See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/227337621 Invasion of Rhynchosporium commune onto wild barley in the Middle East Article in Biological Invasions · February 2010 DOI: 10.1007/s10530-010-9808-6 CITATIONS READS 5 43 5 authors, including: Kiros Meles 6 PUBLICATIONS 42 CITATIONS SEE PROFILE Bruce A McDonald ETH Zurich 254 PUBLICATIONS 8,075 CITATIONS SEE PROFILE Amor Hassine Yahyaoui Celeste C Linde 32 PUBLICATIONS 571 CITATIONS 116 PUBLICATIONS 2,851 CITATIONS Consultative Group on International Agricult… SEE PROFILE Australian National University SEE PROFILE Some of the authors of this publication are also working on these related projects: The rise of Pyricularia graminis-tritici sp. nov. as the wheat blast pathogen in Brazil: sympatric speciation inferred from multilocus gene phylogeny, pathogenicity spectra and avirulence genes evolution View project All content following this page was uploaded by Bruce A McDonald on 02 February 2015. The user has requested enhancement of the downloaded file. All in-text references underlined in blue are added to the original document and are linked to publications on ResearchGate, letting you access and read them immediately. Biol Invasions DOI 10.1007/s10530-010-9808-6 ORIGINAL PAPER Invasion of Rhynchosporium commune onto wild barley in the Middle East A. Kiros-Meles • D. Gomez • B. A. McDonald A. Yahyaoui • C. C. Linde • Received: 21 January 2010 / Accepted: 17 June 2010 Ó Springer Science+Business Media B.V. 2010 Abstract 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 C. C. Linde (&) Evolution, Ecology & Genetics, Research School of Biology, Australian National University, Bldg. 116, Daley rd, Canberra, ACT 0200, Australia e-mail: celeste.linde@anu.edu.au A. Kiros-Meles Tigrai Institute of Agricultural Research, Mekele, Tigray, Ethiopia D. Gomez CSIRO Entomology, GPO Box 1700, Canberra, ACT 2601, Australia B. A. McDonald Plant Pathology Group, Institute of Integrative Biology, ETH Zurich/LFW, Universitätstrasse 2, 8092 Zurich, Switzerland A. Yahyaoui International Center for Agricultural Research in the Dry Areas (ICARDA), P. O. Box 5466, Aleppo, Syria 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. Keywords Pathogen invasion
Hordeum
Microsatellites
Agriculture
Founder populations
Population structure
Rhynchosporium secalis Introduction Plant pathogen invasions are well documented, especially when they cause obvious effects on the naı̈ve host population (Anderson et al. 2004). A well-known example is the devastating effect Cryphonectria parasitica had on native North American chestnut trees after the pathogen was introduced from Asia (Anagnostakis 1987). Invasive pathogens also have the potential to affect plant diversity and ecosystem structure in ways more subtle than destroying the entire plant species. For example, the introduction of a viral pathogen to California reversed the competitive balance between native and exotic grasses, because the viral pathogen allowed exotic grasses to invade 123 A. Kiros-Meles et al. and become dominant (Borer et al. 2007). This example also highlights one possible side-effect of a pathogen introduction into a naı̈ve plant system. International trade related to agriculture (e.g. in seed, germplasm, grains, fruits and vegetables) provides many opportunities for pathogen invasion. Pathogens from the region where a crop originated may invade new areas, or new pathogens may be introduced into a crop’s region of origin after acquiring an exotic pathogen from elsewhere. The effects of pathogen emergence and invasion on plant biodiversity have been poorly studied even though diseases of cultivated plants are likely to threaten wild populations of related plants. The case of Torreya taxifolia in Florida presents an example of a pathogen introduction on a cultivated conifer host that led to the crash of native T. taxifolia populations (Schwartz et al. 1995). Rhynchosporium commune is an economically important pathogen that causes a foliar disease, called scald, on barley. R. commune was previously known as Rhynchosporium secalis, however recent phylogenetic and pathogenicity analyses led to the description of R. commune as the taxon associated with Hordeum (barley) and Bromus spp. (Zaffarano et al. 2010). Currently, R. commune has a worldwide distribution and is present wherever barley is cultivated. Apart from cultivated barley, R. commune can also infect other Hordeum species as well as Bromus driandrus (Ali and Boyd 1974; Caldwell 1937; Zaffarano et al. 2008). It was assumed that R. commune co-evolved with its Hordeum