On the origin and domestication history of Barley (Hordeum vulgare)

Remains of barley (Hordeum vulgare) grains found at archaeological sites in the Fertile Crescent indicate that about 10,000 years ago the crop was domesticated there from its wild relative Hordeum spontaneum. The domestication history of barley is revisited based on the assumptions that DNA markers effectively measure genetic distances and that wild populations are genetically different and they have not undergone significant change since domestication. The monophyletic nature of barley domestication is demonstrated based on allelic frequencies at 400 AFLP polymorphic loci studied in 317 wild and 57 cultivated lines. The wild populations from Israel-Jordan are molecularly more similar than are any others to the cultivated gene pool. The results provided support for the hypothesis that the Israel-Jordan area is the region in which barley was brought into culture. Moreover, the diagnostic allele I of the homeobox gene BKn-3, rarely but almost exclusively found in Israel H. spontaneum, is pervasive in western landraces and modern cultivated varieties. In landraces from the Himalayas and India, the BKn-3 allele IIIa prevails, indicating that an allelic substitution has taken place during the migration of barley from the Near East to South Asia. Thus, the Himalayas can be considered a region of domesticated barley diversification.     

Population-based resequencing reveals that the flowering time adaptation of cultivated barley originated east of the Fertile Crescent

Gene resequencing and association analysis present new opportunities to study the evolution of adaptive traits in crop plants. Here we apply these tools to an extensive set of barley accessions to identify a component of the molecular basis of the flowering time adaptation, a trait critical to plant survival. Using an association-based study to relate variation in flowering time to sequence-based polymorphisms in the Ppd-H1 gene, we identify a causative polymorphism (SNP48) that accounts for the observed variation in barley flowering time. This polymorphism also shows latitude-dependent geographical distribution, consistent with the expected clinal variation in phenotype with the nonresponsive form predominating in the north. Networks, genealogies, and phylogenetic trees drawn for the Ppd-H1 haplotypes reveal population structure both in wild barley and in domesticated barley landraces. The spatial distribution of these population groups indicates that phylogeographical analysis of European landraces can provide information relevant to the Neolithic spread of barley cultivation and also has implications for the origins of domesticated barley, including those with the nonresponsive ppd-H1 phenotype. Haplotypes containing the nonresponsive version of SNP48 are present in wild barley accessions, indicating that the nonresponsive phenotype of European landraces originated in wild barley. The wild accessions whose nonresponsive haplotypes are most closely similar to those of landraces are found in Iran, within a region suggested as an area for domestication of barley east of the Fertile Crescent but which has previously been thought to have contributed relatively little to the diversity of European cultivars.     

Molecular phylogeography of domesticated barley traces expansion of agriculture in the Old World

Barley (Hordeum vulgare ssp. vulgare) was first cultivated 10,500 years ago in the Fertile Crescent and is one of the founder crops of Eurasian agriculture. Phylogeographic analysis of five nuclear loci and morphological assessment of two traits in >250 domesticated barley accessions reveal that landraces found in South and East Asia are genetically distinct from those in Europe and North Africa. A Bayesian population structure assessment method indicates that barley accessions are subdivided into six clusters and that barley landraces from 10 different geographical regions of Eurasia and North Africa show distinct patterns of distribution across these clusters. Using haplotype frequency data, it appears that the Europe/North Africa landraces are most similar to the Near East population (F ST = 0.15) as well as to wild barley (F ST = 0.11) and are strongly differentiated from all other Asian populations (F ST = 0.34-0.74). A neighbor-joining analysis using these F ST estimates also supports a division between European, North African, and Near East barley types from more easterly Asian accessions. There is also differentiation in the presence of a naked caryopsis and spikelet row number between eastern and western barley accessions. The data support the differential migration of barley from two domestication events that led to the origin of barley--one in the Fertile Crescent and another farther east, possibly at the eastern edge of the Iranian Plateau--with European and North African barley largely originating from the former and much of Asian barley arising from the latter. This suggests that cultural diffusion or independent innovation is responsible for the expansion of agriculture to areas of South and East Asia during the Neolithic revolution.     

