Re: How does one make a wild banana becomeb a domesticated banana?

Dear Fabrice,

wild bananas belong to the genus Musa, and their native range spans from Polynesia to India. Domesticated bananas are now grown across the humid tropics worldwide, and in many countries they are a major export (Heslop-Harrison and Schwarzacher 2007). Over a thousand different varieties of cultivated bananas are recognized (Heslop-Harrison and Schwarzacher 2007). The major difference between cultivated and wild bananas is that wild bananas have seeds, while cultivated bananas are sterile. Here is an image of Musa velutina, one of the c. 40 species of wild bananas, showing its numerous hard black seeds in cross-section. What we think of as “seeds” in a cultivated banana fruit are actually just the undeveloped ovules (Fortescue 2005).

Why do cultivated bananas lack seeds? Almost all the cultivars of bananas are triploid; that is, they have three copies of each chromosome in every cell. Humans (and most animals) are diploid: we have two copies of each chromosome in every cell, one from our mother and one from our father. In plants, there is a little more variation in the numbers of chromosomes that each individual has in its cells. (The grand champion is a fern in the genus Ophioglossum, with up to 48 copies of each chromosome; Faseena and Thoppil 2007.) In a breeding population of plants or animals, though, the number of chromosomes must always be even. With an odd number of chromosomes, an organism is unable to undergo meiosis properly, and therefore unable to make viable gametes (seeds/pollen in plants, or eggs/sperm in animals). This is why the ovules of cultivated bananas don’t develop.

How can a plant without seeds reproduce and spread across the landscape? This is where humans come into the picture. Cultivated bananas are propagated by cloning the vegetative parts of the plant (Heslop-Harrison and Schwarzacher 2007). Originally, bananas were propagated by planting suckers that sprout from the base of the plant, but in modern times, sterile cell culture is often used to prevent the spread of diseases (Heslop-Harrison and Schwarzacher 2007). The early domestication of the banana probably took place in Polynesia, New Guinea, and Indonesia in the past c.3000-4000 years (De Langhe and Maret 1999). It is impossible to know exactly how hunter-gatherers began to domesticate the plants around them, but here’s a plausible scenario that fits with all the evidence: a hunter-gatherer discovers a grove of banana plants producing large, delicious fruit without seeds. (The wild species of Musa are known to hybridize considerably, and most cultivars grown today come from spontaneous mutations, so it’s quite possible that a wild population of seedless bananas could arise; Heslop-Harrison and Schwarzacher 2007.) He or she protects the grove to ensure a food supply for the village, and perhaps notices that when a plant finishes flowering and dies back, one of the suckers beside the trunk sprouts up to take its place (Heslop-Harrison and Schwarzacher 2007). When the hunter-gatherers move to a new village, our hypothetical protagonist digs up one of these suckers and takes it with him, to carry the tasty banana to his new home... and so the domestication of the banana begins.

The changes in the banana that allowed it to be cultivated are the results of a natural process—mutation and polyploidy—but the continued propagation of the triploid banana, which would potentially have a hard time surviving on its own in the wild, depends on humans. I hope this answers your question! Here are some more resources on banana genetics:

De Langhe, E. and P. de Maret. 1999. Tracking the banana: its significance in early agriculture. Pp 377-396 in The Prehistory of Food: Appetites for Change, edited by C. Gosden and J. Hather. Routledge, London and New York.

Faseena, M., and J.E. Thoppil. 2007. A high polyploid chromosome complement of Ophioglossum nudicaule L.f. Cytologia 72: 161-164.

Fortescue, J.A., and D.W. Turner. 2005. The anatomy of ovule ontogeny of banana, plantain, and enset (Musaceae). Scientia horticulturae 104: 479-492.

Heslop-Harrison, J.S., and T. Schwarzacher. 2007. Domestication, genomics, and the future for banana. Annals of Botany 100: 1073-1084. free download available.

Here is another article about banana genetics by Pat Heslop-Harrison.

Here is a good article on bananas from the UCLA botanical garden.