Genomic prediction for agronomic traits in a diverse Flax (Linum usitatissimum L.) germplasm collection.

Breeding programs require exhaustive phenotyping of germplasms, which is time-demanding and expensive. Genomic prediction helps breeders harness the diversity of any collection to bypass phenotyping. Here, we examined the genomic prediction's potential for seed yield and nine agronomic traits using 26,171 single nucleotide polymorphism (SNP) markers in a set of 337 flax (Linum usitatissimum L.) germplasm, phenotyped in five environments. We evaluated 14 prediction models and several factors affecting predictive ability based on cross-validation schemes. Models yielded significant variation among predictive ability values across traits for the whole marker set. The ridge regression (RR) model covering additive gene action yielded better predictive ability for most of the traits, whereas it was higher for low heritable traits by models capturing epistatic gene action. Marker subsets based on linkage disequilibrium decay distance gave significantly higher predictive abilities to the whole marker set, but for randomly selected markers, it reached a plateau above 3000 markers. Markers having significant association with traits improved predictive abilities compared to the whole marker set when marker selection was made on the whole population instead of the training set indicating a clear overfitting. The correction for population structure did not increase predictive abilities compared to the whole collection. However, stratified sampling by picking representative genotypes from each cluster improved predictive abilities. The indirect predictive ability for a trait was proportionate to its correlation with other traits. These results will help breeders to select the best models, optimum marker set, and suitable genotype set to perform an indirect selection for quantitative traits in this diverse flax germplasm collection.

Homozygosity mapping identified loci and candidate genes responsible for freezing tolerance in Camelina sativa.

Homozygosity mapping is an effective tool for detecting genomic regions responsible for a given trait when the phenotype is controlled by a limited number of dominant or co-dominant loci. Freezing tolerance is a major attribute in agricultural crops such as camelina. Previous studies indicated that freezing tolerance differences between a tolerant (Joelle) and susceptible (CO46) variety of camelina were controlled by a small number of dominant or co-dominant genes. We performed whole genome homozygosity mapping to identify markers and candidate genes responsible for freezing tolerance difference between these two genotypes. A total of 28 F3 RILs were sequenced to ∼30× coverage, and parental lines were sequenced to >30-40× coverage with Pacific Biosciences high fidelity technology and 60× coverage using Illumina whole genome sequencing. Overall, about 126k homozygous single nucleotide polymorphism markers were identified that differentiate both parents. Moreover, 617 markers were also homozygous in F3 families fixed for freezing tolerance/susceptibility. All these markers mapped to two contigs forming a contiguous stretch of chromosome 11. The homozygosity mapping detected 9 homozygous blocks among the selected markers and 22 candidate genes with strong similarity to regions in or near the homozygous blocks. Two such genes were differentially expressed during cold acclimation in camelina. The largest block contained a cold-regulated plant thionin and a putative rotamase cyclophilin 2 gene previously associated with freezing resistance in arabidopsis (Arabidopsis thaliana). The second largest block contains several cysteine-rich RLK genes and a cold-regulated receptor serine/threonine kinase gene. We hypothesize that one or more of these genes may be primarily responsible for freezing tolerance differences in camelina varieties.

Genetic mapping and genomic prediction of sclerotinia stem rot resistance to rapeseed/canola (Brassica napus L.) at seedling stage.