Genetic resources of wild barley in the Near East: structure, evolution and application in breeding

Genetic diversity and structure of populations of the wild progenitor of barley, Hordeum spontaneum, from three countries, Israel, Turkey and Iran, in the Near East Fertile Crescent, are compared and contrasted. The analysis is based on electrophoretically discernible allozymic variation in proteins encoded by 27 shared loci in 2125 individuals representing 52 populations of wild barley. The results indicate that: (a) H. spontaneum in the Near East Fertile Crescent is very variable genetically; (b) genetic differentiation of populations includes some clinal but primarily regional and local patterns often displaying sharp geographic differentiation over short distances; (c) the average relative genetic differentiation (G_st) was 54% within populations, 39% among populations, and 8% between the three countries; (d) allele distribution is characterized by a high proportion of unique alleles (51%), and a high proportion of common alleles that are distributed either locally or sporadically; (e) discriminant analysis by allele frequencies successfully clustered wild barley of each of the three countries (96% correct classification); (f) a substantial portion of the patterns of allozyme variation in the wild gene pool is significantly correlated with the environment and is predictable ecologically, chiefly by a combination of humidity and temperature variables; (g) natural populations of wild barley are, on the average, more variable than two composite crosses and land races of cultivated barley. The spatial patterns and environmental correlates and predictors of genetic variation of H, spontaneum in the Fertile Crescent indicate that genetic variation in wild barley populations is not only rich but at least partly adaptive and predictable by ecology and allozyme markers. Consequently, conservation and utilization programmes should optimize sampling strategies by following the ecological-genetic factors and allozyme markers as effectively predictive guidelines.     

The Horn of Africa as a centre of barley diversification and a potential domestication site

According to a widely accepted theory on barley domestication, wild barley (Hordeum vulgare ssp. spontaneum) from the Fertile Crescent is the progenitor of all cultivated barley (H. vulgare ssp. vulgare). To determine whether barley has undergone one or more domestication events, barley accessions from three continents have been studied (a) using 38 nuclear SSR (nuSSRs) markers, (b) using five chloroplast SSR (cpSSR) markers yielding 5 polymorphic loci and (c) by detecting the differences in a 468 bp fragment from the non-coding region of chloroplast DNA. A clear separation was found between Eritrean/Ethiopian barley and barley from West Asia and North Africa (WANA) as well as from Europe. The data from chloroplast DNA clearly indicate that the wild barley (H. vulgare ssp. spontaneum) as it is found today in the “Fertile Crescent” might not be the progenitor of the barley cultivated in Eritrea (and Ethiopia). Consequently, an independent domestication might have taken place at the Horn of Africa. Jihad Orabi and Gunter Backes have contributed equally to this work.     

'Tocol-omic' diversity in wild barley, short communication

Hordeum spontaneum, wild barley, is the direct progenitor of domestic barley, Hordeum vulgare, an economically important ingredient of animal feed, beer, soy sauce, and more recently, of nutraceuticals. Domestic barley has also been used in the past as a medicine. Barley is a rich source of tocotrienols, with α-tocotrienol being the most prevalent. Wild barley seeds were harvested from ecogeographically diverse areas across the Fertile Crescent, and the tocopherol (α-δ) and tocotrienol (α-δ) contents were determined. Diversity differences in individual and total 'tocol' values were significant between and within specific countries, and were significantly correlated with temperature. Wild barley may be used in the future to improve functional qualities of domestic barley. 'Tocolome' and 'tocolomics' are proposed to encompass all tocols and potentially synergy-enhancing 'entourage' compounds that may occur in tocols' 'metabolomic neighborhoods', aiding the standardized manufacture of complex barley derivatives for nutraceutical and pharmaceutical functions.     

