marker-assisted selection in wheat - ictsd

56Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishare unreliable. Interestingly, unlike theexperience with many alien introgressionsegments, no obvious negative effects of itspresence on agronomic performance haveyet been detected either in InternationalMaize and Wheat Improvement Center(CIMMYT) trials (Ayala et al., 2001b) or atRAGT Seeds (Cambridge, UK).Durable resistance to yellow rustYellow rust is historically the most damagingof the leaf fungal pathogens in temperateEurope. Control has been achieved in thepast largely by a combination of fungicideapplication and of combinations ofmajor seedling resistance genes, of whicha significant number have been describedin the literature. However, like most racespecificresistances, most of these majorgenes have lost their effectiveness, and thishas led to a renewed effort in the definitionof partial or adult plant resistances tothis disease. The French variety Cappelle-Desprez dominated the wheat crop acrossFrance and the United Kingdom during the1960s and 1970s, and maintained its levelof adult resistance to yellow rust over thewhole of this period. A major part of thegenetic basis for this durable resistance waslocated to a translocated wheat chromosome(Law and Worland, 1997), and this hasbeen confirmed by a rigorous QTL analysis(Mallard et al., 2005), which has provided anumber of informative SSR markers for thiseffect. Other independent sources of adultresistance have been identified in Frenchand Eastern European germplasm at RAGTSeeds, and the major QTL responsible havebeen defined and marked.OBMOBM larvae feed on developing grain andheavy infestations result in a significantreduction in grain quality and some loss inyield. As for many sporadic pests, phenotypicscreening is unreliable and an indirectmeans of selection would be valuable.The gene Sm1 confers resistance to OBM(Sitodiplosis mosellana) by the expression ofan antibiotic that kills or slows the developmentof larvae. Thomas et al. (2005) definedthe map position of Sm1 and proposed aclose linkage with an SSR locus Xbarc35.This linkage remains to be validated inUnited Kingdom breeding populations, asit remains unclear whether the antibioticeffect shown by a few United Kingdomwheat varieties is conferred by Sm1.SBMVSBMV is one of two known viral pathogenstransmitted by the soil fungus Polymyxagraminis (another one being yellow mosaicvirus [YMV]), and can be an important agentof yield loss in some areas. Chemical controlis not feasible, and once soil is infected bythe virus-bearing host, the only solutionspossible are to abandon wheat culture orto use resistant varieties. Phenotyping isparticularly difficult as plant infection isenvironmentally sensitive, and the detectionof infection is laborious and prone to error.A proprietary assay for resistance to SBMVoriginating from European germplasm hasbeen in routine use at RAGT Seeds since2000 with a very high level of marker/phenotype association. More recently, abulk segregant analysis along with a QTLapproach has allowed the definition of aresistance locus to YMV from Chinesegermplasm, and a number of linked SSRmarkers have been identified (Liu et al.,2005).High-throughputinfrastructuresTechnical considerations of DNA acquisition,laboratory information management