Many cultivated barleys, especially landraces from northern Europe, are daylength nonresponsive, and so continue vegetative growth until flowering later in the summer [ 8 ], allowing them to take advantage of the longer growing season in northern Europe. We compared phenotypic data for ear row number, caryopsis structure, growth habit and flowering time with the population memberships Table 1. No other similarities between the range of phenotypes displayed by different populations were apparent.
The spread of agriculture involved the dispersal of barley well beyond the native range of the wild species into the variety of environments found in Europe. Adaptation to these new conditions is reflected in a north-south clinal distribution of landraces with the daylength responsive and nonresponsive genotypes of the photoperiod gene PPD-H1 , nonresponsive forms more common in the cooler northern latitudes [ 8 ].
With wild barley, there is a strong correlation between population structure and temperature and precipitation [ 27 ]. It might therefore be anticipated that similar climatic correlations may be discernable in the population structure of cultivated barley. Analysis of a series of climate variables supported these expectations Table 2. Between-population variance was significantly higher than within-population variance for both spring and winter barleys. This trend was apparent at all months of the year, but for spring barleys was strongest during the growing season.
For winter barleys the seasonal trend was less clear. The results indicate that the accessions in each population are adapted, at least to some extent, to their environment, but do not reveal whether this adaptation was a factor in the origin of individual populations, or merely reflects the more recent evolution of landraces to the environments in which they are being grown. The relationships inferred from the groupings revealed by neighbour-joining Figure 4 , along with the phenotypic and geographic data, enable possible origins for the populations to be deduced.
Populations are closely related, forming a distinct group in the neighbour-joining tree, and have identical phenotypes, virtually all of their members being two-rowed and hulled with spring growth habit and daylength nonresponsiveness Table 1.
We have previously shown that the nonresponsive phenotype of European barleys originated in the eastern Fertile Crescent and that the first nonresponsive plants probably entered Europe after the initial spread of agriculture [ 8 ]. This population of nonresponsive plants would almost certainly have had a distinct genetic makeup compared with the barley already present in Europe, which originated in the western Fertile Crescent.
Populations are almost exclusively nonresponsive of the 99 accessions from these three populations that were typed, 97 possessed a nonresponsive haplotype and could be the descendents of this original population of nonresponsive plants.
These three populations possess the wild phenotypes for ear row number and caryopsis structure, but have acquired a spring growth habit, whereas their wild progenitors would have been winter types. The presence of some members of population 7 in the same region of the neighbour-joining tree as populations is indicative of past cross-hybridisation between these populations, which we discuss below.
Population 4 is also made up entirely of daylength nonresponsive accessions. This population is located some distance from populations in the tree topology.
Population 4 has a narrow geographical distribution in Switzerland and the Carpathian mountains Figure 6 and is the only population in which the majority of accessions have naked rather than hulled grains. The apparent lack of a close relationship between population 4 and populations might indicate that the former is not directly descended from the latter. Instead, population 4 could have become homogeneous for daylength nonresponsiveness via a founder effect operating on a population that contained a mixture of responsive and nonresponsive types.
Population 5 forms a separate cluster in the neighbour-joining tree, but has a mixture of phenotypes, including two- and six-row barleys, hulled and naked forms, spring and winter habits and both daylight responsive and nonresponsive.
There is little uniformity to the combination of phenotypes possessed by individual accessions, and the two deeply rooted groups within the population 5 cluster are equally mixed. These features, along with the broad geographical distribution, suggests that this population has not been subject to selection.
With a crop such as barley, one way in which a distinct genetic population might arise is by geographical partitioning during or soon after the initial spread of agriculture. Populations might be expected to arise in this way if the process of spread involves two or more trajectories that isolate different parts of the crop so that cross-hybridization between the nascent populations is restricted.
The original spread of agriculture into Europe is thought to have followed at least two trajectories, one along a northern route through the Balkans, Hungary and Danube and Rhine valleys, and the other through the Mediterranean basin to Italy and Iberia [ 28 — 30 ]. The lack of evidence for human or environmental selection might therefore indicate that population 5 is a relict of a population that originated from the geographical partitioning that occurred during this initial period of spread along the northern trajectory.
Another candidate as a relict is population 9, as the core area of distribution of this population lies within those regions of Mediterranean Europe where crops are thought to have spread via the southern trajectory. If the spread of cultivation along this trajectory resulted in evolution of a distinct population of barley then that population, at least initially, would have had a geographical distribution very similar to that displayed today by population 9.
