Probing the peanut’s past

Many readers in developed countries probably regard Arachis hypogaea — if they regard it at all — as a salty snack, maybe a source of clarty peanut butter. The peanut or groundnut, however, is a major staple crop in many parts of the world, a valuable source of protein and energy. So of course scientists are interested in its ancestry, not least to help them breed better varieties. A recent paper by Guillermo Seijo and his colleagues confirms what many have long suspected; that the cultivated peanut is a hybrid between A. duranensis and A. ipaensis. ((Seijo, G., Lavia, G.I., Fernàndez, A., Krapovickas, A., Ducasse, D.A., Bertioli, D.J., Moscone, E.A. (2007). Genomic relationships between the cultivated peanut (Arachis hypogaea, Leguminosae) and its close relatives revealed by double GISH. American Journal of Botany, 94(12), 1963-1971.))

The thing is, like many domesticated plants, peanuts have a complicated genome. Peanut has 40 chromosomes. But it is an amphidiploid, an allotetraploid, meaning that it has two sets of chromosomes from two different ancestors, each of which almost certainly had 20 chromosomes. The genome is described as AABB. But which species did the As and Bs come from? Many attempts have been made to find out, most of them involving attempting to cross existing modern species. Based on all that, the most recent monograph on Arachis ((Krapovickas, A., Gregory, W.C. Translated by David E. Williams and Charles E. Simpson (2007). Taxonomy of the genus Arachis (Leguminosae). Bonplandia, 16(Supplement), 1-205.)) names A. duranensis, A. ipaensis and A. Batizocoi as the wild species that grow where cultivated peanuts have the most characters considered primitive. This kind of evidence is generally taken as indicating the site of domestication.

As in many cases, however, there is a powerful belief abroad that if it is in the DNA it is somehow truer. One of the techniques that addresses the DNA directly, and that is especially useful when chromosomes are believed to come from different species, is called genomic in situ hybridization, or GISH. ((Raina, S.N., Rani, V. (2001). Methods in Cell Science, 23(1/3), 83-104. DOI: 10.1023/A:1013197705523)) In essence, this technique allows researchers to see which parts of which chromosomes match a particular target. Seijo and his colleagues used it to see how seven wild peanut species with 20 chromosomes paired up with the chromosomes of the cultivated peanut. Cut to the chase: “Of all the genomic DNA probe combinations assayed, A. duranensis (A genome) and A. ipaensis (B genome) appeared to be the best candidates for the genome donors.”

That rather vindicates the original conclusion. But it raises a couple of rather interesting questions. One will have to wait for another time. But the other is worth posing now. Why has it proven so difficult — impossible, in fact, so far — to reproduce the original cross that gave rise to the domesticated peanut? Synthetic wheat, made by combining three, not two, genomes, has been a huge boon to breeders, giving them access to a whole range of genetic diversity that they couldn’t readily find in existing wheats. Synthetic peanuts might be expected to do the same. But as yet no new domesticated peanuts have been synthesized by crossing the wild relatives. Why not?