marker-assisted selection in wheat - ictsd

Chapter 14 – Marker-assisted selection in Eucalyptus 267propagation ability such as adventitiousrooting, stump sprouting and in vitro shootmultiplication have also been detected(Grattapaglia, Bertolucci and Sederoff,1995; Marques et al., 1999), as has a majorQTL for early flowering (Missiaggia,Piacezzi and Grattapaglia, 2005). In addition,QTL for insect resistance and essentialoil traits were mapped (Shepherd, Chaparroand Teasdale, 1999) and recently a majorQTL for Puccinia psidii rust resistance withquasi Mendelian inheritance was found andmapped in E. grandis (Junghans et al., 2003).Major QTL were also found for rotationage traits such as volume growth, woodspecific gravity, bark thickness and stemform (Grattapaglia et al., 1996; Verhaegenet al., 1997; Thamarus et al., 2004; Kirst etal., 2004, 2005b).Although QTL of relatively large effectshave been detected for growth traits, whenit comes to potential application in MASthe best opportunities for QTL mappingare those related to specialized wood propertiesthat have a direct impact on industrialprocesses. These traits are usually difficult tomeasure both because they require destructivewhole stem sampling and because theyare traits that are expressed late. Myburg(2001) demonstrated the application of indirect,high-throughput phenotyping of woodquality traits in Eucalyptus by near infraredreflectance spectroscopy (NIRS) for QTLmapping in a hybrid E. grandis x E. globulusbackcross population. Approximately300 individuals that had been previouslygenotyped with AFLP markers were analysedby NIRS, and predictions made forpulp yield, alkali consumption, basic density,fibre length and coarseness, and severalwood chemical properties (lignin, celluloseand extractives). A variety of molecularmarker classes and pedigree types wereused in these experiments. QTL weredetected in F 1 , inbred or outbred F 2 andhalf-sib families with or without clonalreplicates. Also looking at wood qualitytraits, Thamarus et al. (2004) used novelhigh-throughput and traditional methodsto quantify wood density, fibre length, pulpyield and microfibril angle (MFA) in twofull-sib families of E. globulus that shareda common parent. Pulp yield and cellulosecontent were determined by NIRS, andMFA was quantified by SilviScan. Exceptfor fibre length, QTL for all traits couldbe detected in both populations, includingthree QTL in common genetic regions onboth crosses for wood density, one for pulpyield and one for MFA. The proportionof phenotypic variation explained by theQTL identified in both crosses ranged from3.2 to 15.8 percent.Recently QTL analysis of transcriptlevels of lignin-related genes showed thattheir mRNA abundance is regulated bytwo genetic loci co-localized with QTL forgrowth, suggesting that the same genomicregions are regulating growth, lignin contentand composition (Kirst et al., 2004).In a subsequent study, Kirst et al. (2005b)showed that one identified expression QTLexplained up to 70 percent of the transcriptlevel variation for over 800 genesand that hotspots with co-localized expressionQTL were identified on single treeAFLP typically containing genes associatedwith specific metabolic and regulatorypathways, suggesting coordinated geneticregulation. The correlation of gene expressionprofiles in segregating progeny can alsoextend knowledge about genes involved inthese pathways. Complementary DNAsrepresenting previously uncharacterized orhypothetical genes, whose transcript levelsare strongly correlated with those of geneswith known functions, may be associatedwith the same pathway or biological process.