shown that two cultivars —TMS-IBA961089A and TMS-IBA000388—had either a significantly higher marketable yield of fresh roots or else performed as well as Kiroba, the improved control variety. Furthermore, the two cultivars showed no quantifiable virus concentrations. Due to their outstanding performance, TMS-IBA961089A and TMS-IBA000388 have been earmarked for on-farm evaluation across several sites after which they will be included in NPTs just before official release in Tanzania. If they consistently perform well, these cultivars will be used as new sources of resistance to generate new varieties in future that are truly resistant to CBSD.