In hexaploid bread wheat (Triticum aestivum L. em. Thell), ten members of the IWMMN (International Wheat Microsatellites Mapping Network) collaborated in extending the microsatellite (SSR = simple sequence repeat) genetic map. Among a much larger number of microsatellite primer pairs developed as a part of the WMC (Wheat Microsatellite Consortium), 58 out of 176 primer pairs tested were found to be polymorphic between the parents of the ITMI (International Triticeae Mapping Initiative) mapping population W7984 × Opata 85 (ITMIpop). This population was used earlier for the construction of RFLP (Restriction Fragment Length Polymorphism) maps in bread wheat (ITMImap). Using the ITMIpop and a framework map (having 266 anchor markers) prepared for this purpose, a total of 66 microsatellite loci were mapped, which were distributed on 20 of the 21 chromosomes (no marker on chromosome 6D). These 66 mapped microsatellite (SSR) loci add to the existing 384 microsatellite loci earlier mapped in bread wheat.
Grain protein content (GPC) is an important factor in pasta and breadmaking quality, and in human nutrition. It is also an important trait for wheat growers because premium prices are frequently paid for wheat with high GPC. A promising source for alleles to increase GPC was detected on chromosome 6B of Triticum turgidum var. dicoccoides accession FA-15-3 (DIC). Two previous quantitative trait locus (QTL) studies found that the positive effect of DIC-6B was associated to a single locus located between the centromere and the Nor-B2 locus on the short arm of chromosome 6B. Microsatellite markers Xgwm508 and Xgwm193 flanking the QTL region were used in this study to develop 20 new homozygous recombinant substitution lines (RSLs) with crossovers between these markers. These 20 RSLs, plus nine RSLs developed in previous studies were characterized with four new RFLP markers located within this chromosome segment. Grain protein content was determined in three field experiments organized as randomized complete block designs with ten replications each. The QTL peaks for protein content were located in the central region of a 2.7-cM interval between RFLP markers Xcdo365 and Xucw67 in the three experiments. Statistical analyses showed that almost all lines could be classified unequivocally within low-and high-protein groups, facilitating the mapping of this trait as a single Mendelian locus designated Gpc-6B1. The Gpc-6B1 locus was mapped 1.5-cM proximal to Xcdo365 and 1.2-cM distal to Xucw67. These new markers can be used to reduce the size of the DIC chromosome segment selected in markerassisted selection programs. Markers Nor-B2 and Xucw66 flanking the previous two markers can be used to select against the DIC segment and reduce the linkage drag during the transfer of Gpc-6B1 into commercial bread and pasta wheat varieties. The precise mapping of the high GPC gene, the high frequency of recombinants recovered in the targeted region, and the recent development of a tetraploid BAC library including the Gpc-6B1 DIC allele are the first steps towards the map-based cloning of this gene.