KEY MESSAGE: GWAS detected ninety-eight significant SNPs associated with Sclerotinia sclerotiorum resistance. Six statistical models resulted in medium to high predictive ability, depending on trait, indicating potential of genomic prediction for disease resistance breeding. The lack of complete host resistance and a complex resistance inheritance nature between rapeseed/canola and Sclerotinia sclerotiorum often limits the development of functional molecular markers that enable breeding for sclerotinia stem rot (SSR) resistance. However, genomics-assisted selection has the potential to accelerate the breeding for SSR resistance. Therefore, genome-wide association (GWA) mapping and genomic prediction (GP) were performed using a diverse panel of 337 rapeseed/canola genotypes. Three-week-old seedlings were screened using the petiole inoculation technique (PIT). Days to wilt (DW) up to 2 weeks and lesion phenotypes (LP) at 3, 4, and 7 days post-inoculation (dpi) were recorded. A strong correlation (r = - 0.90) between DW and LP_4dpi implied that a single time point scoring at four days could be used as a proxy trait. GWA analyses using single-locus (SL) and multi-locus (ML) models identified a total of 41, and 208 significantly associated SNPs, respectively. Out of these, ninety-eight SNPs were identified by a combination of the SL model and any of the ML models, at least two ML models, or two traits. These SNPs explained 1.25-12.22% of the phenotypic variance and considered as significant, could be associated with SSR resistance. Eighty-three candidate genes with a function in disease resistance were associated with the significant SNPs. Six GP models resulted in moderate to high (0.42-0.67) predictive ability depending on SSR resistance traits. The resistant genotypes and significant SNPs will serve as valuable resources for future SSR resistance breeding. Our results also highlight the potential of genomic selection to improve rapeseed/canola breeding for SSR resistance.

Linkage disequilibrium and population structure in a core collection of Brassica napus (L.).

Estimation of genetic diversity in rapeseed is important for sustainable breeding program to provide an option for the development of new breeding lines. The objective of this study was to elucidate the patterns of genetic diversity within and among different structural groups, and measure the extent of linkage disequilibrium (LD) of 383 globally distributed rapeseed germplasm using 8,502 single nucleotide polymorphism (SNP) markers. We divided the germplasm collection into five subpopulations (P1 to P5) according to geographic and growth habit-related patterns. All subpopulations showed moderate genetic diversity (average H = 0.22 and I = 0.34). The pairwise Fst comparison revealed a great degree of divergence (Fst > 0.24) between most of the combinations. The rutabaga type showed highest divergence with spring and winter types. Higher divergence was also found between winter and spring types. Admixture model based structure analysis, principal component and neighbor-joining tree analysis placed all subpopulations into three distinct clusters. Admixed genotype constituted 29.24% of total genotypes, while remaining 70.76% belongs to identified clusters. Overall, mean linkage disequilibrium was 0.03 and it decayed to its half maximum within < 45 kb distance for whole genome. The LD decay was slower in C genome (< 93 kb); relative to the A genome (< 21 kb) which was confirmed by availability of larger haplotype blocks in C genome than A genome. The findings regarding LD pattern and population structure will help to utilize the collection as an important resource for association mapping efforts to identify genes useful in crop improvement as well as for selection of parents for hybrid breeding.

Genome-wide association mapping and genomic prediction for adult stage sclerotinia stem rot resistance in Brassica napus (L) under field environments.

Sclerotinia stem rot (SSR) is a fungal disease of rapeseed/canola that causes significant seed yield losses and reduces its oil content and quality. In the present study, the reaction of 187 diverse canola genotypes to SSR was characterized at full flowering stage using the agar plug to stem inoculation method in four environments. Genome-wide association study (GWAS) using three different algorithms identified 133 significant SNPs corresponding with 123 loci for disease traits like stem lesion length (LL), lesion width (LW), and plant mortality at 14 (PM_14D) and 21 (PM_21D) days. The explained phenotypic variation of these SNPs ranged from 3.6 to 12.1%. Nineteen significant SNPs were detected in two or more environments, disease traits with at least two GWAS algorithms. The strong correlations observed between LL and other three disease traits evaluated, suggest they could be used as proxies for SSR resistance phenotyping. Sixty-nine candidate genes associated with disease resistance mechanisms were identified. Genomic prediction (GP) analysis with all the four traits employing genome-wide markers resulted in 0.41-0.64 predictive ability depending on the model specifications. The highest predictive ability for PM_21D with three models was about 0.64. From our study, the identified resistant genotypes and stable significant SNP markers will serve as a valuable resource for future SSR resistance breeding. Our study also suggests that genomic selection holds promise for accelerating canola breeding progress by enabling breeders to select SSR resistance genotypes at the early stage by reducing the need to phenotype large numbers of genotypes.

