The WtmsDW Locus on Wheat Chromosome 2B Controls Major Natural Variation for Floret Sterility Responses to Heat Stress at Booting Stage

Heat stress at booting stage causes significant losses to floret fertility (grain set) and hence yield in wheat (Triticum aestivum L.); however, there is a lack of well-characterized sources of tolerance to this type of stress. Here, we describe the genetic analysis of booting stage heat tolerance in a cross between the Australian cultivars Drysdale (intolerant) and Waagan (tolerant), leading to the definition of a major-effect tolerance locus on the short arm of chromosome 2B, Wheat thermosensitive male sterile Drysdale/Waagan (WtmsDW). WtmsDW offsets between 44 and 65% of the losses in grain set due to heat, suggesting that it offers significant value for marker-assisted tolerance breeding. In lines lacking the WtmsDW tolerance allele, peaks in sensitivity were defined with reference to auricle distance, for various floret positions along the spike. Other (relatively minor) floret fertility response effects, including at the Rht-D1 dwarfing locus, were considered likely escape artifacts, due to their association with height and flowering time effects that might interfere with correct staging of stems for heat treatment. Heat stress increased grain set at distal floret positions in spikelets located at the top of the spike and increased the size of spikelets at the base of the spike, but these effects were offset by greater reductions in grain set at other floret positions. Potentially orthologous loci on chromosomes 1A and 1B were identified for heat response of flowering time. The potential significance of these findings for tolerance breeding and further tolerance screening is discussed.

The WtmsDW Locus on Wheat Chromosome 2B Controls Major Natural Variation for Floret Sterility Responses to Heat Stress at Booting Stage

Heat stress at booting stage causes significant losses to floret fertility (grain set) and hence yield in wheat (Triticum aestivum L.); however, there is a lack of well-characterized sources of tolerance to this type of stress. Here, we describe the genetic analysis of booting stage heat tolerance in a cross between the Australian cultivars Drysdale (intolerant) and Waagan (tolerant), leading to the definition of a major-effect tolerance locus on the short arm of chromosome 2B, Wheat thermosensitive male sterile Drysdale/Waagan (WtmsDW). WtmsDW offsets between 44 and 65% of the losses in grain set due to heat, suggesting that it offers significant value for marker-assisted tolerance breeding. In lines lacking the WtmsDW tolerance allele, peaks in sensitivity were defined with reference to auricle distance, for various floret positions along the spike. Other (relatively minor) floret fertility response effects, including at the Rht-D1 dwarfing locus, were considered likely escape artifacts, due to their association with height and flowering time effects that might interfere with correct staging of stems for heat treatment. Heat stress increased grain set at distal floret positions in spikelets located at the top of the spike and increased the size of spikelets at the base of the spike, but these effects were offset by greater reductions in grain set at other floret positions. Potentially orthologous loci on chromosomes 1A and 1B were identified for heat response of flowering time. The potential significance of these findings for tolerance breeding and further tolerance screening is discussed.

QTL mapping identifies a major locus for resistance in wheat to Sunn pest (Eurygaster integriceps) feeding at the vegetative growth stage

This research provides the first report of a major locus controlling wheat resistance to Sunn pest. It developed and validated SNP markers that will be useful for marker-assisted selection. Sunn pest (Eurygaster integriceps Puton) is the most destructive insect pest of bread wheat and durum wheat in West and Central Asia and East Europe. Breeding for resistance at the vegetative stage of growth is vital in reducing the damage caused by overwintered adult populations that feed on shoot and leaves of seedlings, and in reducing the next generation of pest populations (nymphs and adults), which can cause damage to grain quality by feeding on spikes. In the present study, two doubled haploid (DH) populations involving resistant landraces from Afghanistan were genotyped with the 90k SNP iSelect assay and candidate gene-based KASP markers. The DH lines and parents were phenotyped for resistance to Sunn pest feeding, using artificial infestation cages at Terbol station, in Lebanon, over three years. Quantitative trait locus (QTL) analysis identified a single major locus on chromosome 4BS in the two populations, with the resistance allele derived from the landrace accessions, IG139431 and IG139883. The QTL explained a maximum of 42 % of the phenotypic variation in the Cham6 × IG139431 and 56 % in the Cham6 × IG139883 populations. SNP markers closest to the QTL showed high similarity to rice genes that putatively encode proteins for defense response to herbivory and wounding. The markers were validated in a large, unrelated population of parental wheat genotypes. All wheat lines carrying the 'C-G' haplotype at the identified SNPs were resistant, suggesting that selection based on a haplotype of favourable alleles would be effective in predicting resistance status of unknown genotypes.

