Pests and diseases are major production constraints for the global banana industry costing significant time and money each year in control measures. There are a number of banana improvement programs occurring around the world that seek to address these constraints by developing resistant banana varieties through conventional plant breeding, mutation and selection programs, and biotechnology.

At the recent 2017 Australian Banana Industry Congress, Dr Frédéric Bakry gave insights in the banana breeding program being conducted by CIRAD, the French agricultural research and international cooperation organization whose goal is the sustainable development of tropical and Mediterranean regions. Pests and diseases are major production constraints for the global banana industry costing significant time and money each year in control measures. There are a number of banana improvement programs occurring around the world that seek to address these constraints by developing resistant banana varieties through conventional plant breeding, mutation and selection programs, and biotechnology.

At the recent 2017 Australian Banana Industry Congress, Dr Frédéric Bakry gave insights in the banana breeding program being conducted by CIRAD, the French agricultural research and international cooperation organization whose goal is the sustainable development of tropical and Mediterranean regions. 

There are a number of factors that determine the success of a banana breeding program. These include:

·         Access to a wide variety of genetic strains of banana (germplasm) for use in the plant-improvement process. 

·         Basic scientific understanding of the diversity and genetics of bananas, and available technologies with which to study them.

·         Efficient breeding practices including

o   determining which combinations of parents are best to cross, and 

o   the influence of the environmental factors on successful production of seeds. Many desirable banana varieties have low reproductive fertility.

·         Early selection of favourable progeny and efficient identification desired traits in the field in various locations e.g. resistance, productivity, fruit quality.  Many progeny are slow to propagate.

Bananas have variable numbers of chromosomes which makes understanding their genetics complicated. Unlike humans which have two sets of chromosomes (diploid), bananas are polyploid meaning they have multiple sets of chromosomes. There are more than 2000 banana cultivars, most of which have originated from hybridisation between the wild parents Musa acuminata (A genome) and Musa balbisiana (B genome). The majority of cultivated varieties are triploids which have three sets (e.g. Cavendish and Gros Michel AAA, Lady Finger AAB, Ducasse ABB, and Plantains AAB. Almost all export bananas are derived from Cavendish cultivar (AAA) (Li et al. 2014)and the present wild relatives are diploid as well as some cultivated types e.g Monkey bananas (AA).

Evolutionary studies since 2005 have suggested that AAA varieties like Cavendish and Gros Michel arose from independent hybridisation events between Mlali (AA) and Khaï (AA) banana diploid sub-groups. Though originating thousands of years ago in South-East Asia, man has subsequently dispersed these cultivars around the world. Today Mlali types are only found in east Africa and neighbouring islands while Khaï types are still found in South East Asia.

Sources of resistance

Identifying sources of disease resistance is a crucial step in the generation of resistant varieties.

Great genetic diversity occurs amongst wild banana types in the world and sources of naturally occurring pest and disease resistance have been identified in both wild and cultivated types (Table 1).

Table 1 The frequency with which resistance to major disease has been identified in bananas.

Source	Banana Disease
	FOC1 Race 1	FOC1 Tropical Race 4 (TR4)	Yellow Sigatoka	Black Sigatoka	Burrowing Nematode	Lesion Nematode	Banana Bunchy Top Virus	Banana Streak Virus
Wild bananas	Frequent	Not rare	Frequent	Frequent	Intermediate	Intermediate	No	Yes
Cultivated Varieties	Frequent	Rare	Intermediate	Intermediate	Rare	Rare	No	Tolerance

¹ Fusarium oxysporum f.sp. cubense – also known as Panama disease or Fusarium Wilt.

Joint research conducted by CIRAD and Wageningen University & Research Centre (Netherlands), investigating levels of resistance to FOC TR4, is in an early phase. Initial results however have shown that for the Mlali and Khaï subgroups, all the Mlali types were susceptible to TR4 while members of the Khai subgroup showed a range of resistance to TR4, some being fully susceptible to some being fully resistant.

Disease resistance in bananas is generally a dominant trait over susceptibility which makes the introduction of resistance genes into progeny much easier.

CIRAD breeding strategies: breeding by reconstruction

A major challenge for banana improvement is to produce disease resistant, seedless hybrids that meet consumer expectations, from diploid bananas. CIRAD’s strategy for breeding improved bananas strives to reconstruct dessert varieties by imitating the natural evolution and domestication of the triploid bananas. The strategy relies on:

·         Synthesis of AAA varieties from AA parents

·         Introduction of pest and disease resistance genes by making crosses between edible varieties and disease resistant fertile clones

·         The evaluation of different parental combinations based on the performance of their progenies.

One edible diploid that has been identified as a strategic genetic resource is IDN110 / Pisang Rejang (Cv Rose) (AA) (Fig 2). This banana line has many favourable traits including:

·         Resistance to Yellow Sigatoka and Black Sigatoka diseases.

·         Resistance to FOC R1 and TR4 in controlled conditions (Garcia-Bastidas et al., pers coms 2017).

·         Resistant to burrowing and lesion nematodes (R. similis and P. coffeae – Quénéhervé et al. 2009).

·         Female and male fertility that ranges from low – medium.

·         Suitable as an organic cropping system only for local consumption (because of fruit fragility).