Genome-Wide Association Studies and Transcriptome Changes during Acclimation and Deacclimation in Divergent Brassica napus Varieties.

Information concerning genes and signals regulating cold acclimation processes in plants is abundant; however, less is known about genes and signals regulating the deacclimation process. A population of primarily winter B. napus varieties was used to conduct a genome-wide association study and to compare the transcriptomes from two winter B. napus varieties showing time-dependent differences in response to cold acclimation and deacclimation treatments. These studies helped to identify loci, candidate genes, and signaling processes impacting deacclimation in B. napus. GWAS identified polymorphisms at five different loci associated with freezing tolerance following deacclimation. Local linkage decay rates near these polymorphisms identified 38 possible candidate genes. Several of these genes have been reported as differentially regulated by cold stress in arabidopsis (Arabidopsis thaliana), including a calcium-binding EF-hand family protein (encoded by BnaCnng10250D) that was also differentially expressed during deacclimation in this study. Thousands of other genes differentially expressed during the acclimation and deacclimation treatments implicated processes involving oxidative stress, photosynthesis, light-regulated diurnal responses, and growth regulation. Generally, responses observed during acclimation were reversed within one week of deacclimation. The primary differences between the two winter B. napus varieties with differential deacclimation responses involved protection from oxidative stress and the ability to maintain photosynthesis.

Genetic diversity analysis of a flax (Linum usitatissimum L.) global collection.

BACKGROUND: A sustainable breeding program requires a minimum level of germplasm diversity to provide varied options for the selection of new breeding lines. To maximize genetic gain of the North Dakota State University (NDSU) flax breeding program, we aimed to increase the genetic diversity of its parental stocks by incorporating diverse genotypes. For this purpose, we analyzed the genetic diversity, linkage disequilibrium, and population sub-structure of 350 globally-distributed flax genotypes with 6200 SNP markers. RESULTS: All the genotypes tested clustered into seven sub-populations (P1 to P7) based on the admixture model and the output of neighbor-joining (NJ) tree analysis and principal coordinate analysis were in line with that of structure analysis. The largest sub-population separation arose from a cluster of NDSU/American genotypes with Turkish and Asian genotypes. All sub-populations showed moderate genetic diversity (average H = 0.22 and I = 0.34). The pairwise Fst comparison revealed a great degree of divergence (Fst > 0.25) between most of the combinations. A whole collection mantel test showed significant positive correlation (r = 0.30 and p < 0.01) between genetic and geographic distances, whereas it was non-significant for all sub-populations except P4 and P5 (r = 0.251, 0.349 respectively and p < 0.05). In the entire collection, the mean linkage disequilibrium was 0.03 and it decayed to its half maximum within < 21 kb distance. CONCLUSIONS: To maximize genetic gain, hybridization between NDSU stock (P5) and Asian individuals (P6) are potentially the best option as genetic differentiation between them is highest (Fst > 0.50). In contrast, low genetic differentiation between P5 and P2 may enhance the accumulation of favorable alleles for oil and fiber upon crossing to develop dual purpose varieties. As each sub-population consists of many genotypes, a Neighbor-Joining tree and kinship matrix assist to identify distantly related genotypes. These results also inform genotyping decisions for future association mapping studies to ensure the identification of a sufficient number of molecular markers to tag all linkage blocks.

Shovelomics for phenotyping root architectural traits of rapeseed/canola (Brassica napus L.) and genome-wide association mapping.