Genetic control of grain yield and grain physical characteristics in a bread wheat population grown under a range of environmental conditions

Genetic analysis of the yield and physical quality of wheat revealed complex genetic control, including strong effects of photoperiod-sensitivity loci. Environmental conditions such as moisture deficit and high temperatures during the growing period affect the grain yield and grain characteristics of bread wheat (Triticum aestivum L.). The aim of this study was to map quantitative trait loci (QTL) for grain yield and grain quality traits using a Drysdale/Gladius bread wheat mapping population grown under a range of environmental conditions in Australia and Mexico. In general, yield and grain quality were reduced in environments exposed to drought and/or heat stress. Despite large effects of known photoperiod-sensitivity loci (Ppd-B1 and Ppd-D1) on crop development, grain yield and grain quality traits, it was possible to detect QTL elsewhere in the genome. Some of these QTL were detected consistently across environments. A locus on chromosome 6A (TaGW2) that is known to be associated with grain development was associated with grain width, thickness and roundness. The grain hardness (Ha) locus on chromosome 5D was associated with particle size index and flour extraction and a region on chromosome 3B was associated with grain width, thickness, thousand grain weight and yield. The genetic control of grain length appeared to be largely independent of the genetic control of the other grain dimensions. As expected, effects on grain yield were detected at loci that also affected yield components. Some QTL displayed QTL-by-environment interactions, with some having effects only in environments subject to water limitation and/or heat stress.

Genetic variation and possible SNP markers for breeding wheat with low-grain asparagine, the major precursor for acrylamide formation in heat-processed products

BACKGROUND

In products made from wheat (Triticum aestivum) flour, acrylamide formation is almost exclusively determined by the level of free asparagine in the grain. Genetic variability for grain asparagine content was evaluated in order to assess the potential for acrylamide mitigation by breeding.

RESULTS

Free asparagine levels in the grains of 92 varieties varied from 137 to 471 mg kg⁻¹, representing an approximate threefold difference between the low- and high-asparagine genotypes. Heritability was low, with a value of 32%, indicating that breeding cultivars with inherently low grain asparagine would be a challenge. A genome-wide scan with single-nucleotide polymorphism (SNP) markers identified nine SNPs that were significantly (P < 0.001) associated with variation in free asparagine. The significant SNPs were localized on chromosome 5A, and explained between 14% and 24% of the observed variation. These putative SNPs are candidates for further studies to develop molecular markers.

CONCLUSION

Significant genetic variation exists for reducing acrylamide precursors in wheat flour, indicating that breeding and genetics could play an important role in mitigating the acrylamide risk in wheat products. The study identified a region on chromosome 5A that could provide a basis for further research to develop functional markers.

QTL dissection of the loss of green colour during post-anthesis grain maturation in two-rowed barley

Ability to genetically manipulate the loss of green colour during grain maturation has potentials for increasing productivity, disease resistance, and drought and heat tolerance in crop plants. Two doubled haploid, two-rowed barley populations (Vlamingh × Buloke and VB9524 × ND11231*12) were monitored over 2 years for loss of green colour during grain filling using a portable active sensor. The aims were to determine the genomic regions that control trait heritability by quantitative trait locus (QTL) analysis, and to examine patterns of QTL-environment interactions under different conditions of water stress. In the Vlamingh × Buloke cross, broad-sense heritability estimate for loss of green colour (measured as the difference in sensor readings taken at anthesis and maturity, ∆SRI) was 0.68, and 0.78 for the VB9524 × ND11231*12 population. In the VB9524 × ND11231*12 population, rapid loss of green colour was positively associated with grain yield and percent plump grains, but in the Vlamingh × Buloke population, a slower loss of green colour (low ∆SRI) was associated with increased grain plumpness. With the aid of a dense array of single nucleotide polymorphisms (SNPs) and EST-derived SSR markers, a total of nine QTLs were detected across the two populations. Of these, a single major locus on the short arm of barley chromosome 5H was consistently linked with trait variation across the populations and multiple environments. The QTL was independent of flowering time and explained between 5.4 and 15.4 % of the variation observed in both populations, depending on the environment, and although a QTL × E interaction was detected, it was largely due to a change in the magnitude of the effect, rather than a change in direction. The results suggest that loss of green colour during grain maturation may be under the control of a simple genetic architecture, but a careful study of target populations and environments would be required for breeding purposes.

