marker-assisted selection in wheat - ictsd

38Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishFigure 1Progress of microsatellite marker development in riceMcCouch et al., 2002Temnykh et al., 2001Temnykh et al., 2000Chen et al., 1997Akagi et al., 1996Panaud, Chen andMcCouch, 1996Wu and Tanksley, 1993Zhao and Kochert, 19930 500 1000 1500 2000Number of microsatellite markersScreened libraries Genes/ESTs BAC-ends Shotgun sequences Finished BACstracks the publication of rice microsatellitemarkers derived from screening libraries ofclones and from the various categories ofsequences deposited in public databases. Theearliest method of developing microsatellitemarkers in rice was by using microsatellitesequences as probes to isolate clones fromgenomic libraries (Zhao and Kochert, 1993;Wu and Tanksley, 1993; Panaud, Chen andMcCouch, 1996; Akagi et al., 1996; Chenet al., 1997; Temnykh et al., 2000). In 1996,Akagi et al. used microsatellite repeatsfound in rice sequences from databasesearches to develop 35 new markers andin 2000, Temnykh et al. published 91 newmicrosatellite markers developed fromexpressed sequence tag (EST) sequences.Temnykh et al. (2001) developed 200 newmarkers, mostly from end sequences of ricebacterial artificial chromosomes (BACs).However, the most dramatic increase inmicrosatellite markers (2 240 new markersin 2002 and 25 000 in 2004) was madepossible primarily though the use of wholegenome shotgun sequences (McCouch et al.,2002; G. Wilson, personal communication).Complete genome sequence provides anadditional advantage in electronicallydetermining the position of new markerson genetic and physical maps. However,full genomic sequence is not a requirementfor microsatellite marker development, andthere are a number of microsatellite markersthat have been developed for a wide arrayof crop species (Table 3) without the benefitof full genomic sequence.Marker-assisted selectionstrategies and examplesMAS in a breeding context involves scoringindirectly for the presence or absenceof a desired phenotype or phenotypiccomponent based on the sequences orbanding patterns of molecular markerslocated in or near the genes controlling thephenotype. The sequence polymorphismor banding pattern of the molecular markeris indicative of the presence or absence of aspecific gene or chromosomal segment thatis known to carry a desired allele.DNA markers can increase screeningefficiency in breeding programmes in a