Edible bananas have very few seeds. Wild bananas are packed with seeds; there’s almost nothing there to eat. So how did edible bananas come to be cultivated? The standard story is that some smart proto-farmer saw a spontaneous mutation and then propagated it vegetatively. Once the plant was growing, additional mutants would also be seen and conserved. In fact this “single-step domestication” is considered the standard story for many vegetatively-propagated plants, such as potato, cassava, sweet potato, taro and yam. And while it may be true for those other crops, evidence is accumulating that it may not be the whole story for bananas.
Edmond de Langhe and his colleagues pose a question: Did backcrossing contribute to the origin of hybrid edible bananas? And their answer is, “yes, we’re pretty sure it did”. [1] Truth be told you could probably count the number of people who are really interested in (and able to fully understand) the details of how they got there on one hand. For the rest of us, here’s my take on it.
The hybrid bananas they refer to, our edible bananas, are almost all the results of a cross, either between two wild species, Musa acuminata (A for short) and M. balbisiana (B), or within just one of the species that nevertheless gave rise to a plant that doesn’t need pollen to trigger the growth of a fruit (it is parthenocarpic) and doesn’t itself usually make seeds, although it may produce pollen. Some cultivated bananas are diploid, with two A chromosomes, just two are AB, and none, of more than a thousand, is BB. The rest are all triploid, with three sets of chromosomes: AAA, AAB and ABB with, again, no BBB. Stay with me.
On that basis, Simmonds and Shepherd [2] put the characteristics of the two wild relatives at the opposite ends of a 15 point scoring system to characterise all bananas. Unfortunately, the bananas themselves don’t fall neatly into the categories one might expect them to.
De Langhe and his colleagues looked at the chromosomes and DNA in more detail, using important observations that were not available to Simmonds and Shepherd. Most importantly, for bananas where the parentage is known with certainty, the mitochondria are inherited from the father, or pollen parent, while the chloroplasts come from the mother, or ovule parent. There is also good evidence for exchange among the A and B chromosomes in banana varieties, which would also explain the failure of many varieties to sort neatly under the Simmonds and Shepherd scheme. This kind of evidence allows De Langhe and colleagues to propose alternative, more complex routes to the seedless bananas of today.
Most of these involve a more-or-less fertile AB hybrid being fertilized by A pollen, and then a little nuclear DNA jiggery-pokery (meiotic restitution) and perhaps some rearrangement of the DNA. And that could happen — and more importantly could be noted — if those proto-farmers were growing their newly found edible bananas in close proximity to their wild relatives, as they would have been in southeast Asia. Something very like that is going on today among cassava farmers, for example; they allow volunteer seedlings, the product of sexual reproduction between already favoured clones and wild relatives, to flourish in their fields and then select among them. [3] Banana farmers could easily have done the same.
The details really are not for the faint-hearted; they do, however, make sense of most of the observations on bananas today, including the rarity of certain chromosome combinations and the anomalies in the banana scoring system. And the paper goes out of its way to suggests methods that might verify the backcross hypothesis, including various approaches to direct examination of the DNA.
The big question, of course, is “what does any of this matter?”. And the surprise is that it really does. Banana breeding is difficult at the best of times; no seeds, no pollen, you can imagine. But if the backcross hypothesis is true, then the current approach to banana breeding, which De Langhe et al. describe as “substituting an A genome allele by an alternative derived from a AA diploid source of resistance or tolerance to biotic and abiotic stress”, might be misguided. If the chromosomes are not “pure” A or B, and if backcrosses were involved in the origin of banana varieties, maybe breeders should look again at some of the diploid offspring from their crosses and see whether they could be further backcrossed to come up with types that are more use to farmers.
Notes:
- De Langhe, E., Hribova, E., Carpentier, S., Dolezel, J., & Swennen, R. (2010). Did backcrossing contribute to the origin of hybrid edible bananas? Annals of Botany DOI: 10.1093/aob/mcq187 [↩]
- Simmonds, N., & Shepherd, K. (1955). The taxonomy and origins of the cultivated bananas. Botanical Journal of the Linnean Society, 55 (359), 302-312 DOI: 10.1111/j.1095-8339.1955.tb00015.x [↩]
- Pujol, B., Mühlen, G., Garwood, N., Horoszowski, Y., Douzery, E., & McKey, D. (2005). Evolution under domestication: contrasting functional morphology of seedlings in domesticated cassava and its closest wild relatives New Phytologist, 166 (1), 305-318 DOI: 10.1111/j.1469-8137.2004.01295.x [↩]
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Thanks for this interesting summary.
One thing I can’t understand, and it seems to be conveniently ignored in scientific papers, is how these “smart proto-farmers”, who presumably had no prior experience of farming and plant domestication (both as individuals and as a culture), and (again, presumably) had to work in conditions which were not conducive to experimenting with plants (due to both lack of knowledge and lack of technical sophistication), figured these things out 8000 (or 10000) years ago, although it still seems to be a puzzling challenge even for modern scientists armed with their knowledge of genetics and extensive cultural (if not individual) experience with farming.
I sense something amiss here. Why is nobody addressing this issue?