EST-SSR markers derived from an elite barley cultivar (Hordeum vulgare L. 'Morex'): polymorphism and genetic marker potential

Microsatellites or simple sequence repeats have become the markers of choice for marker-assisted selection because of their low template DNA requirement, high reproducibility, and high level of polymorphism. This study investigated a new set of barley (Hordeum vulgare L.) EST-derived SSR markers designed to target gene sequences expressed during grain development, as they are more likely to be important in determining grain quality. The EST sequences (HVSMEh and HVSMEi) were derived from cDNA libraries of the elite six-rowed cultivar Morex, made from spikes harvested at 5 to 45 days after pollination. Approximately half of the 110 SSR markers derived from the ESTs were polymorphic in a panel of 8 diverse barley genotypes, with PIC values between 0.19 and 0.79. Twenty of the new markers were mapped to chromosomal locations using 2 doubled haploid populations. To demonstrate marker potential, quantitative trait locus (QTL) analyses were carried out with phenotypic data on wort beta-glucan content and beta-glucanase activity, two traits with a long history of genetic studies. Most of the EST-SSR markers mapped to within 10 cM of the cellulose synthase (HvCesA) and cellulose synthase-like (HvCslF) genes, which provides highly informative functional markers for tracking these genes in breeding programs. It was also observed that on any given chromosome, the QTL for beta-glucan content and beta-glucanase activity were rarely coincident but tended to occur in adjacent intervals along chromosomal regions, which agreed with their independent genetic basis; the adjacent localization may be important for coordination of cell wall degradation during germination and malting.

GENSTAT programs for performing Muir's alternative partitioning of genotype-by-environment interaction

Genotype-by-environment (GE) interaction exists when different cultivars or strains have different phenotypic responses to environmental variation (Merila and Fry 1998). The phenomenon is of major concern in plant breeding, as it can limit gains in selecting superior cultivars. In animal breeding, the problem is also an important issue because breeding stocks are developed by a few companies but are used worldwide (Lin and Togashi 2002).

Dinucleotide microsatellites from Eucalyptus sieberi: inheritance, diversity, and improved scoring of single-base differences

Eight dinucleotide microsatellites were developed in Eucalyptus sieberi L. Johnson (silvertop ash), a member of the subgenus Eucalyptus. Transfer of six of these to the subgenus Symphyomyrtus and their Mendelian inheritance are demonstrated using a full-sib cross in Eucalyptus nitens. Genetic diversity parameters are presented for the eight loci based on a sample of 100 old-growth E. sieberi trees from a single natural stand. One locus, Es266, had an atypically high fixation index, and significantly deviated from Hardy-Weinberg equilibrium genotypic proportions, indicating the likely presence of null alleles. Two of the loci, Es076 and Es140, had many alleles that differed in size by only a single base pair, possibly because of short poly(A) or poly(T) stretches in their flanking regions. These two loci were by far the most polymorphic, but were difficult to score reliably on a capillary DNA sequencer. Reliability of scoring of these two one-base microsatellite loci was markedly improved by the incorporation of internal reference alleles into each sample analysed.Key words: SSRs, single base pair alleles, null alleles, internal reference alleles.

Dinucleotide microsatellites from Eucalyptus sieberi: inheritance, diversity, and improved scoring of single-base differences

Eight dinucleotide microsatellites were developed in Eucalyptus sieberi L. Johnson (silvertop ash), a member of the subgenus Eucalyptus. Transfer of six of these to the subgenus Symphyomyrtus and their Mendelian inheritance are demonstrated using a full-sib cross in Eucalyptus nitens. Genetic diversity parameters are presented for the eight loci based on a sample of 100 old-growth E. sieberi trees from a single natural stand. One locus, Es266, had an atypically high fixation index, and significantly deviated from Hardy-Weinberg equilibrium genotypic proportions, indicating the likely presence of null alleles. Two of the loci, Es076 and Es140, had many alleles that differed in size by only a single base pair, possibly because of short poly(A) or poly(T) stretches in their flanking regions. These two loci were by far the most polymorphic, but were difficult to score reliably on a capillary DNA sequencer. Reliability of scoring of these two one-base microsatellite loci was markedly improved by the incorporation of internal reference alleles into each sample analysed.