spp. hosts in the Fertile Crescent, the origin of barley (Salamini et al. 2002), but recent global population genetic studies have shown that northern Europe is the likely centre of origin for R. commune (Linde et al. 2009; Zaffarano et al. 2006) and not the Middle East as was expected. It was postulated that the pathogen was introduced into the Middle East through trade in agricultural products rather than being present as a pathogen throughout regions where barley was domesticated (Linde et al. 2009; Zaffarano et al. 2006, 2008; 2009). Under this scenario, R. commune is an invasive pathogen in the Middle East and pathogen populations on cultivated barley and subsequently on wild Hordeum species both represent founder populations, that is, they have undergone recent genetic bottlenecks resulting in low levels of genetic diversity. We hypothesized that the effects of genetic drift on the 123 pathogen would be more pronounced in populations infecting wild Hordeum species because the larger number of resistance genes found in wild barley compared to cultivated barley (Abbott et al. 1992) would allow only a fraction of the pathogen population introduced on the cultivated host to infect the wild host. Hordeum is a genus of 32 species that grow in temperate and adjacent subtropical areas. Centres of diversity for Hordeum, based on areas containing the highest number of species, are found in Southwest Asia (Middle East), Central Asia, western North America, and southern South America (von Bothmer et al. 1995). Members of the genus can also be found in Africa and northern Africa is considered a secondary center of diversity for barley (Salamini et al. 2002). Hordeum vulgare ssp. spontaneum (hereafter referred to as H. spontaneum) is the wild progenitor of cultivated barley (Hordeum vulgare ssp. vulgare, hereafter referred to as H. vulgare). The endemic range of H. spontaneum extends from Turkey, Syria and the Jordan Valley towards Pakistan and Afghanistan (von Bothmer et al. 2003; Zohary and Hopf 2000). Populations of H. spontaneum are frequently observed growing on uncultivated land, along roadsides, bordering barley fields and in some areas within cultivated barley fields. Populations of R. commune occur on both H. spontaneum and H. vulgare in the Middle East, but the role of these populations in contributing to the genetic structure of pathogen populations and to the epidemiology on cultivated or wild barley in the Middle East is unknown. It is foreseeable that R. commune will negatively affect naı̈ve hosts such as wild barley populations in the Middle East, even though some resistance genes have been described in wild barley (Abbott et al. 1992; Genger et al. 2003). The aim of this study was to determine whether the population structure of R. commune in Syria and Jordan is consistent with a founder population that has been disseminated by agricultural activities resulting in a homogenous pathogen population across the region. We hypothesised that the pathogen population on the two hosts H. spontaneum and H. vulgare would be similar with little population differentiation, with the effects of genetic drift more pronounced on the populations of H. spontaneum, a wild Hordeum species and progenitor of cultivated barley. Invasion of Rhynchosporium commune onto wild barley in the Middle East Materials and methods Collection of R. commune from cultivated and wild barley populations Hordeum vulgare and H. spontaneum leaves infected with R. commune were sampled in Jordan and Syria (Table 1), separated by a maximum of 700 km. Scald samples from cultivated barley were collected using a hierarchical sampling method (McDonald et al. 1999). For H. spontaneum, one leaf per plant was taken with a minimum sampling distance of one meter. Isolates obtained from H. vulgare and H. spontaneum occurring in Madaba-East were collected from the same field site within a 400 m2 area. At this site, scald lesions occurring on H. vulgare were collected hierarchically (McDonald et al. 1999), while for H. spontaneum, 60 infected leaves were collected from two diagonals of the same sampling area, at an interval of a least 1 m distance. The JUST population was collected from a field where H. spontaneum was re-established at least 15 years ago. Isolation, cultivation and maintenance of R. commune isolates from Hordeum leaves were conducted as described previously (McDonald et al. 1999). In total, 586 isolates were collected from cultivated barley and H. spontaneum in Jordan and Syria (Table 1). Microsatellite analyses DNA was extracted by either a CTAB method (von Korff et