marker-assisted selection in wheat - ictsd

42Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishcritical information about how specificcombinations of genes and alleles interactin relevant varietal backgrounds and allowsbreeders to compare the phenotypic effectof genes or chromosomal segments that havebeen inherited from a common ancestor andselected in multiple-cross combinations.In addition to the use of MAS in traditionalcrossing and selection programmes,breeders also have opportunities to adjustparticular traits or phenotypes via theintroduction of genes using a transgenicapproach (Ye et al., 2000; James, 2003;Nuffield Council on Bioethics, 2004). Onceintroduced into the gene pool, a transgenecan be tracked with the aid of molecularmarkers (designed to tag the transgenesequence itself) through subsequent crosses,just as would be done for any other gene ofinterest in a breeding programme.Another use of molecular markers invariety improvement involves markerassistedgermplasm evaluation (Xu, Ishiiand McCouch, 2003). Population structureanalysis offers insight about how diversityis partitioned within a species and canhelp define clusters, or subpopulations, ofgermplasm that are likely to contain highfrequencies of particular alleles (Garris,McCouch and Kresovich, 2003). This typeof analysis can also guide allele miningefforts aimed at identifying valuableaccessions in a germplasm collection foruse as parents in a breeding programme.Such approaches have the potential to makeparental selection more efficient, to expandthe gene pool of modern cultivars andultimately to speed up the development ofproductive new varieties. As information isgenerated about which genes and alleles areassociated with phenotypic characters ofagronomic importance, and as the complexinteractions among genes are enumerated inthe context of specific gene pools and theenvironments to which they are adapted,breeders are increasingly empowered tomake predictions about how to combinediverse alleles productively.To exploit molecular breeding strategiesfully, information resources must be developedso that the overwhelming amount ofinformation about genes, alleles and naturalgenetic variation can be funnelled into a usefultool for breeding applications. This willinvolve a very different approach to informationresources than currently employedby the large genome databases, which areoriented towards genomics researchersand molecular biologists rather than thebreeding community. Nonetheless, a fewexamples offer beacons of inspiration in thisarea, including the emerging InternationalRice Information System (IRIS) database(Bruskiewich et al., 2003; www.icis.cgiar.org/), the GeneFlow database (www.geneflow.com), the marker-assisted selectionwheat (MASwheat) database (http://maswheat.ucdavis.edu/)and software such asReal Time QTL (http://zamir.sgn.cornell.edu/Qtl/Html/home.htm).In conclusion, genomics research is generatinginformation about the location andphenotypic consequences of specific genesand alleles in a wide range of species. Thisinformation can be translated into tools forbreeders. Molecular marker technology canbenefit breeding objectives by increasing theefficiency and reliability of selection and byproviding essential insights into how genesbehave in different environments and indifferent genetic backgrounds. Once genesand QTL are identified, markers allowinteresting alleles to be traced throughthe pedigrees of breeding programmes ormined out of germplasm collections to serveas the basis for future varietal improvement.Using markers in combination withboth QTL and association approaches, the