Revisiting domestication

One of the crucial pieces of evidence in studies of cereal crop domestication is the DNA mutation that keeps seeds attached to their stalk.1 Staying attached — not shattering — is important because it allows people to harvest the seeds much more easily. You can gather bundles out in the field, carry them back to the village and do the processing there. If the seeds shatter, you have to harvest early, before they are fully ripe, and thus risk not getting their full food value or do all the processing out in the field or else risk losing much of the harvest on the way back to the village.

The loss of shattering is thus a crucial step in the process from cultivation to domestication.

The shattering mutation itself is extremely rare. In rice, for example, all types of rice share essentially the exact same mutation, crucial evidence that in rice it occurred only once, and then spread from Japonica types back into the wild and from them into the Indica types, domesticated a little later.

Now, we (and others) occasionally play the game of why aren’t any new species being domesticated. Sometimes the answer is that too many people are too satisfied with the few crop species that support humanity. Other times, it is that it is just too hard or too time-consuming, especially if one is hanging around waiting for a non-shattering mutation to arise. Mostly, both.

But hey! We know the gene that is mutated in domesticated rice, and wheat, and sorghum, and maize, and other species too, probably. And we know the nature of the mutation. And we know that the more we know, the easier it is to find out about new species. So, what if some smartypants isolated the gene from an as-yet-undomesticated species, say Coix , mutated it so it no longer functioned to shatter the seeds, and stuck it back in?2 Then just give loads of samples to loads of farmers and let them get on with the business of selection.

Could we engineer a post-cambrian explosion of crop diversity?

  1. Discussion can be a bit convoluted, because the process of separation is usually called shattering, and the crucial mutation is therefor an anti-shattering allele, but most people talk about it as the shattering gene, or shattering allele. The key point is that domesticated crops do not shatter. []
  2. It is important to do it within the species’ own DNA, thus avoiding accusations of God-like meddling. []

7 Replies to “Revisiting domestication”

  1. Although we associate shattering/non-shattering alleles with genes that control the falling of seeds, I wonder if similar genes control the falling of fruit on tree crops such as citrus or avocado, where fruit at different stages of development up until full ripeness or near full ripeness stay safely on the tree until needed. The genes that control abscision layer formation could be similar in very different crops, even if they are expressed in different tissues. A couple of tropical tree crops for which falling fruit are needed and perhaps positively selected for are Macadamia and Canarium, as the trees can be too tall for easy harvesting. There are potentially many wild or semi-wild tropical tree crops that have never been effectively bred for favorable fruit drop characteristics.

    1. The whole issue of fruit drop is an interesting one that I don’t know nearly enough about. I seem to recall that avocados never drop, or maybe that they never ripen until removed from the tree. I agree though that finding out more about abscission for some tree species could lead to improved ability to harvest their products.

  2. I wanted to point out that for shattering, the mutant is likely to be recessive. If you stuck a mutant allele into a normal plant you would likely get a normal plant. So I think what would work better is to make an RNAi construct using the shattering gene as the template, that way it would silence the genes involved and be a dominant trait.

    Our increased understanding of genetics is making stuff like this possible. For example, similar to domesticating a new species, genetic engineering allows us to make previously inedible parts of plants instead edible. I wrote about one example being worked on – and that is removing the gossypol from cotton seeds so that they are edible by non-ruminants.

    Besides the possibility of having more diversity on the farm, we could certainly see more diversity on the dinner plate.

  3. Conventional wisdom places great emphasis in the non-shattering mutation as a prerequisite, or perhaps only a feature, of domestication. But is the assumption valid?

    A number of domesticated plant species are still shattering. Oil-seed rape, sorghum and teff (‘teff’ means lost in Amharic) are examples.

    I read many years ago that African women would comb Oryza glaberrima plots with baskets and that the process was fairly efficient. In contrast, the harvesting and threshing of non-shattering cereals is very time and energy consuming. One must thus assume that, at one point in time in human history, the extra effort called for by non-shattering plants started to pay off; alternatively, the preferential harvesting of the shattering type progressively combed them away and selected the non-shattering one in the natural stands.

    I would suggest that the ‘post-cambrian explosion of crop diversity’ would require both yields and cooking recipes which would enable the new crops to compete with the existing ones; in some instances a new Parmentier (the man who popularised the potato in France) would no doubt help. In fact, we have a lot of orphan crops, and most of them are orphan because one of the aforesaid requirements (or both) is missing.

  4. Andre is suggesting that the development of suitable culinary knowledge would have helped new crops to compete with existing ones, in the process of domestication. The reverse side of this coin is that a loss of culinary knowledge is now leading to the loss of existing crops and crop diversity. Modern cooking writers usually follow market trends, basing recipes on what city consumers are likely to find in city markets. On a global scale, rural populations, where mother to child transmission of food knowledge is probably best maintained, are now outnumbered by urban populations. This has created pressure on our production systems by consumers who are no longer in direct contact with food sources, and who are therefore unable to translate food knowledge into the selection and maintenance of food sources. Instead of being part of a decentralised and dynamic plant domestication process, crops have become idealised universal types, and a small number have become frozen food, literally and figuratively.

    Please see further comments in relation to the problem of ‘Overdependence on few crop species‘.

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