al. 2004) or a DNeasy Plant Mini kit (Qiagen, Hilden Germany) according to the specifications of the manufacturer. A set of seven specific primer pairs was used to amplify microsatellite loci Rh80 , Rh9, Rh10, Rh11, Rh12, Rh13, Rh14 as described previously (Linde et al. 2005). Fragment sizes were determined on an ABI 3100 sequencer using the GENESCAN v3.7 software (Applied Biosystems). Data analysis Isolates sharing the same alleles across all seven loci were identified as having the same multilocus haplotype (MLHT) and were considered clones. The number of unique MLHTs was calculated in GENALEX (Peakall and Smouse 2006). Genotypic diversity, Ĝ (Stoddart and Taylor 1988) where the maximum possible value of Ĝ occurs when the number of unique MLHTs is equal to the number of individuals in the sample, was calculated for each population. The proportion of the maximum possible genotypic diversity (Ĝm = Ĝ/N where N = population sample size), was calculated to allow for meaningful comparisons of Ĝ across varying sample sizes. The significance of differences in genotype diversity was calculated using a t-test (Chen et al. 1994). The clonal fraction was calculated as the occurrence and frequency of clones within a population, N N G, where N is the sample size and G is the number of haplotypes. Because of sensitivity to sample size of the previous indices (Grunwald et al. 2003), genotype diversity was also calculated with the Shannon– Weaver index, Eh0 (Shannon and Weaver 1949). To determine the probability that individuals with identical alleles at all 7 loci were derived from separate sexual reproductive events (Psex), analyses were conducted in GENCLONE 2.0 (Arnaud-Haond and Belkhir 2007). Comparisons of gene diversity among field populations and between host groups were determined in GENALEX (Peakall and Smouse 2006) by calculating Shannon’s diversity index (Lewontin 1972), Nei’s gene diversity (Nei 1973), number of alleles and the effective number of alleles in each locus (Kimura and Crow 1964). Analysis of molecular variance (AMOVA) was performed to assess the distribution of genetic variation across host and spatial (population) groups in GENALEX (Peakall and Smouse 2006). AMOVA estimates variance components for haploid genotypes and partitions genotypic variation within and among hierarchical groupings based on the proportion of allelic differences between all pairs of individuals. The parameters estimated by AMOVA, referred to as U statistics (Excoffier et al. 1992), are analogous to Wright’s F-statistics (Wright 1969) describing the level of population differentiation. The degree of differentiation between each pair of populations was also measured by calculating pairwise U statistics. Under the null hypothesis of no genetic differentiation, the level of migration is expected to be high because differentiation is inversely proportional to gene flow. Bayesian clustering, implemented in the software STRUCTURE v2.2 was used to identify population structure based on the assignment of individuals to 123 A. Kiros-Meles et al. Table 1 Microsatellite derived population genetic parameters for populations of Rhynchosporium commune from Hordeum spontaneum and cultivated barley collected in Jordan and Syria Year Nb Nuc Ĝdm collected Eh0 e Clonal Naf fraction Neg Hh Ii IjA Population Country of origina Host Ramtha-W Jordan, Irbid H. spontaneum 2003 24 11 0.33 2.22 0.54 4.00 2.72 0.51 0.98 0.60 Madaba-E Jordan, Madaba H. spontaneum 2003 39 25 0.41 3.03 0.36 4.14 2.18 0.44 0.85 0.06* JUST Jordan, Irbid H. spontaneum 2003 15 12 0.65 2.22 0.20 3.00 2.20 0.44 0.80 -0.02* RS04_AlBara Syria, Idlib H. spontaneum 2004 22 7 0.24 1.80 0.68 3.14 2.63 0.49 0.88 1.50 Sy04Kansa Syria, Idlib H. spontaneum 2004 23 2 0.08 0.65 0.91 1.14 1.12 0.07 0.09 N/A RS04SyRasa Syria, Al-Hasaka H. spontaneum 2004 21 9 0.30 2.00 0.57 3.00 2.23 0.40 0.73 1.32 RS04SyHsTT Syria, Al-Hasaka H. spontaneum 2004 19 1 0.05 0.00 0.95 1.00 1.00 0.00 0.00 N/A RS02Sy6 Syria, Ar Raqqah H. spontaneum 2002 30 20 0.42 2.79 0.33 6.00 3.79 0.60 1.29 0.72 Goreen Syria, Hamah H. spontaneum 2004 58 2 0.02 0.33 0.97 1.86 1.20 0.16 0.29 N/A Jalilabad Syria, Idlib H. spontaneum 2004 30 6 0.11 1.47 0.80 3.29 2.21 0.47 0.85 0.62* Madaba-E_cult Jordan, Madaba RS02Sy1 Syria, Halab H. vulgare H. vulgare 2003 2002 53 28 67 16 0.27 3.02 0.47 0.10 1.87 0.76 4.43 2.64 0.48 0.96 0.15* 5.00 2.26 0.42 0.83 1.52 RS02Sy2 Syria, Halab H. vulgare 2002 56 15 0.09 1.72 0.73 4.14 1.93 0.39 0.77 1.34 RS02Sy3 Syria, Ar Raqqah H. vulgare 2002 67 35 0.26 3.24 0.48 6.71 3.58 0.57 1.25 0.80 RS02Sy4 Syria, Ar Raqqah H. vulgare 