Category: DNA

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Mashing up banana wild relatives

Over at the Vaviblog is a detailed discussion (though not nearly as detailed as the paper) of a new paper outlining a new theory for the origin of the cultivated banana. ((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))

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.

Leaving the details aside, De Langhe and his colleagues propose that instead of a single step, at least two were involved, with a proto-cultivated banana back-crossing with one of its wild relatives and then being seen by the proto-farmer as an improvement to be added to her proto-portfolio of agricultural biodiversity. 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. ((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)) Banana farmers could easily have done the same.

To quote again from The Vaviblog:

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.

Now, what I really need is for one of the handful of people who really understand this stuff to tell me where I’ve misunderstood it.

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The diverse uses of genebank collections

It’s easy to assume that it’s only plant breeders that use genebank collections. But in fact there are clever people out there who are delving into genebanks for all kinds of different reasons. Two recent articles on rice illustrate this nicely.

In the latest Rice Today Tom Hargrove tells the story of the global odyssey of the rice varieties called Carolina Gold and White.

Carolina Gold and White show how genes of good crop varieties spread. The seeds made a remarkable journey: from Indonesia to Madagascar by boat almost 2,000 years ago, then to the wealthy and slave-driven Carolina plantations. Her seeds seem to have helped war-weary Confederate veterans start a new life along the Amazon in South America. Freed slaves may have taken her seeds back to Africa, which she once called home. Carolina Gold recently started a new life in South Carolina, and her white-hulled sister is a parent of an improved variety for upland rice farmers in Colombia and Panama.

Hargrove describes how it was by comparing contemporary seeds with genebank samples that part of that epic story was pieced together, and how it was from genebank samples that Carolina Gold cultivation was revived, albeit it on a much smaller scale, in the State which gave it its name, after almost a hundred years.

Meanwhile, EurekAlert has a piece on another famous, though rather more modern, rice variety: IR8. One of the Green Revolution varieties,

IR8 used to produce 9.5 to 10.5 tons per hectare, significantly more than other varieties in the 1960s when average global rice yields were around only 2 tons per hectare. But, when grown today, IR8 can yield only around 7 tons per hectare.

Why? Is it nature or nurture? Researchers “grew rice from original IR8 seeds preserved in the International Rice Genebank and compared it to rice grown from IR8 seeds continuously grown and harvested over the last few decades.” Genetically, they were found to be similar, so the 15% decrease in yield must be down to environmental changes, possibly including hotter nights:

the findings demonstrate the need for ongoing or “maintenance” breeding because it allows rice plants to cope with a changing environment.

So, even when genebank accessions are not used by breeders, the results can end up being of use to breeders.

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