By Isabel Schmidt
Walking through the fresh produce aisle at your local supermarket, you will likely see many varieties of foods such as apples, melons or tomatoes. One fruit, however, only tends to come in one variety: the Cavendish banana. While other fruits may vary in colour and texture (for example, compare a Granny Smith apple to a Red Delicious), this banana variant is a solitary staple of the fruit bowl.
Considering the ubiquity of today’s Cavendish banana, it is actually a fairly recent replacement for another variety of banana. The Gros Michel, a more flavourful variety of banana that dominated supermarket shelves until the 1950s, is a cautionary tale of the dangers of a lack of genetic biodiversity. Most importantly, it warns against a fate that could also befall our beloved Cavendish and change the look of supermarket shelves permanently.
The Seedless Banana
An average supermarket banana consists of a yellow peel and soft, fleshy inside. Yet as a typical fruit it is missing a key component: seeds. Seeds are the part of a fruit that contain genetic information to pass onto offspring, and when fertilised they will grow an entirely new plant. Imagine a banana full of large black seeds – this is what an average wild banana looks like. But these seeds leave little room for the edible banana flesh. Removing the seeds from the banana is therefore key to producing an edible product.
In a wild banana plant population, reproduction leads to genetic diversity. Successful reproduction of the wild banana plant requires the pollen of one banana plant and the unfertilised seed of another. These two ingredients are known as gametes. When combined, such as when a bat brushes pollen from one plant onto the flower of another, the unfertilised seed becomes fertilised. This fertilised seed will then be able to grow a new banana plant. The new plant will get half of its genetic information from the plant that made the pollen and half from the plant that made the unfertilised seed.
This promotes variation in the genetics of wild banana plants, as there can be infinite ways to combine different genes. Consider the wild Musa acuminata banana strain, which has 22 chromosomes. Chromosomes are simply a set of genes; when these are transcribed, the resulting plant displays certain physical characteristics from each parent plant depending on which chromosomes are inherited. If 11 chromosomes come from each parent plant at random, the resulting plant will have two different sets of each chromosome adding up to 22. Alternatively, some banana strains will have four sets of each chromosome, with two sets of each chromosome were present in each gamete.
Seeds were removed from the modern banana through the process of creating a triploid plant. Triploid plants are those that contain three copies of chromosomes.
Seeds were removed from the modern banana through the process of creating a triploid plant. Triploid plants are those that contain three copies of chromosomes. These can originate naturally due to mutations, but unlike plants with either two or four sets of chromosomes they are unable to reproduce. This is because they cannot split their chromosomes properly to make a new fertilised seed containing an even set of chromosomes from each parent plant. For each chromosome set, one gamete will contribute two chromosomes while the other contributes only one. As a result, they usually do not have the ability to produce more plants, hence the lack of seeds.
The previously mentioned Musa acuminata banana strain was bred to produce these seedless triploid plants, which then became widely dispersed. The breeding of a banana in this way eventually led to the Gros Michel. This became a very common variety of banana which was produced and sold on a large scale as the triploid variety was easier to eat and more appealing to consumers.
The Banana Pandemic
As the new triploid plant was sterile, Gros Michel banana plants could not be planted using seeds; therefore, they had to be produced by cloning. To clone a plant, farmers cut small pieces of the original banana tree and propagate these to grow into mature plants. As the mature plants only originate from one parent, their sets of genetic information are identical. They will all look the same and have the same properties unless a mutation occurs.
This is extremely important when it comes to disease. In a genetically diverse population, plants with different traits will react differently to disease. If a fungus infects a plantation of banana trees, a few could survive due to a gene that makes them immune to the effects of the fungus. These remaining banana trees could then reproduce and create more banana trees resistant to the infection. ‘Survival of the fittest’ ensures that the plant with the most advantageous properties (such as resistance to disease) reproduces, while less viable plants die out before reproduction takes place.
The identical nature of the banana clones led to a big problem when a disease began to infect the crops. Panama disease, a fungal infection that causes banana trees to stop producing fruit, began to infect Gros Michel banana crops. As the original plant was vulnerable to disease, every cloned plant was identically vulnerable and soon entire crops became infected. As Panama disease spread, Gros Michel bananas became increasingly scarce as no plants had the genetic variation required to prevent onset of disease.
The identical nature of the banana clones led to a big problem when a disease began to infect the crops… As the original plant was vulnerable to disease, every cloned plant was identically vulnerable and soon entire crops became infected.
The devastation of Gros Michel crops due to Panama disease led banana companies such as Dole and Chiquita to stock the current Cavendish banana, another triploid variety, as a replacement. Despite being admittedly blander, it was similar to the Gros Michel and its properties made it easy to scale up production. It was also initially thought to be resistant to Panama disease, as the Cavendish and Gros Michel banana varieties had different chromosomes leading to different properties.
Plantations were tightly controlled to prevent disease from spreading in the same way as the previous Panama disease. Nonetheless, the Cavendish banana ultimately faced the same problems due to its lack of genetic diversity. Panama disease mutated to be more dangerous to the Cavendish variety, and the variant TR4 was eventually found to infect the Cavendish banana in Columbia in 2019. While it has not yet spread to the same extent as the initial wave, this new wave of disease could see the Cavendish banana joining the Gros Michel in the history books.
Saving the Banana
Clearly, the identical genes of the banana crops mean they are all similarly vulnerable to dangerous diseases such as Panama disease. Moreover, the Cavendish is a staple food and losing it would be catastrophic. Approaches to protect it, such as the use of disease-resistant soils containing nitrate, and quarantine, have been explored, but a long-term solution is urgently required. Some scientists have looked into naturally mutated plants or mutants originating from individual cells used for propagation, while others have tried to create genetically modified organisms (GMOs).
GMOs are organisms that have had their genetic code modified through the use of genetic engineering. Through this process, scientists take a successful gene from another organism (for example, a plant which has resistance to a particular disease) and insert it into the genetic makeup of a seed. This seed can then produce a plant which displays the desired trait. GMOs are common in produce to increase crop yields, improve flavour or increase shelf life. An example of a successful genetically modified Cavendish is a TR4 resistant version where scientists chose a gene from another TR4 resistant banana and inserted it into the Cavendish. This approach has also protected the Gros Michel against the disease Black Sigatoka by including a gene found in rice.
Scientists take a successful gene from another organism (for example, a plant which has resistance to a particular disease) and insert it into the genetic makeup of a seed. This seed can then produce a plant which displays the desired trait.
Another solution could be to introduce new varieties of banana. While the Cavendish is by far the most common banana globally, other varieties such as Dwarf Red, Lady Fingers or Blue Java are also available and edible. Diversifying the variety of bananas available could lessen the impact of diseases like Panama disease. As all these varieties have a different genetic makeup, it is less likely that one disease will impact them all. Nonetheless, this requires consumers to get used to a variety of different flavours and change their perception of bananas.
The simple banana is a great example of how a lack of biodiversity can quickly become catastrophic. The creation of the seedless banana meant the banana plants could no longer reproduce with gametes from both plants. Both the Gros Michel and Cavendish varieties show how a crop of genetically identical plants can be easily threatened by disease. As the Cavendish is now being threatened by the same disease that devastated the Gros Michel, scientists are looking into genetic modification or new varieties of banana as potential solutions. Maybe, though, the banana we take for granted could soon become a thing of the past.