2002 62 24 0.18 2.76 0.61 5.00 2.89 0.49 1.01 0.66 a The country of origin, followed by the region from which collections were made b Sample size c Number of Multilocus haplotypes d Stoddart and Taylor (1988) genotypic diversity e Shannon-Wiener index (Shannon and Weaver 1949) f Number of alleles g Effective number of alleles h Nei’s gene diversity (Nei, 1973) i Shannon’s Information index (Lewontin 1972) j Index of Association * Populations were at linkage equilibrium (P [ 0.05), thus representing randomly mating populations likely ancestral groups (Pritchard et al. 2000). This method estimates the probability of an individual belonging to an ancestral group and individuals assigned to one group or jointly to two or more groups if admixed. Five independent runs of STRUCTURE were performed by setting the number of groups, K, from 1 to 15, with 10,000 ‘‘burn-in’’ replicates and a run length of 40,000 steps. For each independent run, the posterior probability, Ln(K), of the data for a set value of K is calculated and used as a guide to estimate K. The value of K when Ln(K) no longer improves is used as the best estimate of K. The second order rate of change in Ln(K), denoted DK, as described by Evanno et al. (2005), provides a good indicator of the ‘‘real’’ value of K and was calculated to provide consensus with estimations of K 123 derived from Ln(K). Analyses were performed using an admixture model without prior population information. The genetic relatedness among all MLHTs was also determined using multivariate statistics. Genetic relatedness was visualized using nonmetric multidimensional scaling (NMDS), an ordination analysis that can be useful for recovering nonhierarchical patterns of genetic variation (Lessa 1990). The stress value associated with NMDS indicates how well the distribution of points on the plot matches the actual distances between observations (individuals). Stress values of \0.2 correspond to meaningful representation of the data with little chance of misrepresentation, whereas values of [0.3 generally indicate poor representation and suggest that care should be taken Invasion of Rhynchosporium commune onto wild barley in the Middle East in interpreting the ordination (Clarke and Warwick 2001). NMDS plots were based on Nei’s genetic distance (Nei 1978) calculated for all pairwise comparisons of unique MLHTs identified from each population. NMDS plots were generated using the multivariate statistical software package PRIMER-R v6 (PRIMER-E Ltd., Plymouth Marine Laboratory, Plymouth, UK). Isolation by distance between geographic and genetic distances among populations was assessed with a Mantel test in GENALEX (Peakall and Smouse 2006). To test for random mating, multilocus linkage disequilibrium was measured by calculating the index of association (IA) using the Multilocus v1.3 software (Agapow and Burt 2001). The IA observed for each population was compared with the IA expected under random mating which was simulated through the reshuffling of alleles within each clone-corrected population data set over 1,000 permutations. Each population was clone-corrected to prevent biased calculations of IA caused by the repeated sampling of the same MLHT in any population. Results Genetic diversity One hundred and 46 distinct multilocus haplotypes of R. commune were identified from a total of 586 isolates collected from the Middle East (Table 1). A total of 34 MLHTs were shared among spatial populations. Population analysis of shared MLHTs revealed that populations shared at least one through to a maximum of seven MLHTs (data not shown). The exception to this was the Goreen population that did not share MLHTs with other populations, but had two haplotypes which were unique to that population. The most common MLHT shared by eight populations constituted approximately 14% of the total number of isolates in this study. Nine MLHTs were shared between the Jordanian and Syrian populations while 23 MLHTs were common to populations of R. commune recovered from H. vulgare and H. spontaneum. Results of the GENCLONE analysis indicated that the probability that individuals which were identical at all 7 loci were derived from separate sexual reproductive events, Psex, was low, with values ranging from 1.8 9 10-6 to 4.5 9 10-12. Therefore, these individuals were considered to be derived from asexual reproduction. Across the fifteen populations, levels of genotypic diversity and gene diversity ranged between 0.02 and 0.65, 0.0 and 0.60, respectively. Shannon’s information index was lowest for population RS04SyHsTT and greatest for