Analysis of molecular diversity, population structure and linkage disequilibrium in a worldwide survey of cultivated barley germplasm (Hordeum vulgare L.)

BACKGROUND: The goal of our study was a systematic survey of the molecular diversity in barley genetic resources. To this end 953 cultivated barley accessions originating from all inhabited continents except Australia were genotyped with 48 SSR markers. Molecular diversity was evaluated with routine statistics (allelic richness, gene diversity, allele frequency, heterozygosity and unique alleles), Principal Coordinate Analysis (PCoA), and analysis of genome-wide linkage disequilibrium. RESULTS: A genotyping database for 953 cultivated barley accessions profiled with 48 SSR markers was established. The PCoA revealed structuring of the barley population with regard to (i) geographical regions and (ii) agronomic traits. Geographic origin contributed most to the observed molecular diversity. Genome-wide linkage disequilibrium (LD) was estimated as squared correlation of allele frequencies (r(2)). The values of LD for barley were comparable to other plant species (conifers, poplar, maize). The pattern of intrachromosomal LD with distances between the genomic loci ranging from 1 to 150 cM revealed that in barley LD extended up to distances as long as 50 cM with r(2 )> 0.05, or up to 10 cM with r(2 )> 0.2. Few loci mapping to different chromosomes showed significant LD with r(2 )> 0.05. The number of loci in significant LD as well as the pattern of LD were clearly dependent on the population structure. The LD in the homogenous group of 207 European 2-rowed spring barleys compared to the highly structured worldwide barley population was increased in the number of loci pairs with r(2 )> 0.05 and had higher values of r(2), although the percentage of intrachromosomal loci pairs in significant LD based on P < 0.001 was 100% in the whole set of varieties, but only 45% in the subgroup of European 2-rowed spring barleys. The value of LD also varied depending on the polymorphism of the loci selected for genotyping. The 17 most polymorphic loci (PIC > 0.80) provided higher LD values as compared to 19 low polymorphic loci (PIC < 0.73) in both structured (all accessions) and non-structured (European 2-rowed spring varieties) barley populations. CONCLUSION: A global population of cultivated barley accessions was highly structured. Clustering highlighted the accessions with the same geographic origin, as well as accessions possessing similar agronomic characters. LD in barley extended up to 50 cM, and was strongly dependent on the population structure. The data on LD were summarized as a genome-wide LD map for barley

Studies in the Phaeotremella foliacea group (Tremellomycetes, Basidiomycota)

The taxonomy of the Phaeotremella foliacea group is revised based on morphological, ecological, geographic and DNA data. The name P. foliacea is retained for the gymnosperm-dwelling species associated with Stereum sanguinolentum in Eurasia and North America. Tremella neofoliacea and Cryptococcus skinneri are considered synonyms of P. foliacea s. str. Three other species in the complex inhabit deciduous trees. Of them, Phaeotremella fimbriata, comb. nov., is associated with Stereum rugosum; this species possesses blackening basidiocarps and small basidiospores, and it occurs in Europe. Its close relative is the East Asian Phaeotremella eugeniae, sp. nov., inhabiting Quercus mongolica and having larger basidiospores. The third species, Phaeotremella frondosa, comb. nov., produces the largest basidiospores in the genus and is associated either with S. rugosum (mainly in North Europe) or with other Stereum species (temperate Eurasia and North America). Additionally, Tremella nigrescens is typified and placed in the synonyms of P. frondosa, and two species, T. fuscosuccinea and T. roseotincta, are combined to Phaeotremella.Peer reviewe

On Craterocolla and Ditangium (Sebacinales, Basidiomycota)

In the present paper, we select a neotype for Tremella cerasi and prove that it is conspecific with Ditangium insigne (the genus type of Ditangium). We argue that Ditangium should be restored as a correct genus for T. cerasi, while the currently used generic name Craterocolla is to be treated as its younger synonym. Morphological, ecological and genetic variability of Ditangium cerasi is discussed, and two other Ditangium species are described—D. altaicum, sp. nov. and D. incarnatum, comb. nov. In addition, Exidia fulva is combined in Globulisebacina.Peer reviewe