·         Used as parent in numerous CIRAD hybrids.

Six AAA advanced selections from progeny derived from crosses using IDN110 as a parental line were sent to Australia in 2014 for inclusion in field trial evaluations. These include CIRAD: 918–924–925-931-938 and 940 (Fig 3). All are dessert bananas which were selected in the tropical conditions of the French West Indies-Guadeloupe. All six varieties are resistant to yellow and black sigatoka and FOC R1. They display different patterns of resistance to FOC TR4 and nematodes and their productivity and fruit quality also varies. These varieties have potential for tropical areas of Australia but require further evaluation in Australian conditions. A more recent selection (PRAM01), which is resistant to FOC R1 and TR4 is planned to be sent in Australia in the near future.

In other breeding work conducted by CIRAD, the inheritance of TR4 resistance has been studied in sweet acid AABs hybrids which are close to the sub-group of our Lady Fingers. These may have suitability for the subtropics.   In crosses conducted between a Kunnan tetraploid (AABB) and a IDN110 Cultivar Rose (AA), a L9 hybrid (AAB) was resistant to R1 but susceptible to TR4 while X17 hybrid (AAB) was immune to both races of FOC (Fig 5).CIRAD is also developing a new range of Cavendish-type hybrids but are still reinforcing the disease resistance of these hybrids. This will be achieved by pre-breeding within the Khaï diploid subgroup by introducing resistant genes from wild seedy relative in particular Musa acuminata subsp. malaccensis.

Fig 5. The inheritance of FOC TR4 resistance in triploid AAB breeding.  AABB (Kunnan T) ♀ crossed with AA (wildtype or cultivated variety) to give sweet acid AAB hybrids.

Future work from banana breeding

Work at CIRAD has great potential, and breeding for resistance to many banana pest and diseases is achievable. However further research needs to be conducted to:

·         Find additional sources of resistance in wild and edible diploid bananas

·         Understand the fundamental mechanisms and genetic control of disease resistance traits so they can be incorporated into breeding programs

·         Conduct pre-breeding studies to understand the inheritance of new resistance genes at different ploidy levels.

References

Bakry F., Carreel F., Jenny C., Horry J.P. (2009). Genetic improvement of banana. In : Jain Shri Mohan (ed.), Priyadarshan P.M. (ed.). Breeding plantation tree crops : tropical species. New York : Springer [Etats-Unis], p. 3-50. http://dx.doi.org/10.1007/978-0-387-71201-7_1

Bakry F., Horry J.P. (2016). Advances in genomics: applications to banana breeding. In : Van den Bergh I. (ed.), Smith I. (ed.), Picq C. (ed.). XXIX International Horticultural Congress on Horticulture: Sustaining Lives, Livelihoods and Landscapes (IHC2014): IX International Symposium on Banana: ISHS-ProMusa Symposium on Unravelling the Banana’s Genomic Potential. Louvain : ISHS, p. 171-180. (Acta Horticulturae, 1114). International Horticultural Congress. 29, 2014-08-17/2014-08-22, Brisbane (Australie). http://dx.doi.org/10.17660/ActaHortic.2016.1114.24

Hippolyte I., Jenny C., Gardes L., Bakry F., Rivallan R., Pomies V., Cubry P., Tomekpé K., Risterucci A.M., Roux N., Rouard M., Arnaud E., Kolesnikova-Allen M., Perrier X. (2012). Foundation characteristics of edible Musa triploids revealed from allelic distribution of SSR markers. Annals of Botany, 109 (5) : p. 937-951. http://dx.doi.org/10.1093/aob/mcs010

Li W.M., Dita M., Wu W., Hu G.B., Xie J.H., and Ge X.J. (2014) Resistance sources to Fusarium oxysporum f.sp. cubense tropical race 4 in banana wild relatives. Plant Pathology Doi:10.1111/ppa.12340.

Perrier, X. et al. (2011). Multidisciplinary perspectives on banana (Musa spp.) domestication. PNAS, 108(28):11311-11318.

Perrier, X., Bakry, F., Carreel, F., Jenny, C., Horry, J.P., Lebot, V. and Hippolyte, I. (2009). Combining biological approaches to shed light on the evolution of edible bananas. Ethnobotany Research and Applications 7:199-216.

Quénéhervé, P., Salmon, F., Topart, P., & Horry, J. P. (2009). Nematode resistance in bananas: screening results on some new Mycosphaerella resistant banana hybrids. Euphytica, 165(1), 137-143.

Quénéhervé, P., Valette, C., Topart, P., Du Montcel, H. T., & Salmon, F. (2009). Nematode resistance in bananas: screening results on some wild and cultivated accessions of Musa spp. Euphytica, 165(1), 123-136.

Raboin L.M., Carreel F., Noyer J.L., Baurens F.C., Horry J.P., Bakry F., Tézenas Du Montcel H., Ganry J., Lanaud C., Lagoda P. (2005). Diploid ancestors of triploid export banana cultivars : Molecular identification of 2n restitution gamete donors and n gamete donors. Molecular Breeding, 16 (4): p. 333-341. http://dx.doi.org/10.1007/s11032-005-2452-7

 

 

 

*Written by Dr Rosie Godwin, from Dr Bakry’s Congress presentation.