marker-assisted selection in wheat

Chapter 19 – Technical, economic and policy considerations on marker-assisted selection in crops 397economically important crop species. Forspecies such as maize, rice and soybeans,robust SNP-based assay platforms alreadyexist in the private sector as well as in somepublic sector enterprises. The added advantageof SNP-based marker systems is thatthey avoid gel-based allele separations forvisualization and have the potential forautomation in high-throughput assay platforms.These ongoing research efforts willinevitably lead to the development of morerobust, high-throughput assays that areboth simple and cost effective (Jenkins andGibson, 2002).When is it advantageous to use MAS?In addition to the cost and time savingsdescribed above, for a number of breedingscenarios, MAS methods are likely to offersignificant advantages compared with conventionalselection methods. These scenariosassume the availability of markers for multipletraits and take into consideration theadvantages of MAS under optimum situations(Dreher et al., 2002; Dudley, 1993).• Gene stacking for a single trait. MASoffers potential savings compared withconventional selection when it allowsbreeders to identify the presence of multiplegenes/alleles related to a single trait,and the alleles do not exert individuallydetectable effects on the expression ofthe trait. For example, when one geneconfers resistance to a specific diseaseor pest, breeders would be unable to usetraditional phenotypic screening to addanother gene to the same cultivar in orderto increase the durability of resistance.In such cases, MAS would be the onlyfeasible option, provided markers areavailable for such genes.• Early detection. MAS offers potential savingscompared with conventional selectionwhen it allows alleles for desirabletraits to be detected early, well beforethe trait is expressed and can be detectedphenotypically. This benefit can be particularlyimportant in species that growslowly, for example, tree crops.• Recessive genes. MAS offers potential savingscompared with conventional selectionwhen it allows breeders to identifyheterozygous plants that carry a recessiveallele of interest whose presence cannotbe detected phenotypically. In traditionalbreeding approaches, an extra step ofselfing is required to detect phenotypesassociated with recessive genes.• Heritability of traits. Up to a point, gainsfrom MAS increase with decreasing heritability.However, due to the difficultiesencountered in QTL detection, the gainsare likely to decline beyond a certainthreshold heritability estimate.• Seasonal considerations. MAS offerspotential savings compared with conventionalselection when it is necessaryto screen for traits whose expressiondepends on seasonal parameters. Usingmolecular markers, at any time of theyear, breeders can screen for the presenceof an allele (or alleles) associatedwith traits that are expressed only duringcertain growing seasons. For example,CIMMYT’s wheat breeding stationin northern Mexico is usually used forscreening segregating germplasm for leafrust resistance. However, expression ofleaf rust is not uniform in all growingseasons. The same concept is true forfield screening for drought tolerance.When there are seasons with low expressionof leaf rust or less intense droughtdue to unexpected rainfall, markers, ifavailable, can be a valuable alternative asa tool for screening.• Geographical considerations. MAS offerspotential savings when it is necessary