marker-assisted selection in wheat

272Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishFigure 7Proposed scheme for early marker-assisted selection of plus trees to be used as clonesMAS STAGEXF 1F 1Mating between plus hybrid trees tomaximize segregation for several traitsin the outbred F 2 Deployment of a large number (> 1000) of F 2progeny individuals toQTL MAPPING STAGEQTL mapping for wood properties traitssuch as lignin, fibreand wood densityembra_622*embra_28*embra_94*embra_627*EG_62*embra_372*embra_32*embra_31*embra_233*embra_50*embra_8*embra_328*embra_324*embra_173*embra_51*embra_25*embra_106*embra_248*embra_258*embra_105*embra_646*embra_175*ES_157*embra_277*embra_362*embra_189embra_115EG_94embra_227embra_122*EG_98*embra_125*embra_114embra_361F 2embra_189*EG_94*embra_227*embra_350*embra_321*EG-98*embra_125*embra_629*EN_14*embra_109embra_197embra_57EN_1embra_53EG_67embra_103embra_624ES_140embra_18embra_131embra_210.1*embra_210embra_204*embra_217*embra_365*embra_701*embra_19embra_393embra_66*embra_78*embra_137*ES_54*embra_645*QTL mapping informationto be used in MASF 2maximize probabilityof generatinga recombinant individual with asuperior multipleQTL allele contentGenotyping > 1000 seedlings witha small (~ 6 to 10) set of flankingmarkers for a targetednumber of QTL for wood qualitytraitsEarly MAS. Selection intensity is increased by MAS for late expressing traitsbut number of trees commonly deployed in progeny (~ 100) test iskept the same, thus allowing large variation to select for othertraits such as volume growth, form and branching habitQTL mapping is carried out and flanking markers in linkage disequilibrium with favourable alleles at major effect QTLalleles are identified. These markers are then used for early within-family selection in expanded progeny sizes at veryearly age to select for late expression traits such as wood density and lignin content.deploying clones to capture non-additivegenetic variation, it is reasonable to state thateucalypt is the forest tree crop for whichMAS has the best prospects of application.Quantitative theory as well as commonsense suggest that MAS in forest treesshould help, particularly in situations wheretrait heritability is low and selection occursat the level of the individual tree. However,implementing MAS for such traits is a challengingtask as extremely precise QTLmapping information is required and thiscan only be derived from experiments withlarge progeny sizes (in the order of severalhundred individuals), clonal replicatesfor increased precision, representative andmultiple genetic backgrounds and environments.To date, mapped QTL in foresttrees still do not fall into this descriptionalthough improved experiments are underway (Grattapaglia, 2004). Most experimentshave mapped QTL for traits thatdisplay intermediate to high heritabilityand probably did not tag the top alleles thatexist in the breeding populations as only avery limited sample of crosses were conducted.Furthermore, given the relativelysmall progeny sizes used for QTL detection(around 100 to 200 individuals), theestimated magnitude of the effects werelargely overestimated following the wellknown “Beavis effect” (Beavis, 1998).