population RS02Sy6 (Table 1). There was a high degree of clonality and thus lower genotypic diversity in populations such as, but not limited to, Goreen and RS04SyHsTT. At least half of the H. spontaneum derived R. commune populations were exclusively (RS04SyHSTT) or nearly exclusively clonal (Table 1). For example, only two MLHTs were observed in the Goreen population with one haplotype occurring 52 times among 58 individuals. Clonal fractions were also high in two of the five H. vulgare derived populations (Table 1). Genotypic diversities of populations in pairwise comparisons, regardless of host origin or geography, were always not significantly different (data not shown). Genotypic diversity (Ĝm and Eh0 ), gene diversity (H) and effective number of alleles were marginally greater for populations recovered from H. spontaneum (Table 2). However, these and the average number of allele differences were not significant (P [ 0.5) with a t-test, suggesting that host species did not influence the levels of genetic diversity of R. commune populations. Population structure AMOVA revealed low and insignificant levels of differentiation between isolates from H. spontaneum and H. vulgare (U = 0.012; Table 3). In addition to this, the host grouping failed to account for any variation in the hierarchical analysis. Similarly, AMOVA analyses revealed low and insignificant levels of differentiation between R. commune populations representing geographical groups Jordan, Central Syria and Northeastern Syria (U = 0.003; P = 0.085). In contrast, 24% of variation could be explained among R. commune populations within host groups or within geographical groups. Variation among isolates within populations explained 75% of the genetic variance in the hierarchical analyses concerning hosts, and 76% in the regional analysis. Ninety-five percent of pairwise comparisons of U were calculated as between 0 and 0.6, suggesting migration varied between field sites. 123 A. Kiros-Meles et al. Table 2 Comparison of microsatellite allele counts, gene diversity and genotypic diversity between Rhynchosporium commune isolates from Hordeum spontaneum and Hordeum vulgare Host population Na Nub Ĝcm Eh0 d Nae Nef Hg Ih H. spontaneum 281 80 0.06 3.62 9.7 3.5 0.57 1.32 H. vulgare 305 89 0.05 3.55 10.0 3.0 0.51 1.16 a Sample size b Number of multilocus haplotypes c Stoddart and Taylor (1988) genotypic diversity d Shannon-Wiener index (Shannon and Weaver 1949) e Number of alleles f Effective number of alleles g Nei’s gene diversity (Nei 1973) h Shannon’s Information index (Lewontin 1972) No significant difference (t-test; P [ 0.05) was observed in calculations of Ĝm, Na, Ne, H and I between the two host populations Table 3 AMOVA among 586 isolates of Rhynchosporium commune collected across 15 field populations on two Hordeum species occurring in Jordan and Syria Comparison Between host groups Among populations within host groups Within populations Degrees of freedom Sums of squares Estimated variance % Variation U P 1 30.3 0.024 1 0.012 0.001 13 248.9 0.467 24 0.239 0.001 571 847.8 1.485 75 0.249 0.001 Assignment of individuals to common ancestries, using the STRUCTURE software, revealed that isolates could be grouped into eight likely clusters with good consensus between the two different methods used to estimate K (Fig. 1). However, both STRUCTURE (data not shown) and NMDS plots (Fig. 2) revealed poor correlation among cluster assignments when isolates were grouped according to their host and geographic population labels. These results indicated populations were sufficiently admixed to a point where clustering patterns, based on a priori population labels, could not be readily identified. This suggests that field populations of R. commune are comprised of individuals of varying ancestries and origin. Isolation by distance analyses (R2 = 0.1316; P = 0.030) showed little correlation between geographic distance and genetic distance, indicating a homogenous founder population. (\3) being identified in each of these populations. Evidence for panmixia was observed for only four populations: Madaba-E (IA = 0.06; P = 0.29), JUST (IA = -0.02; P = 0.51), Jalilabad (IA = 0.62; P = 0.07) and Madaba-E_cult (IA = 0.15; P = 0.08). Calculation of IA at the remaining populations indicated significant association among alleles, consistent with non-random mating (P \ 0.05; Table 1). Random mating Fig. 1 Eight ancestral groups (K = 8) of Rhynchosporium commune identified among 586 isolates based on the peak values of the posterior probability, Ln(K) (open square), and the second order rate of