marker-assisted selection in wheat - ictsd

388Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishOther reports describing the manipulationof quantitatively inherited traitsinclude those of Bouchez et al. (2002)for introgressing favourable alleles atthree QTL for earliness and grain yieldin maize, and by Yousef and Juvik (2001)who reported on MAS for seedling emergenceand eating quality characters in sweetcorn. Also, Han et al. (1997) attempted toselect for barley malting traits using MAS.Additional scientific reports are availablethat describe MAS-related efforts for quantitativelyinherited traits.In general, manipulating several QTLassociated with multiple genomic regions insegregating progenies is considerably morechallenging. Often the success in geneticgains depends on the stability of these QTLas well as the cost efficiency of large-scaleMAS applications.Genetic diversity studiesIn addition to being used in MAS activities,molecular markers have been usedextensively for genetic diversity studies.Numerous scientific publications are availablethat describe the use of molecularmarkers in estimating the degree of relatednessof a set of cultivars in many cultivatedcrop species. In common with their use intrait manipulations, the practical outcomesof the numerous genetic diversity studiesusing molecular markers are not clear.Evaluation of genetic relatedness usingmolecular markers will have implicationson understanding the genetic structure ofexisting populations, enabling the design ofstrategies for proper acquisition of germplasmfor conservation purposes. Thegenetic uniqueness of accessions or populationsin germplasm collections can beaccurately estimated by the use of DNAprofiling (Brown and Kresovich, 1996;Smith and Helentjaris, 1996). Molecularmarkers have also been used for identifyingredundancies in existing germplasm collectionsin rice (Xu, Beachell and McCouch,2004) and sorghum (Dean et al., 1999). Incassava, Chavarriaga-Aquirre et al. (1999)used morphological traits, isozyme profilesand agronomic criteria to identify a core setof 630 accessions from a base collection ofapproximately 5 500 accessions.Modern farming in advanced countriesis based on high performing, geneticallyuniform new cultivars, which are generallyderived from well adapted, geneticallyrelated parental material. Tanksley andMcCouch (1997) have concluded that mostmodern soybean cultivars grown in theUnited States can be traced back to a verylimited number of strains from a small areaof northeastern China, while a majority ofhard red winter wheats is derived from a fewlines originated in Poland and the RussianFederation. The genetic basis of modernrice varieties grown in the United States isalso considered narrow (Dilday, 1990).Another application in the area ofgenetic diversity is the use of markersin identifying heterotic groups. Molecularmarkers have been used extensively in theconstruction of heterotic groups since the1990s in many different crop species ofeconomic importance. Heterotic groups areclusters of germplasm usually with similarcharacteristics and a high degree of relatednessthat, when crossed with materials fromanother heterotic group, tend to give riseto progeny with high levels of heterosis.Although markers randomly distributed inthe genome can be used to develop heteroticgroups, their usefulness in determininghybrid performance is not clear. While it isreasonable to assume that heterosis dependson the interactions among favourable allelesbelonging to the two parents, unless molecularmarkers that are known to be linked to