Analysis of simple sequence repeats (SSRs) in wild barley from the Fertile Crescent: associations with ecology,geography and flowering time

Wild barley, Hordeum spontaneum C. Koch, is the progenitor of cultivated barley, Hordeum vulgare. The centre of diversity is in the Fertile Crescent of the Near East, where wild barley grows in a wide range of conditions (temperature, water availability, day length, etc.). The genetic diversity of 39 wild barley genotypes collected from Israel, Turkey and Iran was studied with 33 SSRs of known map location. Analysis of molecular variance (AMOVA) was performed to partition the genetic variation present within from the variation between the three countries of origin. Using classification tree analysis, two (or three) specific SSRs were identified which could correctly classify most of the wild barley genotypes according to country of origin. Associations of SSR variation with flowering time and adaptation to site-of-origin ecology and geography were investigated by two contrasting statistical approaches, linear regression based on SSR length variation and linear regression based on SSR allele class differences. A number of SSRs were significantly associated with flowering time under four different growing regimes (short days, long days, unvernalised and vernalised). Most of the associations observed could be accounted for by close linkage of the SSR loci to earliness per se genes. No associations were found with photoperiodic and vernalisation response genes known to control flowering in cultivated barley suggesting that different genetic factors may be active in wild barley. Novel genomic regions controlling flowering time in wild barley were detected on chromosomes 1HS, 2HL, 3HS and 4HS. Associations of SSRs with site-of-origin ecological and geographic data were found primarily in genomic regions determining plant development. This study shows that the analyses of SSR variation by allele class and repeat length are complementary, and that some SSRs are not necessarily selectively neutral.     

Molecular diversity at 18 loci in 321 wild and 92 domesticate lines reveal no reduction of nucleotide diversity during Triticum monococcum (Einkorn) domestication: implications for the origin of agriculture

The diploid wheat Triticum monococcum L. (einkorn) was among the first crops domesticated by humans in the Fertile Crescent 10,000 years ago. During the last 5,000 years, it was replaced by tetraploid and hexaploid wheats and largely forgotten by modern breeders. Einkorn germplasm is thus devoid of breeding bottlenecks and has therefore preserved in unfiltered form the full spectrum of genetic variation that was present during its domestication. We investigated haplotype variation among >12 million nucleotides sequenced at 18 loci across 321 wild and 92 domesticate T. monococcum lines. In contrast to previous studies of cereal domestication, we sampled hundreds of wild lines, rather than a few dozen. Unexpectedly, our broad sample of wild lines reveals that wild einkorn underwent a process of natural genetic differentiation, most likely an incipient speciation, prior to domestication. That natural differentiation was previously overlooked within wild einkorn, but it bears heavily upon inferences concerning the domestication process because it brought forth 3 genetically, and to some extent morphologically, distinct wild einkorn races that we designate here as alpha, beta, and gamma. Only one of those natural races, beta, was exploited by humans for domestication. Nucleotide diversity and haplotype diversity in domesticate einkorn is higher than in its wild sister group, the einkorn beta race, indicating that einkorn underwent no reduction of diversity during domestication. This is in contrast to findings from previous studies of domestication history among more intensely bred crop species. Taken together with archaeological findings from the Fertile Crescent, the data indicate that a specific wild einkorn race that arose without human intervention was subjected to multiple independent domestication events.     

Comment on "AFLP data and the origins of domesticated crops".

We review some concepts and methods of handling and using DNA fingerprinting in phylogenetic analyses related to crop domestication. Particular reference is made to AFLP markers and mode and place of einkorn, barley, and tetraploid wheat domestication in the Neolithic by human communities in the Fertile Crescent. The reconsideration of AFLP databases of domesticated and wild lines demonstrates that phylogenetic tree topologies, originally described for the three species, match closely the new results obtained by principle coordinate analyse.     

