marker-assisted selection in wheat - ictsd

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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
386Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishfor the translocated chromosome segments.If a translocated segment carries a trait ofimportance, markers can then be used totransfer it into different wheats. Diagnosticor perfect markers (i.e. markers with completelinkage to the genes of interest withno possibility of recombination) have beendeveloped for genes conferring resistanceto different biotic stresses in wheat.CIMMYT’s wheat improvement efforts usea set of markers routinely on a seasonalbasis for introgression of a set of genes intohigh-yielding backgrounds. Examples ofthe perfect markers that are currently inuse are:• Cereal cyst nematode (CCN) resistancegene Cre1 (2BL), identified in wheatlandrace AUS10894 and Cre3 (2DL),derived from Triticum tauschii (Lagudah,Moullet and Appels, 1997). These markersare used routinely in segregating populationsto enable selective advancementof lines containing the Cre genes targetedto all environments, but particularly tomarginal ones, where healthy root architectureis essential to allow plants to takeadvantage of minimal soil moisture. Phenotypicevaluation for CCN resistanceis labour intensive as well as expensive.Given that it is impossible to screenfor CCN resistance in Mexico (whereCIMMYT headquarters are located) dueto the lack of required screening facilities,the use of markers is essential forimproving this trait.• Barley yellow dwarf virus (BYDV) resistance,derived from a chromosome segmentintrogressed from Thinopyrumintermedium, on chromosome 7DL(Ayala et al., 2001). BYDV is an importantviral disease in certain wheat growingregions of the world. Environmentalinfluence makes field screening less reliable.The diagnostic marker for the translocatedchromosome segment allows thealien-derived resistance to be combinedwith the BYDV tolerance available inwheat.• Marker for Aegilops ventricosa-derivedresistance to stripe rust (Yr17), leaf rust(Lr37) and stem rust (Sr38 ) (O. Robert,personal communication). The translocationfrom Ae. ventricosa is present onchromosome 2AS. The diagnostic markerfor the translocation is used mainly inbread wheat x durum wheat crosses, toidentify the durum derivatives carryingthe translocation.In addition, CIMMYT uses a set oflinked markers for transferring a locus withmajor effects for boron tolerance (Bo-1),crown rot resistance, scab resistance andstem rust resistance in its MAS efforts.These efforts with linked genes are conductedwith the objective of increasing theallele frequency for desirable alleles in segregatingpopulations (William, Trethowanand Crosby-Galvan, 2007).Gene pyramiding/stackingMAS lends itself well to gene pyramidingefforts for disease resistance. When a cultivaris protected by one gene with majoreffects against a specific disease, it is oftennot possible to introgress additional genesconferring resistance to the same diseasebecause of the difficulty of phenotypicscreening for the presence of additionalgenes (as the plant already shows resistanceto the disease). However, if several genescan be tagged with closely linked molecularmarkers, MAS strategies can be used todevelop lines with stacked genes, giving thecultivar more durable protection than thatafforded by a single resistance gene.Resistance to bacterial blight providesan excellent example of using MAS for genepyramiding. Bacterial blight is caused by
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iiiContentsAcknowledgementsForeword
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Section v - marker-assisted selecti
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viiForewordSince almost the beginni
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ixAbbreviations and acronymsAATFAB-
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xiFIVIMSFNPFSCFSILGABIGASGCAGCPGDPG
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xiiiOBMOECDOIEOPVPAGEPBRsPCRPGRFAPI
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xvContributorsAmalia BaroneProfesso
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xviiElcio Perpétuo GuimarãesSenio
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xixAndrea SonninoSenior Agricultura
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Chapter 1Marker-assisted selection
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Chapter 1 - An overview of the issu
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Chapter 3Molecular markers for use
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Chapter 3 - Molecular markers for u
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Chapter 6Targeted introgression of
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Chapter 6 - Targeted introgression
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Chapter 7 - Marker-assisted selecti
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Chapter 10Strategies, limitations a
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Chapter 10 - Strategies, limitation
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Section VMarker-assisted selectioni
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This book provides a comprehensive
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