marker-assisted selection in wheat - ictsd

266Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishhigher-throughput, higher polymorphismtyping systems such as microsatellites,organized in dense genetic maps (Brondaniet al., 1998; Thamarus et al., 2002). Only 137autosomal microsatellite markers have beenpublished to date for species of Eucalyptus,including 67 from E. globulus, E. nitens,E. sieberi and E. leucoxyon (Byrne et al.,1996; Steane et al., 2001; Glaubitz, Emebiriand Moran, 2001; www.ffp.csiro.au/tigr/molecular/eucmsps.html; Ottewell et al.,2005) and 70 from E. grandis and E. urophylla(Brondani et al., 1998; Brondani,Brondani and Grattapaglia, 2002). Recentlya set of 35 chloroplast DNA microsatelliteswas developed based on the full cp-DNAsequence of E. globulus (Steane, Jones andVaillancourt, 2005). Microsatellite transferabilityacross species of the subgenusSymphyomyrtus, which includes all themost widely planted species, varies between80 and 100 percent depending on the sectionto which they belong. It still remainsaround 50 to 60 percent for species of differentsubgenera such as Idiogenes andMonocalyptus and goes down to 25 percentfor the related genus Corymbia (Kirstet al., 1997). Microsatellite comparativemapping data have also shown that genomehomology across species of the same subgenusSymphyomyrtus is very high, notonly in terms of microsatellite flankingsequence conservation, but also markerorder along linkage maps (Marques et al.,2002). Although some tens of microsatelliteshave been mapped on existing RAPDand AFLP framework maps (Brondani,Brondani and Grattapaglia, 2002; Marqueset al., 2002; Thamarus et al., 2002), thegenus Eucalyptus still lacks a more comprehensivegenetic map widely useful formolecular breeding practice. To fill this gap,a novel set of 230 new microsatellites hasrecently been developed and a consensusmap assembled covering at least 90 percentof the recombining genome of Eucalyptus.This map has 234 mapped loci on 11 linkagegroups, an observed length of 1 568 cM anda mean distance between markers of 8.4cM (Brondani et al., 2006). This representsan important step forward for Eucalyptuscomparative genomics, opening stimulatingperspectives for evolutionary studies andmolecular breeding applications. The generalizeduse of an increasingly larger setof interspecific transferable markers andconsensus mapping information will allowfaster and more detailed investigations ofQTL synteny among species, validationof QTL and expression-QTL across variablegenetic backgrounds, and positioningof a growing number of candidate genesco-localized with QTL, to be tested inassociation mapping experiments.QTL mapping in EucalyptusFollowing the construction of linkagemaps, several groups have reported theidentification of genomic regions that havea significant effect on the expression of economicallyimportant traits in Eucalyptus.QTL mapping experiments have, withoutexception, found a few major effect QTLfor all traits considered in spite of the limitedexperimental precision, the lack ofpre-designed pedigree to maximize phenotypicsegregation, and the relatively smallsegregating populations evaluated. This canbe explained by the undomesticated natureand wide genetic heterogeneity of eucalyptsadded to the fact that most QTL mappingexperiments were carried out in interspecificpopulations thus taking advantage ofcontrasting gene pools. QTL for juveniletraits such as seedling height, leaf area andseedling frost tolerance have been mapped(Vaillancourt et al., 1995b; Byrne et al.,1997a, b), while traits related to vegetative