marker-assisted selection in wheat - ictsd

Chapter 19 – Technical, economic and policy considerations on marker-assisted selection in crops 385are not used to select for the backgroundrecurrent parent genotypes, but only toselect lines carrying the opaque2 mutationallele. Although CIMMYT uses markersfor detecting the presence of the opaque2mutation, markers are not available to selectfor the modifiers, which are important indetermining seed texture and quality andfor which other traditional screening techniquesare being used.A well known example of markerassistedbackcrossing of a qualitative traitinvolves the introgression of the Bt transgeneinto different maize lines (Ragot etal., 1994). Whenever plant transformationtechniques are used to produce geneticallymodified organisms (GMOs), usually thereare some cultivars that are more receptiveto transformation procedures than others.When the cultivar with the best agronomictype is not the most receptive to transformation,it is often possible to transformanother cultivar that is receptive and thenuse the diagnostic marker that detects thetransgene to introgress it into more desirablebackgrounds. This type of MAS-aidedline conversion can be accomplished forany crop species. The presence of markersto detect the transgene enables the detectionof converted progeny with a highdegree of accuracy.Another MAS-related CIMMYT experienceinvolves the case of maize streakvirus (MSV) resistance, for which a majorQTL was identified on maize chromosome1 that explains 50–70 percent of total phenotypicvariation (Pernet et al., 1999a, b).As maize has a well-saturated molecularlinkage map, several microsatellite markersassociated with this QTL were identifiedin the specific chromosomal region. Thesemarkers were tested in three populationsgenerated using three different MSV tolerantlines crossed with one susceptibleline. After screening the F 2 progeny andF 3 families, lines identified by markerswere sent to Africa, where MSV is prevalent.By phenotypic screening of the linesselected by MAS, it was established thatMAS-selected lines were significantly moreresistant to MSV (J-M. Ribaut, personalcommunication).In legumes, resistance to soybean cystnematode (SCN) is one example of aneffective MAS approach. Routinely usedphenotypic assays for SCN screening takeapproximately five weeks and extensivegreenhouse space and labour. Successfulidentification of closely linked microsatellitemarkers has enabled transfer of theresistance gene rhg1 with about 99 percentaccuracy (Cregan et al., 1999; Young1999). Many public and commercial soybeancultivar improvement efforts usethese markers to screen for SCN resistance(Young, 1999). Another example ofsuccessful MAS in common beans wasreported by Yu, Park and Poysa (2000)who used markers associated with commonbacterial blight. These markers identified alocus that explained about 62 percent of thephenotypic variation and have been used inMAS experiments.As described earlier, linkage map constructionin wheat is more challengingthan in species such as rice or maize.The allohexaploid nature allows wheat towithstand chromosomal imbalances as theloss of one chromosome can be compensatedby the presence of a homologouschromosome. As a result, wheat can becrossed with a range of wild relatives (bothintergeneric and interspecific), enablingintrogression of genetic material possessingresistances to different biotic and abioticstresses. When translocations (especiallyintergeneric translocations) are present inwheat, markers can be readily developed