Root system in plants plays an important role in mining moisture and nutrients from the soil and is positively correlated to yield in many crops including rapeseed/canola (Brassica napus L.). Substantial phenotypic diversity in root architectural traits among the B. napus growth types leads to a scope of root system improvement in breeding populations. In this study, 216 diverse genotypes were phenotyped for five different root architectural traits following shovelomics approach in the field condition during 2015 and 2016. A single nucleotide polymorphism (SNP) marker panel consisting of 30,262 SNPs was used to conduct genome-wide association study to detect marker/trait association. A total of 31 significant marker loci were identified at 0.01 percentile tail P value cutoff for different root traits. Six marker loci for soil-level taproot diameter (R1Dia), six loci for belowground taproot diameter (R2Dia), seven loci for number of primary root branches (PRB), eight loci for root angle, and eight loci for root score (RS) were detected in this study. Several markers associated with root diameters R1Dia and R2Dia were also associated with PRB and RS. Significant phenotypic correlation between these traits was observed in both environments. Therefore, taproot diameter appears to be a major determinant of the canola root system architecture and can be used as proxy for other root traits. Fifteen candidate genes related to root traits and root development were detected within 100 kbp upstream and downstream of different significant markers. The identified markers associated with different root architectural traits can be considered for marker-assisted selection for root traits in canola in future.

Analogues of Disulfides from Allium stipitatum Demonstrate Potent Anti-tubercular Activities through Drug Efflux Pump and Biofilm Inhibition.

Disulfides from Allium stipitatum, commonly known as Persian shallot, were previously reported to possess antibacterial properties. Analogues of these compounds, produced by S-methylthiolation of appropriate thiols using S-methyl methanethiosulfonate, exhibited antimicrobial activity, with one compound inhibiting the growth of Mycobacterium tuberculosis at 17 µM (4 mg L-1) and other compounds inhibiting Escherichia coli and multi-drug-resistant (MDR) Staphylococcus aureus at concentrations ranging between 32-138 µM (8-32 mg L-1). These compounds also displayed moderate inhibitory effects on Klebsiella and Proteus species. Whole-cell phenotypic bioassays such as the spot-culture growth inhibition assay (SPOTi), drug efflux inhibition, biofilm inhibition and cytotoxicity assays were used to evaluate these compounds. Of particular note was their ability to inhibit mycobacterial drug efflux and biofilm formation, while maintaining a high selectivity towards M. tuberculosis H37Rv. These results suggest that methyl disulfides are novel scaffolds which could lead to the development of new drugs against tuberculosis (TB).

Norlignans, acylphloroglucinols, and a dimeric xanthone from Hypericum chinense.

Two new norlignans, hyperiones A (1) and B (2), three new acylphloroglucinols, aspidinol C (3) and hyperaspidinols A (5) and B (6), the known compound aspidinol D (4), and the symmetrical dimeric xanthone hyperidixanthone (7) were isolated from Hypericum chinense. Their structures were established by spectroscopic analysis. In an antibacterial assay using a panel of multidrug-resistant (MDR) strains, compounds 3 and 4 exhibited promising activity against the NorA efflux protein overexpressing MDR Staphylococcus aureus strain SA-1199B with a minimum inhibitory concentration (MIC) of 2 µg/mL (8.4 µM) and 4 µg/mL (16.8 µM), respectively. The positive control antibiotic norfloxacin showed activity at MIC 32 µg/mL (100 µM).

Ioniols I and II, tetracyclic diterpenes with antibacterial activity, from Sphaerococcus coronopifolius.

Two naturally occurring diterpenes featuring unprecedented tetracyclic skeletons, ioniols I and II (1 and 2, resp.), along with two previously reported metabolites 3 and 4, were isolated from the organic extract of Sphaerococcus coronopifolius collected from the rocky coasts of Corfu island in the Ionian Sea. The structures of the new natural products, as well as their relative configuration, were elucidated on the basis of extensive spectral analysis, including 2D-NMR experiments. The isolated metabolites were evaluated for their antibacterial activity against a panel of Staphylococcus aureus strains, which included multidrug-resistant (MDR) and methicillin-resistant Staphylococcus aureus (MRSA) strains.