Population 9 is predominantly six-rowed, hulled and daylight responsive, with a mixture of winter and spring types. Population 8 has similar phenotypic features to population 9 but contains a greater proportion of landraces with the winter growth habit and is exclusively daylight responsive, whereas population 9 includes some nonresponsive types. Their geographical distributions are largely non-overlapping, with population 6 centering on the northern Balkans, Hungary and Romania, and population 7 in northern Europe, Scandinavia and the Baltic States.
This suggests that originally they formed a single population spanning most of the eastern half of Europe, subsequently splitting into two, possibly by geographical partitioning. They are largely six-row, entirely hulled and predominantly spring growth habit, but they contain a mixture of daylength responsive and nonresponsive forms.
The latter are located almost exclusively within the lower part of the tree shown in Figure 4 , alongside populations The implication is that cross-hybridization resulted in transfer of the daylength nonresponsive phenotype from populations to some members of populations 6 and 7.
Daylight nonresponsiveness and spring growth habit can be advantageous for the successful growth of barley in the more northerly regions of Europe. Acquisition of daylength nonresponsiveness by a group of early barley landraces that had already evolved a spring growth habit might therefore have been one of the evolutionary adaptations that enabled cultivation of those plants to be extended further north into the regions now occupied by populations 6 and 7.
It might therefore be hypothesized that these populations represent a derived form of barley that evolved during the spread of agriculture into central and northern Europe. We explore these and other archaeological interpretations of the population structure in more detail elsewhere Jones et al. We have shown that barley landraces can be divided into populations based on their microsatellite genotypes, and that these populations have different core distributions in Europe.
The dissection of population structure, combined with examination of their phenotypic attributes and environmental adaptations, enables a rational approach to the identification of landraces that might be used as sources of valuable germplasm for modern breeding programmes.
The accessions of cultivated barley included in this study are listed in Additional file 1 , Table S1. All were described by the germplasm suppliers as landraces or traditional cultivars. The accessions were chosen to give full geographical coverage across Europe.
Information on seasonal growth habit winter or spring , ear row number and caryopsis structure hulled or naked grains were obtained from the passport data for each accession. If not given in the passport data, ear row number and caryopsis structure were identified from the grain morphology.
In order to analyse population structure, a single genotype must be assigned to each accession. Some barley landraces are genetically diverse, and it cannot be assumed that the genotype of a single plant taken at random from the accession will be representative of the landrace as a whole. To avoid such errors, microsatellite genotypes were determined for two bulk samples per accession, each sample composed of a different set of ten coleoptiles, the original seeds chosen at random, and the most frequent allele identified in those cases where a landrace gave a mixed genotype.
Data were recorded and microsatellite allele lengths measured using the Genemapper 3. In those cases where a DNA extract gave peaks for multiple alleles, the amplicon giving the most intense signal was recorded.
The use of duplicate assays allowed an internal check for data quality, reducing the likelihood of a minority allele mistakenly being recorded. This approach is more straightforward than more complex methods for assigning allele frequencies in mixed microsatellite genotypes, such as thresholding [ 32 ] and calibration [ 33 ], and is equally accurate when only the most frequent allele is being recorded.
For each microsatellite, summary data including the number of alleles observed, major allele frequencies, gene diversities and polymorphism information contents PIC , were calculated using Powermarker version 3. The haploid setting and admixture model for ancestry between individuals were chosen, a degree of admixture being a reasonable expectation for populations of landraces that have had opportunities for cross-pollination. ArcGIS 9. You can also search for this author in PubMed Google Scholar.
Correspondence to Robin G. Reprints and Permissions. Allaby, R. Barley domestication: the end of a central dogma?. Genome Biol 16, Download citation. Published : 26 August Anyone you share the following link with will be able to read this content:. Sorry, a shareable link is not currently available for this article. Provided by the Springer Nature SharedIt content-sharing initiative. Skip to main content.
Search all BMC articles Search. Download PDF. Robin G. Abstract Genomic analysis of barley paints a picture of diffuse origins of this crop, with different regional wild populations contributing putative adaptive variations.
Introduction Barley did not come from any one place. Barley: the crop that would not fit Poets and colleagues have recently demonstrated a pattern of genetic diversity in barley that runs entirely counter to the view of a centric origin [ 1 ]. Concluding remarks Is barley domestication the exception or the rule? References 1. Article Google Scholar 3. Article PubMed Google Scholar Allaby Authors Robin G.
Allaby View author publications. Additional information Competing interests The author declares that they have no competing interest. About this article. Cite this article Allaby, R. At present, two hypotheses have been suggested.
