marker-assisted selection in wheat

268Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishSimilarly, new functions can tentatively beassigned to previously characterized genesthat had not been described in the contextof revealing pleiotropic action of thesegenes. However, a major limitation in thistype of study in Eucalyptus is the lack ofa completed genome sequence because,without this, the relative locations of largenumbers of genes and their expressionQTL cannot be determined. This informationis required to assess whether thegenetic control of gene expression variationis in the cis- or trans-form for each gene. Inthe context of MAS in Eucalyptus, a betterunderstanding of such “master expressionQTL” that apparently control cascades ofgene expression of important biochemicalpathways may be very promising targetsfor detailed characterization in associationmapping experiments to uncover relevantpolymorphisms to be used in molecularbreeding practice.In summary, although the number ofreports detecting QTL in Eucalyptus hasgrown and these have become increasinglysophisticated, the large majority of mappedQTL have been localized on RAPD orAFLP maps. Consequently, it is impossibleto compare positions of QTL for the sameor correlated traits, seriously limiting thelong-term value of such mapping for MAS.Exceptions are QTL studies where transferablemarkers such as a few microsatellites(Marques et al., 2002; Thamarus et al.,2004) or candidate genes (Gion et al., 2000;Thamarus et al., 2004) were also mapped sothat it is at least possible to make a roughpreliminary comparison of QTL locationsat the linkage group level. Especially in thegenus Eucalyptus where breeders worldwidetake advantage of interspecific geneticvariation for wood properties and diseaseresistance through hybridization, therecent availability of a robust, genus-widegenetic map with highly transferable microsatellitemarkers (Brondani et al., 2006)should stimulate improved genomic undertakingsincluding QTL validation acrosspedigrees, co-localization of QTL andcandidate genes for guiding associationmapping experiments, positional cloning ofQTL and eventually MAS.MAS in EucalyptusTwenty years have passed since the firstdemonstrations that QTL for major effectscould be mapped with molecular markers(Stuber et al., 1980; Paterson et al., 1988;Lander and Botstein, 1989), and severalreviews have described the potential benefitsand caveats of MAS in the plant geneticsliterature (e.g. Tanksley, 1993; Beavis, 1998;Young, 1999; Mauricio, 2001; Dekkers andHospital, 2002). Yet, large-scale operationalMAS is still largely restricted to very fewcrops and for very specific applications.Maize is probably the best example, wherethe financial returns on hybrid seed developmentcoupled with the ability to controlgermplasm fully, has prompted large-scaleinvestments in MAS by the private sectorbased on high-throughput single nucleotidepolymorphism (SNP) genotyping platforms.Based on a detailed understandingof the molecular architecture of quantitativetraits, current applications include yieldoriented advanced backcross QTL (AB-QTL) systems as well as accelerated lineconversion following trait introgression bymarker-assisted backcrossing (MABC). InEucalyptus and forest tree breeding in general,the application of molecular markersfor directional selection is still an unfulfilledpromise. This is largely due to: the recentdomestication of tree crops and hence thewide genetic heterogeneity of breedingpopulations; the inability to develop inbredlines at least on a short-term basis to allow a