Genetic Markers Help Identify Banana Ancestors

 

Photo by Alistair Smailes on Unsplash

In their search for the ancestors of modern day bananas, botanists and researchers have determined there to be at least three wild banana ancestors responsible for all the various types of bananas found around the world today, represented on the diverse banana family tree. The catch of course is these ancestors have yet to be found. An analysis of modern day banana's genetic material have lead Botanists like Julie Sardos and colleagues to pinpoint genetic markers on their DNA which in turn help with tracing this fruit back to its ancestors. The bananas we commonly see in grocery stores are really a product of years and centuries of crossbreeding and domestication. They only produce sterile seeds and therefore must be grown as hybrids using clones. Since they are all genetically identical, they have low genetic diversity and are susceptible to diseases and higher risks of going extinct. It is for this reason, Botanists and researchers have set out to find the ancestral bananas in hopes that their discovery would mean finding ways to strengthen the crops of modern breeds against threats to their very existence.  

This article is really about how genetic markers found on the DNA samples of numerous types of bananas made it possible to identify the genetic sequences of at least three unknown ancestors. Interestingly enough, we are currently learning about genetic mapping in class and this article connects really well with the topic because genetic markers, according to the National Human Genome Research Institute, are like landmarks in a geographical area. Landmarks are features which mark the area they are in making the area stand out as unique from others. They are helpful when we need to find our way back to somewhere. They are the big red X's on a treasure map that pinpoint the location of the treasure. Hopefully these genetic markers will help researchers successfully find these mysterious banana ancestors and save bananas from threats to their extinction. 

The End of the Cavendish Banana

Tropical Race 4 (TR4) is a strain of the fungus Fusarium oxysporum cubense which kills banana plants by infesting the soil and robbing the bananas of necessary nutrients and water. TR4 is resistant to pesticides and primarily affects the Cavendish banana, a fruit that accounts for more than 40% of the world's banana exports and is the banana that is the most available at grocery stores.

The destruction of common banana species to fungi is nothing new. The Gros Michel banana was once the favored banana amongst Western households due to its sweet taste and creamy texture. However, the Gros Michel succumbed to Tropical Race 1 (TR1), another strain of the Fusarium fungus. When the fungus destroyed the plantations of powerful companies such as the United Fruit Company and the Standard Fruit Company, they switched to Cavendish bananas since they were resistant to TR1. Half a century later, we see history repeat itself with the TR4. The downside is that there is no readily available commercial banana to replace the Cavendish.

Methods to rid of the fungus have proven ineffective since TR4 is spreading in many countries. One solution to save the Cavendish is genetic modification. By isolating a TR4 resistance gene (RGA2) from Musa acuminata malacensis and transferring it to the Cavendish through transgenic bacterial mediums, researchers were able to find a single plant line that had total resistance to the TR4. The other four plant lines were mostly resistant and had low rates of infection. To get the new genetically-modified banana to the market poses another challenge. There are many legislative restrictions on GMO products, and consumers tend to be afraid of GMO products. However, without these genetically modified bananas, the Cavendish banana will share the same history as the Gros Michel.

As an avid banana eater, I don't want to see bananas disappear from grocery shelves. We should change our perspective on GMOs because genetic modification techniques can help solve nutritional deficiencies in many regions and save small family banana farms. Chiquita (United Fruit Company) and Dole (Standard Fruit Company) should have seen this coming and properly prepared for it since they have gone through this before in the 1960s. On a positive note, hopefully GMO techniques can be useful tools in bringing other banana subspecies to the forefront. Personally, I would love to try Blue Java bananas which are blue and taste like vanilla ice cream.

                Cavendish                                       Gros Michel                                Blue Java

https://www.wired.co.uk/article/cavendish-banana-extinction-gene-editing

https://www.nature.com/articles/s41467-017-01670-6

Why are bananas so susceptible to diseases?

One of the most delicious fruits, specifically Cavendish bananas, is at risk. A fungus is spreading and it is out to get the Cavendish banana that is sold in North America and Europe. This disease is known as the Panama disease. Although this disease spreads slowly, it completely wipes out bananas where ever it goes. It is known as Tropical Race 4 or TR4 and it has been spread to China, Indonesia ,Southern Africa, and Asian countries such as Pakistan and Lebanon. Gert Kema examined the fungi from each of the locations where it had spread in order to identify strains of TR4 that infected the plants.

When cells copy, small variations in DNA occur as they reproduce and after a period of time it all adds up. Comparatively, TR4 works differently and does not produce these variations. As Kema gathered DNA samples from each of these locations, he found that each DNA sample was an exact match of each other. Surprisingly, he found that this is because each spore is a clone of the original fungus that invaded Taiwan. This study shows that each fungus is genetically identical. Some bananas could have resisted the fungi if small variations in the DNA occurred. But since the genes in all of the bananas and the disease were very similar, they could not resist the fungi. Cavendish bananas lack any diversity and are all planted together which is what sparked the disease to spread very easily. Scientist have tried to figure out how to stop the disease from spreading, but it is very difficult to do so. TR4 starts from the soil and grows long hyphae after attacking the plants roots. The long hyphae blocks the plants vascular system which reduces the amount of water it receives, eventually causing it to wilt. The fungus than continues to release spores into the soil, which is a main contributor to causing a wide variety of bananas to die as they are planted because once a plantation is infected, it will stay infected for decades. 

For now, scientists hope that they can develop bananas that are resistant to the fungus. But since bananas and TR4 lack genetic diversity on a tremendous scale, it will be very difficult to decrease the amount of bananas that are vulnerable to TR4.

Genetically Modified Bananas

This article published on CNN.com talks about the Ugandan banana blight. The banana is crucial to the diet of the Ugandan people, they eat it bananas for breakfast, lunch, and dinner. More than 10 million bananas are grown each year in Uganda, the second largest producer. Recently banana xanthomonas wilt disease has caused the production of bananas to deplete greatly. This year’s banana production has been decreased by about 30 percent due the banana xanthomonas wilt (BXW).

The National Banana Research Program, has been conducting experiments to grow BXW resistant bananas. They have a special license that allows them to produce genetically modified bananas. Though the genetically modified bananas aren’t available to the general public, many Ugandans are skeptical about the whole idea.