Abstract
BackgroundArchaeobotanical samples of barley (Hordeum vulgare L.) found at Qasr Ibrim display a two-row phenotype that is unique to the region of archaeological sites upriver of the first cataract of the Nile, characterised by the development of distinctive lateral bracts. The phenotype occurs throughout all strata at Qasr Ibrim, which range in age from 3000 to a few hundred years.Methodology and FindingsWe extracted ancient DNA from barley samples from the entire range of occupancy of the site, and studied the Vrs1 gene responsible for row number in extant barley. Surprisingly, we found a discord between the genotype and phenotype in all samples; all the barley had a genotype consistent with the six-row condition. These results indicate a six-row ancestry for the Qasr Ibrim barley, followed by a reassertion of the two-row condition. Modelling demonstrates that this sequence of evolutionary events requires a strong selection pressure.ConclusionsThe two-row phenotype at Qasr Ibrim is caused by a different mechanism to that in extant barley. The strength of selection required for this mechanism to prevail indicates that the barley became locally adapted in the region in response to a local selection pressure. The consistency of the genotype/phenotype discord over time supports a scenario of adoption of this barley type by successive cultures, rather than the importation of new barley varieties associated with individual cultures.
Highlights
The two-row phenotype at Qasr Ibrim is caused by a different mechanism to that in extant barley
Barley (Hordeum vulgare) was among the earliest cereal crops to be exploited during the Mesolithic Neolithic transition [1] and the first to become domesticated, about 9500 years before present (BP) [2]
Consistent with previous findings from Qasr Ibrim, relatively large amounts of DNA were retrieved from the barley grains (Table S1)
Summary
Barley (Hordeum vulgare) was among the earliest cereal crops to be exploited during the Mesolithic Neolithic transition [1] and the first to become domesticated, about 9500 years before present (BP) [2]. The primary role of barley in the origins of agriculture can be attributed to its resilient qualities such as rapid development to maturation [3], xerophytic adaptations such as long awns [4], as well as tolerance of a wide range of edaphic factors such as pH and salinity [5]. These features made it well suited to the early Holocene climate in which the spring growing season was shorter than it is and the summer longer, hotter and drier [3]. The phenotype occurs throughout all strata at Qasr Ibrim, which range in age from 3000 to a few hundred years
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