marker-assisted selection in wheat

Chapter 1 – An overview of the issuesuseful for genetic improvement of suchlong-cycle plants.LivestockAgain, much effort has been put into thedevelopment of molecular marker mapsin this sector. The first reported map inlivestock was for chicken in 1992, whichwas quickly followed by the publication ofmaps for cattle, pigs and sheep. Since then,the search for useful markers has continuedand further species have been targeted,including goat, horse, rabbit and turkey(see www.thearkdb.org/ for the currentstatus regarding some major livestock species).Microsatellite markers have been ofmajor importance.Dekkers (2004) recently reviewed commercialapplications of MAS in livestockand noted that several gene or markertests are available on a commercial basisin different species and for different traits,and that the majority of uses involve GAS,where an important gene (e.g. responsiblefor a congenital defect) has been identifiedor, to a lesser degree, LD-MAS. He pointedout that documentation is poor since,although several genetic tests are available,the extent to which they are used in commercialapplications is unclear, as is themanner in which they are used and whethertheir use leads to greater responses to selection.He concluded that “opportunities forthe application of MAS exist, in particularfor GAS and LD-MAS and, to a lesserdegree, for LE-MAS because of greaterimplementation requirements. Regardlessof the strategy, successful application ofMAS requires a comprehensive integratedapproach with continued emphasis on phenotypicrecording programmes to enableQTL detection, estimation and confirmationof effects, and use of estimates inselection. Although initial expectations forthe use of MAS were high, the current attitudeis one of cautious optimism.”ForestryAs for crops, extensive efforts have beendevoted to construction of molecular markermaps for the major commercial genera,such as eucalypts, pines and acacia. RFLPs,RAPDs, microsatellites and AFLPs havebeen extensively used. The Web site http://dendrome.ucdavis.edu/index.php providesupdated information on the status regardingmolecular marker maps in forestry.Molecular maps have been used to locatemarkers associated with variation in forestrytraits of commercial interest, such asgrowth, frost tolerance, wood properties,vegetative propagation, leaf oil compositionand disease resistance. Since MAS allowsearly selection before traits of interest(e.g. wood quality) are expressed, a majorincentive for using molecular techniquesin tree breeding is to improve the rate ofgenetic gain by reducing the long generationinterval However, Butcher (2003)noted that “MAS has yet to be incorporatedin operational breeding programmes forplantation species” and she referred to thehigh costs of genotyping, the large familysizes required to detect QTL and the lackof knowledge of QTL interactions withgenetic background, tree age and environmentas explanatory factors.In a recent review of biotechnology inforestry, Yanchuk (2002) also highlighted thepotential advantage of early selection usingMAS, but again pointed out that MAS is notyet being applied routinely in tree breedingprogrammes, largely “because of economicconstraints (i.e. the additional genetic gainsare generally not large enough to offset thecosts of applying the technology). Thus it islikely that MAS will only be applied for ahandful of species and situations, e.g. a few