Haplotype diversity and population structure in cultivated and wild barley evaluated for Fusarium head blight responses

Fusarium head blight (FHB) is a threat to barley (Hordeum vulgare L.) production in many parts of the world. A number of barley accessions with partial resistance have been reported and used in mapping experiments to identify quantitative trait loci (QTL) associated with FHB resistance. Here, we present a set of barley germplasm that exhibits FHB resistance identified through screening a global collection of 23,255 wild (Hordeum vulgare ssp. spontaneum) and cultivated (Hordeum vulgare ssp. vulgare) accessions. Seventy-eight accessions were classified as resistant or moderately resistant. The collection of FHB resistant accessions consists of 5, 27, 46 of winter, wild and spring barley, respectively. The population structure and genetic relationships of the germplasm were investigated with 1,727 Diversity Array Technology (DArT) markers. Multiple clustering analyses suggest the presence of four subpopulations. Within cultivated barley, substructure is largely centered on spike morphology and growth habit. Analysis of molecular variance indicated highly significant genetic variance among clusters and within clusters, suggesting that the FHB resistant sources have broad genetic diversity. The haplotype diversity was characterized with DArT markers associated with the four FHB QTLs on chromosome 2H bin8, 10 and 13 and 6H bin7. In general, the wild barley accessions had distinct haplotypes from those of cultivated barley. The haplotype of the resistant source Chevron was the most prevalent in all four QTL regions, followed by those of the resistant sources Fredrickson and CIho4196. These resistant QTL haplotypes were rare in the susceptible cultivars and accessions grown in the upper Midwest USA. Some two- and six-rowed accessions were identified with high FHB resistance, but contained distinct haplotypes at FHB QTLs from known resistance sources. These germplasm warrant further genetic studies and possible incorporation into barley breeding programs.     

Comparative assessment of genetic diversity in wild and primitive cultivated barley in a center of diversity

Wild barley (Hordeum spontaneum K.) and indigenous primitive varieties of cultivated barley (Hordeum vulgare L.), collected from 43 locations in four eastern Mediterranean countries, Jordan, Syria, Turkey and Greece, were electrophoretically assayed for genetic diversity at 16 isozyme loci. Contrary to a common impression, cultivated barley populations were found to maintain a level of diversity similar to that in its wild progenitor species. Apportionment of overall diversity in the region showed that in cultivated barley within-populations diversity was of higher magnitude than the between-populations component. Neighboring populations of wild and cultivated barleys showed high degree of genetic identity. Groups of 3 or 4 isozyme loci were analyzed to detect associations among loci. Multilocus associations of varying order were detected for all three groups chosen for the analysis. Some of the association terms differed between the two species in the region. Although there was no clear evidence for decrease in diversity attributable to the domestication of barley in the region, there was an indication of different multilocus organizations in the two closely related species.     

Domestication and crop evolution of wheat andbarley: Genes,genomics, and future directions

Wheat and barley are two of the founder crops of the agricultural revolution that took place 10,000 years ago in the Fertile Crescent and both crops remain among the world's most important crops. Domestication of these crops from their wild ancestors required the evolution of traits useful to humans, rather than survival in their natural environment. Of these traits, grain retention and threshability, yield improvement, changes to photoperiod sensitivity and nutritional value are most pronounced between wild and domesticated forms. Knowledge about the geographical origins of these crops and the genes responsible for domestication traits largely pre-dates the era of nextgeneration sequencing, although sequencing will lead to new insights. Molecular markers were initially used to calculate distance(relatedness), genetic diversity and to generate genetic maps which were useful in cloning major domestication genes. Both crops are characterized by large,complex genomes which were long thought to be beyond the scope of whole-genome sequencing. However, advances in sequencing technologies have improved the state of genomic resources for both wheat and barley. The availability of reference genomes for wheat and some of its progenitors,as well as for barley, sets the stage for answering unresolved questions in domestication genomics of wheat and barley.     

