Category: Domestication

Posted on

Parasites push promiscuity

Many hermaphrodite plants (and some animals) — including many crops — have what is called a mixed mating reproductive strategy. That means they reproduce by both self- and cross-fertilization, with important consequences for the amount and structure of their genetic variation. The prevalence of mixed mating systems is surprising because inbreeding depression should work to get rid of self-fertilization, resulting in “pure strategies of either outcrossing or selfing.” Now a new study suggests that its natural enemies — pests, parasites, herbivores, etc. — may have a strong effect on the evolution of a plant’s mating systems:

For example, enemies may alter the availability of mates in a population, which may have direct consequences for victim mating system evolution. Enemies may also influence the expression of traits that are important for mating system evolution, thereby improving the evolutionary stability of mixed mating as a reproductive system.

Thus the dynamics of the interactions between a species and the biodiversity that surrounds it can counter the effects of inbreeding depression and lead to the stability of an otherwise doomed evolutionary strategy. I wonder how important this has been in the evolution of agrobiodiversity. After all, concentrating plants in dense near-monocultures like agriculture does is a boon to natural enemies.

Posted on

Reinventing the wheel

More evidence of multiple independent domestication events. Previous work has shown such a pattern for rice in Asia and cucurbits in the America. Now it’s the turn of barley in Eurasia. A paper just out ((Saisho, Daisuke, Purugganan, Michael. (2007) Molecular phylogeography of domesticated barley traces expansion of agriculture in the Old World. Genetics.)) looked at both sequences of 5 genes and also morphological traits in a geographically widespread set of 250-odd landraces. ((From a Japanese university genebank.))

The results suggest that the crop was first domesticated 10,000 years ago somewhere in the Fertile Crescent, from whence it spread to Europe, North Africa and Ethiopia (the material from Ethiopia was somewhat distinct, as has already been documented). However, there was apparently also a second domestication, much later. It occurred in the region encompassing southern Central Asia, the eastern Iranian plataeau and the edge of the Indian subcontinent, and it is material from here that spread eastward starting maybe 2,500 years ago, possibly along the Silk Road, to give rise to the barleys of India, the Himalayas and China.

This is not an unusual pattern in Eurasian agricultural biodiversity. Sheep and cattle DNA data also show “two highly divergent lineages that distinguish European and Asian types, indicating a second independent evolution of these livestock species outside the Near East.” Not unusual, but somewhat puzzling. As the barley authors conclude:

It remains unclear why different cultures sought to re-invent these domesticated species several times rather than simply obtain them through diffusion from other farming societies.

The authors of the barley study speculate that the second domestication happened either because of the transmission of knowledge, or as an independent innovation. I find the second option a bit hard to take. Could it be that the results of the first domestication effort were just not adapted to conditions outside the Fertile Crescent, or there was a barrier to their diffusion? Or maybe it was just a matter of pride for the inhabitants of the Iranian plateau to have their own agrobiodiversity?

Posted on

Linking archaeology and agrobiodiversity

It was probably a silly thing to say. A couple of days ago I briefly mentioned the models that researchers have built, based on present-day genetic data on Europeans, to understand the rate and pattern of human movement into the continent during the Neolithic. And I made the throw-away comment that I wasn’t aware of similar models for crops. I sort of instantly regretted it, and last night did some googling.

At first I thought perhaps I was right after all. I found a recent (2006) paper whose abstract says:

Thus far, no attempts have been made to track the movement of the founder genetic stocks of the first crop plants from their core area based on the genetic structure of living plants.

Further on, though, the authors say they’ve done just that for wheat. And I also found reference to a “Domestication of Europe” project which sought

to determine the extent to which phylogeographical analysis of modern landraces of barley and wheat, combined with examination of ancient DNA in preserved specimens, can reveal genetic information pertaining to the spread and establishment of cereal cultivation from its points of origin in Southwest Asia into and through Europe.

I think the project must have run from 2003-2006. The Glyn Daniel Laboratory for Archaeogenetics at Cambridge was one of the labs involved, and some of the work, and other related research projects, is described on its website.

So there are people out there trying to link up the archaeology and human genetics of agricultural spread in Europe with the genetics of crops and livestock. Is it too early for a Grand Synthesis?

Posted on

The spread of agriculture into Europe

You may recall a post a few days back on how domestic pig keeping spread into Europe. Well, Razib over at Gene Expression, a genetics blog, has a post today which includes a map of the spread of agriculture north and west from the Middle East. He points out that current thinking is that either the practice of agriculture did the spreading (cultural diffusion), or people themselves (demic diffusion) — or both. Both was what the pig work implied, of course. Human genetic studies suggest that Neolithic people have made a fairly low (maybe 20%) contribution to European ancestry, and Razib summarizes the current debate about whether that therefore refutes the hypothesis that movement of people was involved in the spread of agriculture, as well as that of ideas. Bottom line: it probably doesn’t.

He also links to a recent paper that calculates a figure for the rate of movement ((Tracing the Origin and Spread of Agriculture in Europe. Pinhasi R, Fort J, Ammerman AJ. 2005. PLoS Biology Vol. 3, No. 12. doi:10.1371/journal.pbio.0030410)): 1 km/year, give or take. That was arrived at by interpolating radiocarbon dates of Neolithic archaeological sites across Europe, and it fits very well with the results of models using human genetic data. ((As far as I’m aware, however, nobody’s done anything similar with crops.)) An interesting way to think about that speed of movement is that it is roughly a one-day walk per generation.