By: Sophie Gabreldar
Bananas are one of the most popular fruits on the market, and are a personal favourite of mine. Most people have enjoyed a banana split, but are unaware of the long journey this fruit has had. In fact, the modern-day bananas are an entirely different species of banana. The familiar bright yellow Cavendish banana is a staple item in supermarkets and fruit bowls, but it is in danger.
Bananas: Identical Clones?
To understand the danger the Cavendish is in, we must understand how this species reproduces.
Modern bananas as “triploid,” meaning they have three copies of each of their chromosomes. This means they cannot reproduce sexually, because their chromosomes cannot be evenly divided; this is a necessary step in creating a viable sex cell. Triploids occur when there is a mistake during the process of meiosis (the process that forms the sex cells). In this case, the cells are produced with two copies of each chromosome instead of one. When these cells fuse with a normal sex cell, the offspring has two chromosomes from one parent and one from the other, for a total of three copies of chromosomes. In the banana’s case, the plant still produces fruit but cannot make seeds because the extra chromosomes.
Identical Clones: Good or Bad?
On the surface this may appear to be a problem, but plants are not completely dependent on sexual reproduction. As many people know, new plants can be started from cuttings of an adult plant that are replanted. When the cuttings grew into adult plants, it would look completely identical to the parent plant.
This is great as it allows the bananas that are the right size, shape, and flavour to reproduce. Every banana grown as predictable, and this was perfect for the banana industry. There was one serious downside to this: all the bananas were genetically identical. If one tree became diseased, those nearby will also be vulnerable, and it can spread through the whole plantation. This is the exact situation that happened.
Panama Disease in Bananas
Panama disease first started to wipe out banana plantations in 1890. The pathogen responsible for Panama disease is Fusarium oxysporum cubense. There was nothing that indicated its upcoming arrival and there was little hope is slowing the spread. Nearly all the banana plantations were devastated and once devastated, abandoned, because it was quickly figured out that Panama disease, can lurk in the soil for years! This was concerning because the contaminated soil was immune to any chemical treatments. This was the situation facing the banana industry in the 1950s, when Gros Michel plantations around the world were being overwhelmed.
The fungus affected the channels that carry water and minerals from the roots to the leaves and caused them to become blocked. The plant itself plugs them, presumably to stop the fungus spreading. The Cavendish bananas were harder to grow than Gros Michel, but given the Cavendish’s apparent immunity to panama disease, the industry had no alternative but to switch. This is how the Cavendish became the common species of banana.
The Cavendish unfortunately has its own weakness since it susceptible to another disease called Black Sigatoka. The fungus Pseudocercospora fijiensis, affects the plants leaves; it causes cell death, which interferes with photosynthesis and eventually leads to the death of the plant. One lesson to be learnt from the of Gros Michel, is that relying on genetically identical crops is risky.
Although the Cavendish is currently the most popular banana, there are many species of wild bananas. Although most do not have the desired characteristics—such as seed-lessness, or long shelf life—that would make them a direct substitute for the Cavendish, they are a source of untapped potential. Scientists could search within them for resistance genes and other desirable traits to use in engineering and breeding programs. So, the next you enjoy a banana, think about the long journey it took to get to the fruit we know and love.