marker-assisted selection in wheat - ictsd

Chapter 4 – Marker-assisted selection in wheat: evolution, not revolution 53IntroductionWheat is a very important world staplecrop. The 2005 United States Departmentof Agriculture (USDA) estimates for theglobal production of wheat (both breadand durum) and maize are, respectively,627 million tonnes and 708 million tonnes.In Europe, bread wheat is without doubtthe most important broad-acre crop, witha production in the extended EuropeanUnion of 25 states of 115 million tonnes(maize 48 million tonnes). The largest productionand highest productivity of breadwheat are achieved in northwest Europe.Historically, wheat has been bred largelyby government-sponsored national andregional programmes, but the introductionof plant variety rights into Europe inthe 1960s encouraged participation by theprivate sector. Currently, wheat breedingin northwest Europe is almost exclusivelycarried out by private companies, withsome research underpinning by the publicsector. Breeders continue to be successfulin the production of high-yielding, diseaseresistant,high-quality varieties and, in theUnited Kingdom at least, genetic advancesfor yield have been running at between 0.5to 1 percent per annum for many years.Wheat is a naturally inbreeding species,and although a level of heterosis canbe demonstrated, difficulties in enforcingcross-pollination in a reliable and cost-effectiveway have hindered the development ofany significant contribution of F 1 hybridsto the variety pool. Most varietal developmentprogrammes are therefore based onversions of the long-established pedigreebreeding system, where large F 2 populationsare generated and conventional phenotypicselection is carried out in early generationsfor highly heritable, qualitative traits(such as disease resistance) and in later onesfor quantitative traits (primarily yield andquality). Thus, most varieties are bred andgrown as inbred, pure breeding lines. As aresult, the unit value of seed and economicmargins for breeders are low. By contrast,maize is a naturally out-crossing species thatshows highly significant levels of heterosis.This has resulted in the majority of maizebreeding being geared to the productionof F 1 hybrids. In industrialized countries,maize hybrid breeding has for some timebeen dominated by a small number of largeprivate sector companies that are able to sustainprofitability through their control overthe genotype of their varieties. No revenue islost as a result of the use of farm-saved seed,and the inbred components of a successfulhybrid are not available to competitors touse as parental material for their own varietalimprovement programmes. This hasfar-reaching implications on the feasibilityof MAS in maize, and largely explains thelead that maize enjoys over wheat in thedeployment of MAS technology.The continuing development of molecularmarker technology over the last decadehas been a happy by-product of “bigbiology” genomics research. As recentlyas 1996, the definition of 5 000 SSR loci inthe human genome merited a major publicationin Nature (Dib et al., 1996), but thenumber of known human single nucleotidepolymorphisms (SNPs) now runs into millions.Thus, although marker availability,potentially at least, is no longer limitingin crops, and the clear potential benefitsof marker deployment to plant breedingare undisputed, only relatively recentlyhas it begun to make more than a marginalimpact on breeding methodology. Evenin maize, where the level of DNA markerpolymorphism is high, large-scale deploymentof MAS did not gather any significantmomentum until more than 15 years afterthe publication of the first restriction frag-