Revision of Protohydnum (Auriculariales, Basidiomycota)

Three species currently addressed to Protohydnum (Auriculariales) are studied with morphological and DNA methods. The genus Protohydnum is retained for the type species only, P. cartilagineum, recently re-collected in Brazil. The European species, P. piceicola, is not congeneric with P. cartilagineum and, therefore, placed in its own genus, Hyalodon, gen. nov. Another Hyalodon species, H. antui, is described from East Asia. The third member of Protohydnum sensu lato, P. sclerodontium from South-East Asia, is transferred to Elmerina.Peer reviewe

On Protomerulius and Heterochaetella (Auriculariales, Basidiomycota)

The taxonomy of Protomerulius and Heterochaetella is revised based on DNA data and morphological evidence, and their type species, P. brasiliensis and H. dubia, are proved to be congeneric. As a consequence, H. dubia and related species, of which four are described as new, are placed in Protomerulius. Heterochaete microspora is also combined in Protomerulius, and the genus concept is redefined to encompass effused species with smooth or spiny hymenophore and monomitic hyphal structure. Psilochaete multifora, gen. and spec. nov. is a distant relative of Protomerulius spp. found in Norway. Heterochaetella cystidiophora is re-collected in Brazil and placed in the synonyms of Heterochaete sanctae-catharinae. This species does not belong to the Protomerulius-Heterochaetella lineage, and it is transferred to Metulochaete, gen. nov.Peer reviewe

Diversity and Mega-Targets of Selection from the Characterization of a Barley Collection

Germplasm exchange is essential for assuring genetic gain in a breeding program. Two aspects of breeding programs are relevant to making them compatible for germplasm exchange: the amount of genetic diversity within programs and the identifi cation of breeding programs with similar breeding objectives and environments of selection (i.e., mega-targets of selection). The objective of this study was to develop a data-driven method to group breeding programs likely to be compatible for germplasm exchange and to use phenotypic characterization data of barley (Hordeum vulgare L.) from breeding programs to illustrate this method. In two locations in Uruguay we evaluated 20 traits in 353 genotypes of barley from 23 private and public breeding programs distributed worldwide. We found signifi cant amounts of genetic diversity for all traits, but differences in diversity among programs for only seven traits. We identifi ed programs with high (Western Australia Department of Agriculture; University of Saskatchewan; and Svalöf Weibull Ab, Sweden) and low diversity (winter program of Osijek Agricultural Institute, Croatia; spring program of Osijek Agricultural Institute, Croatia; Saatzucht Josef Breun, Germany; Busch Agricultural Resources; USDA-ARS, Aberdeen, ID; and University of Minnesota). We developed a methodology that groups programs with similar performance and response to the environments. We used the methodology to group the 23 breeding programs of barley into sets that might benefi t most from germplasm exchange. The identifi cation of compatible programs for germplasm exchange could be relevant for improving genetic gains in breeding programs

Genetic differentiation and geographical Relationship of Asian barley landraces using SSRs

Genetic diversity in 403 morphologically distinct landraces of barley (Hordeum vulgare L. subsp. vulgare) originating from seven geographical zones of Asia was studied using simple sequence repeat (SSR) markers from regions of medium to high recombination in the barley genome. The seven polymorphic SSR markers representing each of the chromosomes chosen for the study revealed a high level of allelic diversity among the landraces. Genetic richness was highest in those from India, followed by Pakistan while it was lowest for Uzbekistan and Turkmenistan. Out of the 50 alleles detected, 15 were unique to a geographic region. Genetic diversity was highest for landraces from Pakistan (0.70 ± 0.06) and lowest for those from Uzbekistan (0.18 ± 0.17). Likewise, polymorphic information content (PIC) was highest for Pakistan (0.67 ± 0.06) and lowest for Uzbekistan (0.15 ± 0.17). Diversity among groups was 40% compared to 60% within groups. Principal component analysis clustered the barley landraces into three groups to predict their domestication patterns. In total 51.58% of the variation was explained by the first two principal components of the barley germplasm. Pakistan landraces were clustered separately from those of India, Iran, Nepal and Iraq, whereas those from Turkmenistan and Uzbekistan were clustered together into a separate group