Structure and antibacterial activity of brominated diterpenes from the red alga Sphaerococcus coronopifolius.

Four new tetracyclic brominated diterpenes, 1-4, were isolated from the organic extract of Sphaerococcus coronopifolius, collected from the rocky coasts of Corfu Island. The structures of the new natural products, as well as their relative configurations, were elucidated on the basis of extensive spectral analyses, including 2D-NMR experiments. The isolated metabolites were evaluated for their antibacterial activity against a panel of bacteria including multidrug-resistant (MDR) and methicillin-resistant Staphylococcus aureus (MRSA) with MIC values in the range of 16-128 microg/ml.

High throughput genome-specific and gene-specific molecular markers for erucic acid genes in Brassica napus (L.) for marker-assisted selection in plant breeding.

A single base change in the Bn-FAE1.1 gene in the A genome and a two-base deletion in the Bn-FAE1.2 gene in the C genome produce the nearly zero content of erucic acid observed in canola. A BAC clone anchoring Bn-FAE1.1 from a B. rapa BAC library and a BAC clone anchoring Bn-FAE1.2 from a B. oleracea BAC library were used in this research. After sequencing the gene flanking regions, it was found that the dissimilarity of the flanking sequences of these two FAE1 homologs facilitated the design of genome-specific primers that could amplify the corresponding genome in allotetraploid B. napus. The two-base deletion in the C genome gene was detected as a sequence-characterized amplified region (SCAR) marker. To increase the throughput, one genome-specific primer was labeled with four fluorescence dyes and combined with 20 different primers to produce PCR products with different fragment sizes. Eventually, a super pool of 80 samples was detected simultaneously. This dramatically reduces the cost of marker detection. The single base change in the Bn-FAE1.1 gene was detected as single nucleotide polymorphic (SNP) marker with an ABI SNaPshot kit. A multiplexing primer set was designed by adding a polyT to the 5' primer end to increase SNP detection throughput through sample pooling. Furthermore, the Bn-FAE1.1 and Bn-FAE1.2 were integrated into the N8 and N13 linkage groups of our previously reported high-density sequence-related amplified polymorphism (SRAP) map, respectively. There were 124 SRAP markers in a N8 bin in which the Bn-FAE1.1 gene-specific SCAR marker was located and 46 SRAP markers in a N13 bin into which the Bn-FAE1.2 SNP marker was integrated. These three kinds of high throughput molecular markers have been successfully implemented in our canola/rapeseed breeding programs.

Development of SRAP, SNP and multiplexed SCAR molecular markers for the major seed coat color gene in Brassica rapa L.

Seed coat color inheritance in B. rapa was studied in F(1), F(2), F(3), and BC(1) progenies from a cross of a Canadian brown-seeded variety 'SPAN' and a Bangladeshi yellow sarson variety 'BARI-6'. A pollen effect was found when the yellow sarson line was used as the maternal parent. Seed coat color segregated into brown, yellow-brown and bright yellow classes. Segregation was under digenic control where the brown or yellow-brown color was dominant over bright yellow seed coat color. A sequence related amplified polymorphism (SRAP) marker linked closely to a major seed coat color gene (Br1/br1) was developed. This dominant SRAP molecular marker was successfully converted into single nucleotide polymorphism (SNP) markers and sequence characterized amplification region (SCAR) markers after the extended flanking sequence of the SRAP was obtained with chromosome walking. In total, 24 SNPs were identified with more than 2-kb sequence. A 12-bp deletion allowed the development of a SCAR marker linked closely to the Br1 gene. Using the five-fluorescence dye set supplied by ABI, four labeled M13 primers were integrated with different SCAR primers to increase the throughput of SCAR marker detection. Using multiplexed SCAR markers targeting insertions and deletions in a genome shows great potential for marker assisted selection in plant breeding.