Candidate genes and genome-wide association study of grain protein content and protein deviation in durum wheat.

MAIN CONCLUSION: Stable QTL for grain protein content co-migrating with nitrogen-related genes have been identified by the candidate genes and genome-wide association mapping approaches useful for marker-assisted selection. Grain protein content (GPC) is one of the most important quality traits in wheat, defining the nutritional and end-use properties and rheological characteristics. Over the years, a number of breeding programs have been developed aimed to improving GPC, most of them having been prevented by the negative correlation with grain yield. To overcome this issue, a collection of durum wheat germplasm was evaluated for both GPC and grain protein deviation (GPD) in seven field trials. Fourteen candidate genes involved in several processes related to nitrogen metabolism were precisely located on two high-density consensus maps of common and durum wheat, and six of them were found to be highly associated with both traits. The wheat collection was genotyped using the 90 K iSelect array, and 11 stable quantitative trait loci (QTL) for GPC were detected in at least three environments and the mean across environments by the genome-wide association mapping. Interestingly, seven QTL were co-migrating with N-related candidate genes. Four QTL were found to be significantly associated to increases of GPD, indicating that selecting for GPC could not affect final grain yield per spike. The combined approaches of candidate genes and genome-wide association mapping led to a better understanding of the genetic relationships between grain storage proteins and grain yield per spike, and provided useful information for marker-assisted selection programs.

Development of a deletion and genetic linkage map for the 5A and 5B chromosomes of wheat (Triticum aestivum).

The aims of the present study were to provide deletion maps for wheat ( Triticum aestivum L.) chromosomes 5A and 5B and a detailed genetic map of chromosome 5A enriched with popular microsatellite markers, which could be compared with other existing maps and useful for mapping major genes and quantitative traits loci (QTL). Physical mapping of 165 gSSR and EST-SSR markers was conducted by amplifying each primer pair on Chinese Spring, aneuploid lines, and deletion lines for the homoeologous group 5 chromosomes. A recombinant inbred line (RIL) mapping population that is recombinant for only chromosome 5A was obtained by crossing the wheat cultivar Chinese Spring and the disomic substitution line Chinese Spring-5A dicoccoides and was used to develop a genetic linkage map of chromosome 5A. A total of 67 markers were found polymorphic between the parental lines and were mapped in the RIL population. Sixty-three loci and the Q gene were clustered in three linkage groups ordered at a minimum LOD score of 5, while four loci remained unlinked. The whole genetic 5A chromosome map covered 420.2 cM, distributed among three linkage groups of 189.3, 35.4, and 195.5 cM. The EST sequences located on chromosomes 5A and 5B were used for comparative analysis against Brachypodium distachyon (L.) P. Beauv. and rice ( Oryza sativa L.) genomes to resolve orthologous relationships among the genomes of wheat and the two model species.

Genetic and physical mapping of new EST-derived SSRs on the A and B genome chromosomes of wheat.

The availability of genetic maps and phenotypic data of segregating populations allows to localize and map agronomically important genes, and to identify closely associated molecular markers to be used in marker-assisted selection and positional cloning. The objective of the present work was to develop a durum wheat intervarietal genetic and physical map based on genomic microsatellite or genomic simple sequence repeats (gSSR) markers and expressed sequence tag (EST)-derived microsatellite (EST-SSR) markers. A set of 122 new EST-SSR loci amplified by 100 primer pairs was genetically mapped on the wheat A and B genome chromosomes. The whole map also comprises 149 gSSR markers amplified by 120 primer pairs used as anchor chromosome loci, two morphological markers (Black colour, Bla1, and spike glaucousness, Ws) and two seed storage protein loci (Gli-A2 and Gli-B2). The majority of SSR markers tested (182) was chromosome-specific. Out of 275 loci 241 loci assembled in 25 linkage groups assigned to the chromosomes of the A and B genome and 34 remained unlinked. A higher percentage of markers (54.4%), localized on the B genome chromosomes, in comparison to 45.6% distributed on the A genome. The whole map covered 1,605 cM. The B genome accounted for 852.2 cM of genetic distance; the A genome basic map spanned 753.1 cM with a minimum length of 46.6 cM for chromosome 5A and a maximum of 156.2 cM for chromosome 3A and an average value of 114.5 cM. The primer sets that amplified two or more loci mapped to homoeologous as well as to non-homoeologous sites. Out of 241 genetically mapped loci 213 (88.4%) were physically mapped by using the nulli-tetrasomic, ditelosomic and a stock of 58 deletion lines dividing the A and B genome chromosomes in 94 bins. No discrepancies concerning marker order were observed but the cytogenetic maps revealed in some cases small genetic distance covered large physical regions. Putative function for mapped SSRs were assigned by searching against GenBank nonredundant database using TBLASTX algorithms.