change in Ln(K), denoted DK (filled square) at increasing values of K Populations Sy04Kansa, RS04SyHsTT and Goreen were omitted from analysis of IA due to few MLHTs 123 Invasion of Rhynchosporium commune onto wild barley in the Middle East Fig. 2 Genetic relationships among unique MLHTs of Rhynchosporium commune from Hordeum spontaneum (D) and Hordeum vulgare (filled square) as visualized by a NMDS plot (2D stress = 0.08) revealed no discernable clustering patterns when isolates were labeled according to host origin Discussion Since the domestication of barley in the Fertile Crescent more than 10000 years ago (Badr et al. 2000; Salamini et al. 2002), it has become a major cereal crop grown on millions of hectares around the world. As barley cultivation spread, exotic pathogens adapted to this new host and were distributed through agricultural trade and breeding programs. Here we present evidence that one of these exotic pathogens, R. commune, has become widely disseminated in wild barley populations in the Fertile Crescent, presenting an example of pathogen pollution (Anderson et al. 2004) that may pose a threat to wild barley populations that represent an important source of germplasm for barley breeding. From a total of 586 R. commune isolates sampled from cultivated barley and H. spontaneum fields in the Middle East, only 146 distinct genotypes were identified. This represents a low genotypic diversity, typical for a founder population. In fact, in previous studies comparing 34 populations from around the world, R. commune populations from Syria displayed the lowest levels of genotypic diversity (Linde et al. 2009; Zaffarano et al. 2006, 2009). In this study, 23% of MLHTs were widespread and shared among populations. In previous studies, widespread haplotypes have never been found (Linde et al. 2009; McDonald et al. 1999; Zaffarano et al. 2006). However, this is not as surprising as it seems because these populations represent a larger sample size from a relatively smaller geographic region. Also, and perhaps more importantly, the Middle Eastern populations represent founder populations of R. commune, having low levels of genetic diversity relative to other populations studied to date (Linde et al. 2009). Thus, a limited number of founder events and subsequent migration of clones could explain the widespread and shared distribution of MLHTs. Furthermore, R. commune, like most other fungi, has a significant asexual reproduction phase, maintaining the allele combinations found in the original MLHTs. Most populations studied showed a significant association among alleles consistent with non-random mating or asexual reproduction that would facilitate the maintenance of clones. Host genetic diversity can influence pathogen population structure (Zhu et al. 2000) and epidemiology. Based on our earlier findings, we postulated that R. commune originated through a recent host shift onto barley (Zaffarano et al. 2008), leading to a significant reduction in genetic diversity relative to the ancestral pathogen population. We expected that populations of R. commune recovered from wild barley would be genetically less diverse than populations recovered from cultivated barley because there is a larger number of resistance genes present in wild barley than in cultivated barley (Abbott et al. 1992; Genger et al. 2003) and thus only a fraction of the pathogen population introduced on the cultivated host population would be expected to be capable of infecting the wild host population. In Syria and Jordan, barley is most commonly cultivated as landraces as part of a participatory plant breeding program (Ceccarelli et al. 2001). Landraces maintain a high genetic diversity and the genetic makeup of each landrace is determined by natural selection within geographic regions. Even though cultivated barley has a lower genetic diversity than H. spontaneum (Cronin et al. 2007) because of genetic drift that occurred during the domestication process, the genetic diversity of landraces may not be sufficiently different from H. spontaneum to select for different pathogen populations. This study found no difference in pathogen population structure from wild or cultivated barley. Pathogen populations on both hosts had similar low levels of genotypic and gene diversity, low numbers of alleles, and several MLHTs were shared between host populations, suggesting that differences in host 123 A. Kiros-Meles et al. species did not influence the levels of genetic diversity of R. commune populations. The AMOVA analyses showed zero differentiation between host populations