Patterns of polymorphism and linkage disequilibrium in cultivated barley

We carried out a genome-wide analysis of polymorphism (4,596 SNP loci across 190 elite cultivated accessions) chosen to represent the available genetic variation in current elite North West European and North American barley germplasm. Population sub-structure, patterns of diversity and linkage disequilibrium varied considerably across the seven barley chromosomes. Gene-rich and rarely recombining haplotype blocks that may represent up to 60% of the physical length of barley chromosomes extended across the ‘genetic centromeres’. By positioning 2,132 bi-parentally mapped SNP markers with minimum allele frequencies higher than 0.10 by association mapping, 87.3% were located to within 5 cM of their original genetic map position. We show that at this current marker density genetically diverse populations of relatively small size are sufficient to fine map simple traits, providing they are not strongly stratified within the sample, fall outside the genetic centromeres and population sub-structure is effectively controlled in the analysis. Our results have important implications for association mapping, positional cloning, physical mapping and practical plant breeding in barley and other major world cereals including wheat and rye that exhibit comparable genome and genetic features

Linkage disequilibrium and population structure in wild and domesticated populations of Phaseolus vulgaris L.

Together with the knowledge of the population structure, a critical aspect for the planning of association and/or population genomics studies is the level of linkage disequilibrium (LD) that characterizes the species and the population used for such an analysis. We have analyzed the population structure and LD in wild and domesticated populations of Phaseolus vulgaris L. using amplified fragment length polymorphism markers, most of which were genetically mapped in two recombinant inbred populations. Our results reflect the previous knowledge of the occurrence of two major wild gene pools of P. vulgaris, from which two independent domestication events originated, one in the Andes and one in Mesoamerica. The high level of LD in the whole sample was mostly due to the gene pool structure, with a much higher LD in domesticated compared to wild populations. In relation to association studies, our results also suggest that whole-genome-scan approaches are feasible in the common bean. Interestingly, an excess of inter-chromosomal LD was found in the domesticated populations, which suggests an important role for epistatic selection during domestication. Moreover, our results indicate the occurrence of a strong bottleneck in the Andean wild population before domestication, suggesting a Mesoamerican origin of P. vulgaris. Finally, our data support the occurrence of a single domestication event in Mesoamerica, and the same scenario in the Andes

Genetic Diversity and Linkage Disequilibrium in Chinese Bread Wheat (Triticum aestivum L.) Revealed by SSR Markers

Two hundred and fifty bread wheat lines, mainly Chinese mini core accessions, were assayed for polymorphism and linkage disequilibrium (LD) based on 512 whole-genome microsatellite loci representing a mean marker density of 5.1 cM. A total of 6,724 alleles ranging from 1 to 49 per locus were identified in all collections. The mean PIC value was 0.650, ranging from 0 to 0.965. Population structure and principal coordinate analysis revealed that landraces and modern varieties were two relatively independent genetic sub-groups. Landraces had a higher allelic diversity than modern varieties with respect to both genomes and chromosomes in terms of total number of alleles and allelic richness. 3,833 (57.0%) and 2,788 (41.5%) rare alleles with frequencies of <5% were found in the landrace and modern variety gene pools, respectively, indicating greater numbers of rare variants, or likely new alleles, in landraces. Analysis of molecular variance (AMOVA) showed that A genome had the largest genetic differentiation and D genome the lowest. In contrast to genetic diversity, modern varieties displayed a wider average LD decay across the whole genome for locus pairs with r2>0.05 (P<0.001) than the landraces. Mean LD decay distance for the landraces at the whole genome level was <5 cM, while a higher LD decay distance of 5–10 cM in modern varieties. LD decay distances were also somewhat different for each of the 21 chromosomes, being higher for most of the chromosomes in modern varieties (<5∼25 cM) compared to landraces (<5∼15 cM), presumably indicating the influences of domestication and breeding. This study facilitates predicting the marker density required to effectively associate genotypes with traits in Chinese wheat genetic resources