Origin and domestication of the fungal wheat pathogen Mycosphaerella graminicola via sympatric speciation

The Fertile Crescent represents the center of origin and earliest known place of domestication for many cereal crops. During the transition from wild grasses to domesticated cereals, many host-specialized pathogen species are thought to have emerged. A sister population of the wheat-adapted pathogen Mycosphaerella graminicola was identified on wild grasses collected in northwest Iran. Isolates of this wild grass pathogen from 5 locations in Iran were compared with 123 M. graminicola isolates from the Middle East, Europe, and North America. DNA sequencing revealed a close phylogenetic relationship between the pathogen populations. To reconstruct the evolutionary history of M. graminicola, we sequenced 6 nuclear loci encompassing 464 polymorphic sites. Coalescence analyses indicated a relatively recent origin of M. graminicola, coinciding with the known domestication of wheat in the Fertile Crescent around 8,000-9,000 BC. The sympatric divergence of populations was accompanied by strong genetic differentiation. At the present time, no genetic exchange occurs between pathogen populations on wheat and wild grasses although we found evidence that gene flow may have occurred since genetic differentiation of the populations.     

Islands and streams: clusters and gene flow in wild barley populations from the Levant

The domestication of plants frequently results in a high level of genetic differentiation between domesticated plants and their wild progenitors. This process is counteracted by gene flow between wild and domesticated plants because they are usually able to inter‐mate and to exchange genes. We investigated the extent of gene flow between wild barley Hordeum spontaneum and cultivated barley Hordeum vulgare, and its effect on population structure in wild barley by analysing a collection of 896 wild barley accessions (Barley1K) from Israel and all available Israeli H. vulgare accessions from the Israeli gene bank. We compared the performance of simple sequence repeats (SSR) and single nucleotide polymorphisms (SNP) marker data genotyped over a core collection in estimating population parameters. Estimates of gene flow rates with SSR markers indicated a high level of introgression from cultivated barley into wild barley. After removing accessions from the wild barley sample that were recently admixed with cultivated barley, the inference of population structure improved significantly. Both SSR and SNP markers showed that the genetic population structure of wild barley in Israel corresponds to the three major ecogeographic regions: the coast, the Mediterranean north and the deserts in the Jordan valley and the South. Gene flow rates were estimated to be higher from north to south than in the opposite direction. As has been observed in other crop species, there is a significant exchange of alleles between the wild species and domesticated varieties that needs to be accounted for in the population genetic analysis of domestication.     

Invasion of <Emphasis Type="Italic">Rhynchosporium commune</Emphasis> onto wild barley in the Middle East

Rhynchosporium commune was recently introduced into the Middle East, presumably with the cultivated host barley (Hordeum vulgare). Middle Eastern populations of R. commune on cultivated barley and wild barley (H. spontaneum) were genetically undifferentiated and shared a high proportion of multilocus haplotypes. This suggests that there has been little selection for host specialization on H. spontaneum, a host population often used as a source of resistance genes introduced into its domesticated counterpart, H. vulgare. Low levels of pathogen genetic diversity on H. vulgare as well as on H. spontaneum, indicate that the pathogen was introduced recently into the Middle East, perhaps through immigration on infected cultivated barley seeds, and then invaded the wild barley population. Although it has not been documented, the introduction of the pathogen into the Middle East may have a negative influence on the biodiversity of native Hordeum species.     

Genetic relationships among South-East Turkey wild barley populations and sampling strategies of Hordeum spontaneum

To assess the genetic diversity and the genetic structure of Turkish wild barley (Hordeum spontaneum Tell.) populations, 76 genotypes from ten ecologically and geographically different locations were analyzed by means of amplified fragment length polymorphism (AFLP) markers. Five primer combinations produced 187 scorable bands, of which 117 (62.6%) were polymorphic. Several population-specific and genotype-specific bands were identified, which differentiate populations or genotypes. Genetic distance, determined by Nei’s distance coefficient, varied from 0.07 to 0.21 with an average of 0.13. In the UPGMA dendrogram based on Nei genetic distances, the Hordeum spontaneum populations were separated into two major clusters. Genetic diversity was larger among (68%) than within (32%) populations. Eight AFLP bands were strongly correlated to the altitude of the collecting site, while no clear trend was detected between geographical origin and genetic diversity. Our results strongly suggest the need for a change in Hordeum spontaneum sampling strategy: more populations, rather then more individuals within population, should be sampled to appraise and safeguard genetic diversity in the wild barley gene pool.     

