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Banana Biology

When I was giving my lecture to on phylogeny and tetraploidies, I found out not everyone knows why bananas don’t have seeds.

The reason the bananas we eat don’t have seeds is that they are all sterile. A long time ago the Cavendish bananas first came into being when a tetraploid banana (that is a plant that has four copies of every chromosome instead of the normal two) mated with a normal diploid banana. The result, a banana with three copies of every chromosome couldn’t mate or produce seeds. One of the steps in making reproductive cells (the analog of human sperm and egg cells) is the even dividing of a plant’s chromosomes into two reproductive cells.* Normal diploid cells can easily divide into two cells (one copy of each chromosome in each cell), tetraploid plants can divide the same way (two copies of each chromosome in each cell). Hexaploid, three copies in each and so on. Odd numbers of chromosomes don’t work. The plants can’t successfully make the cells it needs to reproduce, if it can’t reproduce it can’t make seeds, and that is why bananas (or seedless watermelons) don’t have seeds.

Every banana has been propagated vegetatively**, making all the bananas of the same type clones (all the bananas available in most grocery stores are a single breed, the Cavendish). The identical genetics of bananas means quality is very consistent, the downside is the sterility of the plants makes them easy targets for diseases and parasites and means the hands of plant breeders, the people who normally breed forms of disease resistance traits into crops, are tied. A plant breeder can’t breed a plant that doesn’t sexually reproduce.

Without plant breeders bringing in new resistance alleles, bananas are very vulnerable. The bananas of our grandparents generation, the Gros Michel, was whipped out by such fungus which attacked the roots of the bananas. The cavendish bananas we eat today are a whole new kind of banana which could resist the fungus, but apparently don’t taste as good (I’m too young to be able to compare the two from firsthand experience.)

Of course today a disease that wiped out bananas could probably be addressed with genetic engineering. The technology can introduce new resistance genes even to bananas that don’t reproduce sexually. Would American and European consumers accept genetically engineered bananas? I don’t know. (Hey, at least we wouldn’t have to worry about pollen drift between genetically engineered and non-genetically engineered bananas. Working with sterile plants has its advantages.)

One place we can be sure genetically engineered bananas can be accepted is East Africa, where they play a vital role in feeding people. People living in Uganda, Rwanda and Burundi eat hundreds of pounds of bananas every year (vs 25 pounds per person per year in the US), yet yields have dropped as much as half because of another fungus called Black Sigatoka. Plant scientists in Uganda are working on developing bananas are more resistant to the fungus (using genetic engineering). Like European scientists, their fields must be protected, but while a scientist working in, say, Germany must worry about anti-GMO activists finding his field and destroying his research, the fields of experimental bananas are much more in danger from theft by farmers hoping to grow their own Black Sigatoka-resistant bananas.

Assorted other banana facts that may be of use someday under a fairly unlikely set of circumstance:

  • Bananas aren’t trees. The giant banana “trees” grow, flower and produce fruit and then die off, re-growing from the roots.
  • Because banana “trees” are made entirely of living tissue, they have a much higher water content than real trees and, as a result, are much more resistant to fire.
  • Banana leaves are waterproof which makes them a useful item in the the shipwreck survivor’s kitchen, and in a pinch they can even by used as an umbrella.

*Actually two copies of the entire genome into four cells, I’m simplifying.

6 Comments

  1. Mary says:

    Every time someone on the internet from Google U with a degree in plant breeding assures me that conventional breeding is always better to solve problems I’ll be sure to point them to this post.

    1. James says:

      Thanks! Conventional breeding and marker assisted breeding have (and continue) to do a lot of good for the world, but there are some problems, like bananas, they just can’t address.

      1. Mary says:

        Oh, of course–the right tool for the right job. It’s so frustrating to talk to people who think you should only be allowed one tool, that tool is 10,000 years old.

        1. James says:

          Absolutely. If you saw the post above this one about nematode resistant soybeans, I found out about it from someone trumpeting it as proof that we didn’t EVER need genetic engineering because breeding had found some resistance QTLs to a specific pathogen.

  2. Josh B says:

    Aren’t Cavendish banana plants still planted and the fruit collected? How are they planted if the fruit has no seeds? Does some other part of the plant have seeds? If so, why is this type of banana considered a fruit?

    1. James says:

      They’re actually vegetatively propagated. Bananas develop small side stalks called “suckers” which can be dug up and replanted somewhere else to develop into a whole second plant. Unlike plants grown from seed, this method of vegetative propagation produces a plant which is genetically identical to the original one, essentially a clone.

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