Most banana varieties grown for consumption are seedless. In the wild, these varieties would not survive and it is only through propagation of corms and suckers or from tissue culture plantlets that bananas can be grown as a crop. This, however, leads to huge problems when it comes to breeding for improved characteristics, such as resistance against pests and diseases, including Fusarium wilt.
Most fertile plants have two sets of chromosomes (diploid, just like humans), or have more but generally even numbered sets of chromosomes. When a flower from a fertile plant develops, the germline cells in the flower go through what is termed as “reduction division”; this means that the chromosome number halves. Consequently, pollen grain cells and egg cells within a flower have half the original set of chromosomes each. When pollination followed by fertilisation occurs these cells fuse to form the full complement of chromosome sets again and give rise to seed.
Cultivated banana is however sterile (seedless) generally due to an uneven number of sets of chromosomes. For instance, Cavendish is triploid which means it has three sets of the “A” genome. So, when flowers are produced in a triploid plant, the process of reduction division is a bit muddled and the chromosome sets are unevenly dispersed. Consequently, no seed develops. It is only because cultivated banana is “parthenocarpic,” which is the formation of fruit without seeds, that fruit is formed at all. It is actually a bit of a freak of nature. The seedless banana fruit has no benefit to the plant.
Wild seeded banana lines do exist and are found in many regions in South East Asia where banana as a species has originated. Some of these wild banana lines have characteristics that could be potentially useful to cultivated banana. Several programs across the world are investigating such wild germplasm for beneficial characteristics. At the University of Queensland (UQ), we have been studying the genetics of resistance to Fusarium wilt in such wild seeded lines, in particular the wild subspecies known as Malaccensis, which we initially identified as a possible source of resistance with our colleagues from Qld DAF.
Through conducting pollinations on these wild Malaccensis lines we have produced seed that have given rise to plants that we tested against the Fusarium wilt fungus. These tests included Race 1 and subtropical Race 4 here at UQ and with TR4 (Tropical Race 4) in collaboration with the NT DPI.
We now have an understanding of the genetics of resistance in the wild Malaccensis plants. The resistance appears to be controlled by a single gene and we have developed several molecular markers closely linked to the resistance gene. That means that we can now screen DNA of the germplasm from breeding programs to determine if those plants are likely to carry that particular source of resistance.
Being able to use markers to select for desirable traits such as fusarium resistance can greatly accelerate the efforts to introduce resistance in a commercial variety.
This can be achieved by selectively screening plants that carry the resistance marker and thus can reduce the need of using the actual fungus to screen the plants and, importantly, avoids the risk of spreading strains of the fungus any further. This work is part of a larger programme led by the International Institute for Tropical Agriculture in East Africa, and contributes towards creating improved pest and disease resistant cooking bananas in Africa.
Our studies have also shown that there seems to be additional sources of resistance in wild banana lines and we are continuing our studies examining additional sources of resistance amongst other wild seeded banana lines. Using multiple sources of resistance within future cultivars would enhance the durability of such cultivars to any changes in the pathogen populations.
** Written by Prof. Elizabeth Aitken and Dr Andy Chen, School of Agriculture and Food Sciences, The University of Queensland