Monophyletic origin of naked barley inferred from molecular analyses of a marker closely linked to the naked caryopsis gene (<Emphasis Type="Italic">nud</Emphasis>)

To elucidate the origin of naked barley, molecular variation of the marker sKT7 tightly linked to the nud locus was examined. A total of 259 (53 wild, 106 hulled domesticated, and 100 naked domesticated) barley accessions were studied. Restriction analysis of the sKT7 PCR-amplified product revealed the alleles I, II, III, and IV. All four alleles were found in wild barley, but allele IV was found only in a single accession from southwestern Iran. Hulled domesticated accessions showed alleles I, II, or III, but all naked domesticated accessions had allele IV. The distribution of allele IV in wild barley and its pervasive presence in naked domesticated lines support the conclusion that naked barley has a monophyletic origin, probably in southwestern Iran. The available results suggest two scenarios for the origin of naked barley: either directly from a wild barley with allele IV or from a hulled domesticated line with allele IV that later became extinct. Naked domesticated accessions from different regions of the world have extremely homogeneous DNA sequences at the sKT7 locus, supporting the monophyletic origin of naked barley. For allele IV, four haplotypes (IVb to IVe) were found in 30 naked accessions: IVb was predominant (66.7%) and widely distributed, while the other three haplotypes, differing by only one nucleotide at different positions relative to IVb, showed a localized distribution. The geographical distribution of the haplotypes of sKT7 allele IV suggests migration routes of naked domesticated barley in central and eastern Asia.Communicated by F. Salamini     

Five ovine mitochondrial lineages identified from sheep breeds of the near East

Archaeozoological evidence indicates that sheep were first domesticated in the Fertile Crescent. To search for DNA sequence diversity arising from previously undetected domestication events, this survey examined nine breeds of sheep from modern-day Turkey and Israel. A total of 2027 bp of mitochondrial DNA (mtDNA) sequence from 197 sheep revealed a total of 85 haplotypes and a high level of genetic diversity. Six individuals carried three haplotypes, which clustered separately from the known ovine mtDNA lineages A, B, and C. Analysis of genetic distance, mismatch distribution, and comparisons with wild sheep confirmed that these represent two additional mtDNA lineages denoted D and E. The two haplogroup E sequences were found to link the previously identified groups A and C. The single haplogroup D sequence branched with the eastern mouflon (Ovis orientalis), urial (O. vignei), and argali (O. ammon) sheep. High sequence diversity (K = 1.86%, haplogroup D and O. orientalis) indicates that the wild progenitor of this domestic lineage remains unresolved. The identification in this study of evidence for additional domestication events adds to the emerging view that sheep were recruited from wild populations multiple times in the same way as for other livestock species such as goat, cattle, and pig.     

Strong correlation of wild barley (Hordeum spontaneum) population structure with temperature and precipitation variation

In this study, we present the genetic analysis of a new collection of wild barley ( Hordeum spontaneum ) using 42 simple sequence repeat (SSR) markers that represent the seven chromosomes. The Barley1K (B1K) infrastructure consists of 1020 accessions collected in a hierarchical sampling mode (HSM) from 51 sites across Israel and represents the wide adaptive niche of the modern barley's ancestor. According to the genetic structure analysis, the sampled sites can be divided into seven groups, and sampled microsites located on opposing slopes or in different soil types did not show significant genetic differentiation. Although the genetic analysis indicates a simple isolation-by-distance model among the populations, examination of the genetic populations' structure with abiotic parameters in an ordination analysis revealed that the combination of elevation, mid-day temperature and rainfall explains a high proportion of the variance in the principal components analysis. Our findings demonstrate that the current populations have therefore been shaped and distinguished by non-selective forces such as migration; however, we suggest that aridity and temperature gradients played major roles as selective forces in the adaptation of wild barley in this part of the Fertile Crescent. This unique collection is a prelude for the investigation of the molecular basis underlying plant adaptation and responsiveness to harsh environments.