One suggested the Chinese cultivated barley was introduced from the Near East Harlan, , ; Badr et al. Recent genome-wide diversity array data suggested that Chinese hulless and six-rowed barleys were domesticated in the Tibetan Plateau and its vicinity Dai et al. Our results showed that 29 out of 61 landraces shared the same haplotype Hap7 with the Tibetan wild barley, and phylogenetic analysis also revealed a close relationship between them. Haplotype Hap2 in the rest of the Chinese landraces was not only present in the Tibetan wild barley population, but also in the Central Asian and Southwest Asian wild barley populations Table 2.
Thus, our results not only supported that the Tibetan wild barley is the ancestor of Chinese domesticated barley, but also suggested that the Near East Fertile Crescent wild barley might have contributed to the origin of Chinese cultivars.
This is in agreement with previous findings that landraces with majority western ancestry were relatively commonly encountered among Asian samples Morrell et al.
Various hypotheses about the world spread of domesticated barley have been proposed. Badr et al. Some studies argued that barley was domesticated in this region and subsequently expanded westward into Europe and North Africa and eastward into Asia years ago Von Bothmer et al.
Morrell and Clegg proposed that the Fertile Crescent domestication contributed the majority of diversity in European and American cultivars, whereas the second domestication, — km farther east contributed most of the diversity in barley from Central Asia to the Far East.
First, a haplotype that is private to the Southwest Asian wild barley population was also detected in the North American and European landrace barley populations Table 2 ; Figure 1 , corroborating assumptions made by Morrell and Clegg that Fertile Crescent domestication contributed the majority of diversity in European and American cultivars.
In addition, we were surprised to see a haplotype that is exclusively found in Tibetan wild barley population is pervasive in all landrace populations Table 2 ; Figure 1. It seems likely that the ancestral carrier s of this haplotype was initially introduced from the Tibet region to other geographic regions, which might explain the high levels of similarity between Eastern malting barley and European cultivars reported by Ordon et al. Our results suggested that the gene pool of Tibetan wild barley has been widely circulated, and has significantly contributed to the gene pool of global cultivated barley.
Moreover, it may be assumed that Central Asia is the sole route for wild barley migration between the Near East and the Qinghai—Tibetan Plateau Dai et al. Thus, our results supported the most likely scenario that the gene pool of the cultivated barley includes contributions of wild barleys from both the Near East and Tibet Dai et al. Meanwhile, we suggest that the gene flow between Eastern and Western cultivars has occurred via the Silk Road, which started from China and moved westward, through the Eurasian civilization zones, Central Asia, and the Roman empire to Europe Ma, The Silk Road might be an important barley transition route between the Orient and the Occident as previously proposed Harold, ; Dai et al.
Ecologically, Tibetan wild barley is adapted to cold and dry environments, these characteristics may also be an important reason for its successfully spread all over the world Dai et al. Crop domestication is the outcome of complex independent or combined processes of artificial and natural selection that lead to plants adapted to cultivation and to meet the requirements of human consumption Dai et al. Gene pools undergoing domestication experienced dramatic changes in allele frequencies due to genetic drift or selection, and some allelic combinations may be lost Wang et al.
In our studies, a total of 10 distinct haplotypes were discovered, only 3 haplotypes were detected in the diverse set of domesticated barleys from across the world, while more haplotypes occurred in wild barley accessions.
This result agreed with previous reports Kilian et al. In addition, reduction in haplotype diversity, nucleotide diversity, and pre-site nucleotide diversity in domesticated lines was in accord with previous findings that H. Insignificance may be attributed to the low number of SNPs Table 4 observed, which weakens the neutrality test Xia et al. GPC, as a key factor for quality in cereals, is influenced to a large extent by both genotype and environment Smith, ; Jaradat, Our findings support the studies that showed H.
GPC in barley is influenced by both genetic and environmental factors Bertholdsson, ; Distelfeld et al. That allelic variation of the NAM-1 gene is an important genetic factor was demonstrated by Distelfeld et al. Jamar et al. The SNP at position is within the coding region and causes a non-synonymous change with aa substitution between Alanine A and Proline P. This substitution occurred in the C subdomain of NAC domain in the N-terminal, and may have an impact on protein folding Jamar et al.
The SNP at position was also identified by Cai et al. In addition, the differences in GPC of a particular group with no polymorphisms at the NAM-1 gene might suggest that expression of the NAM-1 gene or other genes are also important Jamar et al. In summary, our results showed significant genetic differentiation among wild populations. Our data supported that Tibet is a center of origin and domestication centre for cultivated barleys, and suggested that the Silk Road might have played an important role in gene flow between Eastern and Western barley.
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