and therefore no evidence for host mediated selection in H. spontaneum. This suggests that differences between the host species have not limited gene flow between the associated pathogen populations. Host specialization therefore has not influenced pathogen population structure, presumably because the evolutionary time available for such evolution has been too short. Similarly, an isozyme study reported no genetic differentiation among R. commune isolates occurring on barley grass (Hordeum leporinum) and cultivated barley in Australia (Goodwin et al. 1993). In contrast to neutral genetic markers, isolates of R. commune from barley grass in Australia were shown to be pathogenically more diverse than isolates recovered from cultivated barley (Brown 1990). This may reflect differences in the marker system where avirulence genes are expected to evolve faster than neutral loci due to selection. Results from STRUCTURE and NMDS plots indicated that populations were sufficiently admixed to a point where clustering patterns, based on a priori population labels, could not be readily identified. This suggests that field populations of R. commune are comprised of individuals of varying ancestries and origin and that there is free exchange of pathogen genotypes between wild and cultivated host populations. Multiple introductions of the pathogen into the Middle East were most likely achieved with barley trade and breeding programs. However, even though STRUCTURE is robust to modest departures from linkage equilibrium, analyses may be underestimating the amount of uncertainty in the assignments, leading to overestimates of K due to linkage disequilibrium in the data. A caveat of STRUCTURE under linkage disequilibrium is that if a whole group of closely related individuals is sampled, they could be assigned into their own cluster that does not reflect larger-scale population structure (J. Pritchard, STRUCTURE discussion forum). Therefore the number of introductions of the pathogen into the Middle East is unknown, but it is likely to have been overestimated by the STRUCTURE analyses. We provide evidence that populations of R. commune in the Middle East represent founding 123 populations that were uniformly disseminated in Syria and Jordan as suggested by low population differentiation among populations and low correlation between genetic and geographic distances among populations. We propose that the pathogen population was disseminated from a central source population to farmers’ fields once it was introduced, perhaps from ICARDA, the barley breeding centre in Syria. Movement of agricultural products, especially infected barley seed, among countries and regions, most likely facilitated introduction and dissemination of R. commune to the Middle East. This study supports previous evidence that R. commune does not share its centre of origin with cultivated barley (Zaffarano et al. 2006, 2008). This, in addition to evidence for wide host range (Ali and Boyd 1974; Caldwell 1937; Zaffarano et al. 2008), extensive pathogenic variability and ability to generate new virulence phenotypes (Brown 1990; Jackson and Webster 1976) suggests that the invasion of R. commune to the Middle East poses a potential risk to native Hordeum communities, perhaps similar to a community shift observed in California after the introduction of a viral pathogen that was able to infect native grasses (Borer et al. 2007), and the population crash of Torreya taxifolia in Florida as a result of pathogens introduced with cultivated conifers (Schwartz et al. 1995). Furthermore, sources of resistance identified in wild barley populations that have only recently been exposed to the pathogen are less likely to be durable because they have not been selected through a long-term coevolutionary process. More studies are required to quantify the threat that R. commune poses to wild Hordeum spp., which are an invaluable source of genetic material for future improvements of cultivated barley. References Abbott DC, Brown AHD, Burdon JJ (1992) Genes for scald resistance from wild barley (Hordeum vulgare ssp. spontaneum) and their linkage to isozyme markers. Euphytica 61:225–231 Agapow PM, Burt A (2001) Indices of multilocus linkage disequilibrium. Mol Ecol Notes 1:101–102 Ali SM, Boyd WJR (1974) Host range and physiologic specialization in Rhynchosporium secalis. Aust J Agric Res 25:21–31 Invasion of Rhynchosporium commune onto wild barley in the Middle East Anagnostakis SL (1987) Chestnut blight: the classical problem of an introduced pathogen. 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