Category: Genetic diversity

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Lactose tolerance: independent origins and strong selective pressure

Michael Kubisch has submitted another post, based on an article in Nature Genetics. Unfortunately the full article and a News and Views piece about it are behind a paywall. However, I’ve done some sleuthing to find a few links that give more details on the story, which I’ve added at the end. As Michael noted, the article is “not about genetic diversity of agricultural species, but how agriculture has affected human genetic diversity”. That’s good enough for us.

The ability to digest lactose, one of the primary carbohydrates in milk, varies widely among adult human populations. In some European countries nearly 90% of individuals can tolerate lactose, while the incidence in some Asian countries is as low as 1%. The inability to digest lactose is caused by a decline in lactase, the enzyme that breaks down lactose into sugars that can be absorbed into the blood stream. This decline starts shortly after weaning and most likely reflects the fact that until animals were domesticated, milk was simply not a staple of human diets. Lactose tolerance, or lactase persistence as it is sometimes called, in turn is facilitated by a continuous production of lactase throughout adulthood. Not surprisingly, lactase persistence appears to be closely linked to whether a population has traditionally practiced a pastoral or an agricultural lifestyle.

This new study examined the incidence of lactase persistence in several African populations. Based on analysis of genetic markers the authors of the study conclude that the trait appears to have evolved not only independently from Europe, but also more than once in Africa itself. Given that the prevalence of the trait is so high in some populations and domestication of milk-producing animals only goes back 12000 years or so, which is a mere blink of an eye in evolutionary times, milk consumption must have provided a significant benefit for human survival.

Those links:

 

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More sorghum for Zimbabwe

A press release from the Netherlands Organisation for Scientific Research is creating tiny ripples in the blogosphere because it suggests that fertilizer, rather than water, is what poor farmers in semi-arid regions of Zimbabwe need to improve their crops. That’s certainly one conclusion from the research of Dr Bongani Ncube, who successfully defended her doctoral dissertation 10 days ago. (Congratulations, Bongani.) More important, I think, is the demonstration that almost regardless of water, sorghum crops benefit enormously from a preceding grain legume. There’s an interaction with rainfall to be sure. In a dry year, cowpea yields more than groundnut or bambara groundnut, but sorghum is always better after a nitrogen-fixing legume. That’s not to say that farmers shouldn’t make use of a little artificial fertilizer if they can afford it. But a better strategy might be to grow a variety of grain legumes, to buffer any possible effects of rainfall, and then plant sorghum, using agricultural biodiversity instead of cash to increase the harvest.

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Cereals databases

Before I disappear for a few days of immersion in the First International Breadfruit Symposium back in Fiji, let me point to two somewhat complementary online resources on cereals genetic resources that I have come across – no doubt Jeremy will say and about time too – in the past couple of days.

The FIGS database brings together passport and evaluation data on bread wheat landraces from a number of the major genebanks and “allows the user to efficiently interrogate the data associated with this collection and provides the capacity to identify custom subsets of accessions with single and multiple trait(s) that may be of importance to breeding programs.” FIGS stands for “Focused Identification of Germplasm Strategy,” and the focus is on identifying material with resistance to abiotic and biotic stresses.

The other database is that of Israel’s Institute of Cereal Crop Improvement, which includes information on accessions of wild cereal relatives collected over the past 30 years. Again, there’s a particular focus on data on disease resistance.

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Local and exotic crops in Africa

The long dry spell throughout much of February and March, caused by an unexpected El Nino that kept the main rain belt to the north of Zimbabwe, will cause serious hardship in significant areas of the country.

That’s not the only thing, of course, but an article from the Harare Herald ((Posted at allAfrica.com)) makes a plea for farmers to grow local indigenous grains such as “sorghum, mhunga and rapoko” rather than watch maize “wilt and die four years out of five”.

It is a wonderful article, making lots of good points. That food-for-work programmes should be accompanied by intensive training on growing small grains, so that those who need it most can become self-reliant in food and maybe even sell a bit for income. That modern machinery makes preparation much easier, and it isn’t expensive. That an advertising campaign could make a virtue of sadza ((Zimbabwean porridge?)) the way grandmothers made it. That there are benefits for urban consumers too. And finally, “Variety is wonderful. But we should not be rejecting indigenous grains simply because they are not “modern” or “Western”. We should be using them as well”.

I wonder whether anyone is listening?

The Ethiopian Herald, meanwhile, says green gram is becoming the crop of choice in Southern Wollo zone. A legume, green gram (Vigna radiata, maybe most familiar in the West as mung bean) improves soil fertility, ripens more rapidly and doubles or even triples incomes. One farmer is quoted as having replaced his teff crop with green gram, but if everybody does that, who is going to supply the teff flour for njera?

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Multiple founder effect

The common wisdom is that crops are most diverse in their centres (or secondary centres) of domestication, because that’s where people have been playing with them longest. Wild species, too, are often less diverse when they have moved to a new area. That’s down to the founder effect; a small bunch of founding individuals will have a less diversity than the population as a whole and is also more subject to random fluctuations that can change things from the original population. But on the Invasive Species Blog (via this month’s Mendel’s Garden) I recently read that reed canary grass (Phalaris arundinacea) is much more diverse in North America, where it is a recent arrival, than in Europe, its home. Genes from all over the old world are mixed up within single individuals in North America, whereas they are never found together in Europe. The reason, apparently, is that the species has been introduced many times, presumably from many places, and this has brought widely separated populations together and given the opportunity to mingle their genomes.

I wonder whether the same is true for some of the crops that have really travelled around, like tomatoes or peppers.