DNA Evidence for Multiple Introductions of Barley into Europe Following Dispersed Domestications in Western Asia
Jones, G., Charles, M. P., Jones, M. K., Colledge, S., Leigh, F. J., Lister, D. A., Smith, L. M. J., Powell, W., Brown, T. A., Jones, H., Antiquity
DNA analysis of present-day crops is providing a growing body of information on the origins and domestication of cultivated plants, with some results suggesting a monophyletic origin of the major West Asian founder crops (e.g. Heun et al. 1997; Zohary 1999; Badr et al. 2000; Ozkan et al. 2002). The statistical basis of these conclusions has been questioned (e.g. Allaby et al. 2008), however, and other genetic analyses suggest polyphyletic origins for emmer (Allaby et al. 1999), and a more complex domestication history for einkorn (Kilian et al. 2007). Barley, in particular, is generally accepted to have been domesticated at least twice (e.g. Molina-Cano et al. 2005; Azhaguvel & Komatsuda 2007; Morrell & Clegg 2007; but see Badr et al. 2000 for an alternative view). A monophyletic origin suggests a single, relatively rapid domestication, while a polyphyletic origin raises the possibility of a widespread and prolonged exploitation of wild plants resulting in a dispersed and gradual emergence of domesticates (cf. Zeder 2011). This has led to a polarised debate, based on both genetic and archeobotanical evidence, with some arguing for a dispersed, protracted origin of agriculture (e.g. Colledge 1998; Willcox 1998, 2005; Willcox et al. 2008; Fuller et al. 2011) and others for a rapid transition to agriculture in a single localised area of south-eastern Turkey (e.g. Lev-Yadun et al. 2000; Abbo et al. 2010).
This type of phylogenetic research has also been extended to investigation of the early spread of crops in areas outside their region of origin, particularly through Europe (H. Jones et al. 2008, 2011). In an earlier paper (G. Jones et al. 2012) we discussed the adaptation of barley to climatic conditions as it spread through Europe, and the influence this may have had on the rate of agricultural expansion in different parts of Europe. In this paper we explore the routes of multiple barley introductions into Europe from Western Asia, and the contribution this can make to our understanding of early communication networks. Following their domestication in Western Asia, cereal crops were first introduced into southeastern Europe c. 7000 BC. From there they spread through Europe via two main routes, one following a northern trajectory through Central Europe and the other a southern course along the Mediterranean coast (e.g. Bogucki 1996; Tresset & Vigne 2011). Their precise route of spread into south-eastern Europe is less firmly established. Until relatively recently, the available evidence suggested that cultivated crops, such as wheat and barley, reached Greece primarily from Anatolia (e.g. Bogucki 1996). More recent research, however, has raised the possibility of alternative routes for the introduction of plant species into Europe from the east, for example through Cyprus (Colledge et al. 2004) or, later, via a route through the Black Sea region (Valamoti & Jones 2010). This issue lends itself to investigation through the phylogeographic analysis of crop species.
The origin and spread of barley were investigated through genetic analyses of traditional landraces, locally adapted populations of cultivated plants that have not been affected by modern plant breeding programmes. Two types of genetic marker were analysed: (1) neutral markers (simple sequence repeats or SSRs, for 651 landraces) that are not subject to selective pressure (H. Jones et al. 2011); and (2) an adaptive gene (Ppd-H1, for 230 landraces) which is responsible for the control of flowering time in relation to day length (H. Jones et al. 2008). One version (allele) of the adaptive gene causes the plant to respond to lengthening days by coming into flower, and the alternative allele results in a plant that does not respond to lengthening days. A smaller number (72) of wild barleys (and 14 cultivated landraces) from Western Asia was also analysed for the adaptive gene. Both the neutral markers and the adaptive gene were statistically analysed to identify the genetic relationships between cultivated barley landraces and, in the case of the flowering response gene, wild barleys. …