Going bananas!

Pha Tad Ke Botanical Garden has been open for just over a year now, which makes this the 13th monthly blog. For all that time, bananas have been neglected. But this titan among tropical crops makes itself pretty difficult to ignore in Laos, and indeed in most tropical countries.

Bananas (including cooking bananas, also called plantains) are the 4th most important crop in developing countries as a whole, and if we were a race whose sole purpose was to produce bananas, we could certainly give ourselves a pat on the back. Annually, humans grow roughly 150 million tonnes of this fruit. In case you have any doubt that this is quite a few bananas – this equates to over 1 trillion of them. Take a moment to revel in the glorious notion of 1 trillion bananas…

If one took the time to place them end to end, the resultant line would reach to the moon. And back. Almost 250 times! It sounds implausible, but there are 7.6 billion people on the planet, the majority of whom are liberal with their consumption of bananas (direct or otherwise). The banana and plantain genus, Musa, is in the Musaceae plant family, which falls within the order Zingiberales; a group of monocots which includes the gingers (Zingiberaceae).

Deceptively, what may appear to be a banana tree’s trunk or stem is in fact a ‘pseudostem’ formed entirely of whorled leaf stalks (petioles), each of which forms a sheath at its base. This actually makes the banana plant a herb of giant proportions – the largest in the world, in fact. And the misconceptions don’t stop there; it may come as a surprise to learn that, botanically speaking, the fruit is actually a berry.

A banana flower at PTK. This bright red colour is fairly unusual. Photograph by Bryony Smart.

The past of edible bananas is murky, and one which is thought to go back at least 7000 years. The first evidence of cultivation lies in the Malay archipelago, where inhabitants of the myriad islands would select plants producing fleshier, sweeter fruit than the rest. The species they were selecting from was Musa acuminata. Because plants on different islands were separated from each other, distinct sub-species formed. Additionally, farmers moved from island to island, bringing their preferred banana plants with them in case the place they were travelling to did not grow such a tasty variety. Presumably there would also have been some trading involved.

As the various M. acuminata sub-species mixed, many hybrids arose, and some of these happened to have deliciously sweet fruit, with plentiful flesh. They also had reduced fertility, but that was of little concern since bananas are easily cloned by splitting new plants from the parent. It did, however, mean problems occurred during reproduction more often than between the same sub-species. As luck would have it, while these hitches were indeed deviations from the healthy, routine combining of two sets of chromosomes (one from each parent), they sometimes created something incredible: a plant bearing good-sized bananas, with no developed seeds whatsoever. Clearly, some of these genetic mistakes were extremely desirable.

The specific genetic quirk which created bananas such as the ‘Cavendish’ (the one with which westerners will be most familiar) was a third set of chromosomes in M. acuminata plants, on top of the two sets that are normal – a phenomenon called ‘triploidy’. Sex cells in plants and animals (i.e. ovules and pollen, eggs and sperm) multiply by dividing, but mistakes can happen. If the chromosome pairs fail to separate correctly, the result can be one sex cell containing two sets of chromosomes where it should only contain one. When the ‘doubled-up’ pollen or ovule combines with a normal female or male counterpart, the resulting offspring therefore has one extra set of chromosomes. Triploidy and other genetic mistakes affect the plant in dramatic and often unpredictable ways – the fact that triploidy within M. acuminata turned out to be favourable is very fortuitous indeed.

A few different banana types, including ‘Cavendish’ (far right). Photo from Wikimedia Commons.

Other instances of triploidy and tetraploidy (four sets of chromosomes), plus further hybridisation of M. acuminata with Musa balbisiana (the other key banana species), has given rise to the majority of banana and plantain variety we see globally today. But it has also made, quite frankly, a complete and utter genetic mess! Because so much cultivation happens outside of science, the estimated number of banana varieties is anywhere from 400 to more than 1000.

Indeed, binomial nomenclature*, used for nearly all species living and dead, has been all but abandoned. Instead, a combination of A’s and B’s is assigned, where the A indicates a set of chromosomes from M. acuminata, and B a set from M. balbisiana. The number of each letter tells you the number of sets from each species. For example, the Cavendish banana is in the AAA group, having three sets of chromosomes from M. acuminata, and none from M. balbisiana.

So there it is. What may appear to the unsuspecting eye to be an unimpeachable, honest fruit turns out to be a mutant berry produced by a giant herb. And it would not be unreasonable to assume that the next banana you eat will be genetically identical to the one you ate last week. All things considered, one cannot help but wonder, do other stalwarts of our fruit bowls have a similarly intriguing origin? If the information in this article has been at all surprising or unexpected, that might just be a point worth investigating.

*Binomial nomenclature is simply a fancy name for the two-part scientific name assigned to a species, for example Musa acuminata, Musa balbisiana. The first part indicates the genus that the species belongs to, and the second part is the species epithet.

This guest blog was kindly written for us by Jon Wells, who recently finished volunteering at PTK. Edits by Bryony Smart, PTK Botanist.

Image credits:

Selection of bananas: by Timothy Pilgrim via Wikimedia Commons, https://commons.wikimedia.org/w/index.php?curid=1101389

Banana flower at PTK: by Bryony Smart, PTK Botanist