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Early Plant Domestication: Athena Review Vol. 2, no. 1 .
Identification of the Initial Site of Einkorn Wheat Cultivation
If both the wild ancestor of a plant domesticate and its areal distribution are known, can we then safely assume that the original site of its domestication has been identified? This is the question archaeologists are asking in response to the recently published results of an investigation on the origins of domesticated einkorn wheat (Triticum monococcum monococcum) by a multidisciplinary set of European researchers.
Einkorn, the first wheat crop of Old World, Neolithic agriculture, is a hulled wheat that typically produces one grain per spikelet. Wild einkorn is so morphologically similar to the cultivated form that it does not constitute an independent species, but is designated as a subspecies, boeoticum, of T. monococcum L. The status of T. monococcum boeoticum as the progenitor of cultivated einkorn is well-established. Advanced molecular biological techniques, however, allow further classification of the genetic variety within the subspecies. The research team chose for genetic characterization 338 of nearly 1,000 lines of einkorn whose known areas of origin are distributed throughout the Fertile Crescent region and include parts of SE Turkey, northern Syria and Iran and NW Iraq . In this collection, 194 genetic lines were T. m. boeoticum; the remainder, T. m. monococcum and T. m. aegilopoides.
Genetic characterization was based on the marking of amplified fragment length polymorphisms (AFLPs). This method has been likened to the taking of a genetic fingerprint of a genome. The analysis of AFLPs is a painstaking process whereby maps of selected segments of DNA from each line are produced in order to reveal slight differences in DNA structure at particular loci (polymorphisms) among the lines. It is a high resolution technique for the determination of genetic diversity within a plant population, in this case the subspecies T. m. boeoticum inhabiting the Fertile Crescent. The comparison of AFLP profiles, or maps, for T. m. boeoticum, T. m. monococcum, and T. m. aegilopoides provided a basis for calculating the genetic distances among them.
Applying a variety of sophisticated statistical clustering methods to the AFLP data the investigators proved that all T. m. boeoticum lines were more closely related to each other than they were to any other T. monococcum line, and that the degree of genetic similarity within T. m. boeoticum lines is correlated with proximity of distribution. The most relevant issue is, which of the T. m. boeoticum lines was genetically most similar to domesticated einkorn? The study gave a definite answer. The 19 wild einkorn lines most closely related to domesticated einkorn all originated from the Diyarbakir District in SE Turkey, near the volcanic Karacadag Mountains (fig.7). Based on calculations of genetic distance, the researchers conclude that the Karacadag area is the site of initial domestication of einkorn wheat. Yet, the question remains far from settled.
Some experts have responded by saying that closeness of genetic relationship alone is not sufficient proof that the Karacadag region was the first site of einkorn wheat cultivation. Scientists have argued that the underlying assumption that domestication had to have occurred at the core area of a species distribution is a generalization not always true. Support for such views about einkorn wheat must come from the archaeological record.
But here the evidence is not straightforward. South of the Diyarbakir District (fig.7), sites on the upper Euphrates in Syria (Abu Hureyra), and in Palestine and Jordan (Jericho and Beidha) have yielded examples of cultivated einkorn dated centuries earlier than from Turkish sites close to the Karacadag mountains. Nonetheless, the archaeological finds alone are also inadequate for documenting the genetic history of einkorn wheat. Most often archaeological evidence for einkorn cultivation consists of charred grains which lack the anatomical parts necessary to track the transition from wild to cultivated forms. A few finds of einkorn spikelets reported from Turkish sites exemplify transitional forms.
More serious doubts about the inferences drawn from the AFLP studies have been raised by Frank Hole of Yale University. Hole agrees with the timing of einkorn cultivation that the European team gives as approximately 10,000 years BP, but not with the location. Hole advocates a more southern location for einkorn domestication because, he argues, 10,000 years ago southeastern Turkey did not enjoy the Mediterranean-type climate in which annual grasses like einkorn typically flourish. This interpretation implies that the distributions of the einkorn lines may not match those of 10,000 years ago. This objection challenges the researchers belief in the geographic stability of the primary habitat of the wild einkorns. A better understanding of the historical phytogeography of wild einkorn seems to be needed. Indeed, the essence of the arguments against the Karacadag interpretation lies in the manner and degree to which genetic and geographic trends can be related. These problems cannot be resolved solely on the basis of genetic findings.
Reviewed by Virginia M. Betz
[Chapman (ed.), 1992, Grass Evolution and Domestication, Cambridge University Press; Heun et al., Science 278, 14 Nov. 1997; Hole, Science 279, 16 Jan. 1998; Jones et al., Science 279, 16 Jan. 1998; Mlot, Science News 152, 15 Nov. 1997; Zohary and Hopf, 1988 Domestication of Plants in the Old World, Oxford University Press]
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