ABSTRACTtechniques used to characterize wheat cultivars (Vaccino et al., 1993) and assess genetic diversity (Kim andCharacterization of germplasm by means of DNA fingerprinting Ward, 1997; Paull et al., 1998 Tautz and Renz, 1984) and AFLP (Vos et al., 1995).cation Matrix that allowed the discrimination of the 105 cultivars.The SSR technique gained rapid acceptability be- Data obtained from SSR markers were complemented by informationcause of its codominant nature, reproducibility, and high derived from AFLPs. Molecular data were used to quantify genetic information content (De Loose and Gheysen, 1995). than RFLP were found between bread wheat cultivars released in the 1970sand prompted the development of more than 400 SSR 1998; Stephenson et al., 1998). The first SSR markers available were used to characterize eight European cultivars (Devos et al., 1995) and 11 Canadian cultivars I dentification and registration of bread wheat culti-(Lee et al., 1995) of wheat bread. In a more comprehenvars is mainly based on morphologic and physiologic sive study of 40 European bread wheat cultivars using characteristics. Even though these descriptors are use-23 SSR, Plaschke et al. (1995) concluded that a relative ful, they are limited in number and may be affected by small number of SSR was sufficient to discriminate this environmental factors. Molecular markers are a useful set of cultivars. complement to morphological and physiological characThe AFLP technique combines the RFLP reliability terization of cultivars because they are plentiful, indewith the power of PCR to amplify simultaneously many pendent of tissue or environmental effects, and allow restriction fragments (Vos et al., 1995). This technique cultivar identification early in plant development. Mowas used successfully to evaluate genetic diversity and lecular characterization of cultivars is also useful to evalgenetic relationships in wheat (Salamini et al., 1997; uate potential genetic erosion, defined here as a reduc- Barrett and Kidwell, 1998;Domini et al., 2000), bean tion of genetic diversity in time.(Phaseolus vulgaris L.) (Tohme et al., 1996), rice (MacRestriction fragment length polymorphism (RFLP, kill et al., 1996;Virk et al., 2000), tea (Camellia sinensis Bostein et al., 1980) was one of the first DNA marker Kuntze) (Paul et al., 1997), barley (Hordeum vulgare L.) (Qi and Lindhout, 1997), and soybean (Maughan et al., 1996).M.M. Manifesto, A.R.
however, exceptional genotypes that combine excellent yield potential and high GPC, probably by a more effi-Grain Protein Content (GPC) of wheat (Triticum aestivum L. and cient relocation of nitrogen from senescing tissues to T. turgidum L.) is important for improved nutritional value and is grain, or by a more efficient uptake of nitrate and ammoalso one of the major factors affecting breadmaking and pasta quality.
Breeding wheat for resistance is the most effective means to control Septoria tritici blotch (STB), caused by the ascomycete Mycosphaerella graminicola (anamorph Septoria tritici). At least eight genes that confer resistance to STB in wheat have been identified. Among them, the Stb4 locus from the wheat cv. Tadinia showed resistance to M. graminicola at both seedling and adult-plant stages. However, no attempt has been made to map the Stb4 locus in the wheat genome. A mapping population of 77 F10 recombinant-inbred lines (RILs) derived from a three-way cross between the resistant cv. Tadinia and the susceptible parent (Yecora Rojo x UC554) was evaluated for disease resistance and molecular mapping. The RILs were tested with Argentina isolate I 89 of M. graminicola for one greenhouse season in Brazil during 1999, with an isolate from Brazil (IPBr1) for one field season in Piracicaba (Brazil) during 2000, and with Indiana tester isolate IN95-Lafayette-1196-WW-1-4 in the greenhouse during 2000 and 2001. The ratio of resistant:susceptible RILs was 1:1 in all three tests, confirming the single-gene model for control of resistance to STB in Tadinia. However, the patterns of resistance and susceptibility were different between the Indiana isolate and those from South America. For example, the ratio of RILs resistant to both the Indiana and Argentina isolates, resistant to one but susceptible to the other, and susceptible to both isolates was approximately 1:1:1:1, indicating that Tadinia may contain at least two genes for resistance to STB. A similar pattern was observed between the Indiana and Brazil isolates. The gene identified with the Indiana tester isolate was assumed to be the same as Stb4, whereas that revealed by the South American isolates may be new. Bulked-segregant analysis was used to identify amplified fragment length polymorphism (AFLP) and microsatellite markers linked to the presumed Stb4 gene. The AFLP marker EcoRI-ACTG/MseI-CAAA5 and microsatellite Xgwm111 were closely linked to the Stb4 locus in coupling at distances of 2.1 and 0.7 centimorgans (cM), respectively. A flanking marker, AFLP EAGG/ M-CAT10, was 4 cM from Stb4. The Stb4 gene was in a potential supercluster of resistance genes near the centromere on the short arm of wheat chromosome 7D that also contained Stb5 plus five previously identified genes for resistance to Russian wheat aphid. The microsatellite marker Xgwm111 identified in this study may be useful for facilitating the transfer of Stb4 into improved cultivars of wheat.
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