marker-assisted selection in wheat - ictsd

Chapter 14 – Marker-assisted selection in Eucalyptus 263and genetic diversity measures estimatedfrom AFLP and microsatellites. Genetic differentiationestimates consistently showedlow differentiation among provenances andgreat differentiation among families suggestingthat orchard design should be basedon individual or family selection rather thanon provenance selection.Mating and deployment designs basedon genetic distanceGiven the wide genetic diversity and multiplesources of available germplasm foreucalypt breeding, choices typically haveto be made as to which elite parents shouldbe mated. Some selection based on the individual’sown performance or on pedigreeinformation is used before including it ina mating design. Any means of predictingtree performance deserves attention. One ofthe “holy grails” of molecular breeders hasbeen the ability to predict progeny performanceaccurately based on distance estimatesamong parents from genetic marker data.Vaillancourt et al. (1995a) used genetic distancesbased on RAPD markers to predictheterosis in E. globulus progenies. Theability of genetic distance to predict heterosiswas significant but accounted for lessthan 5 percent of the variation in specificcombing ability. Baril et al. (1997) used thestructure of RAPD genetic diversity withinand between E. grandis and E. urophylla towork out prediction equations for the treetrunk volume of individual hybrids at 38months. Surprisingly, this study showedthat a genetic distance based on RAPDmarkers with similar frequencies in the twospecies successfully predicted the value ofa cross. Through this model, the distancecalculated between species explained thegeneral combining ability and the specificcombining ability of volume growth witha global coefficient of determination of81.6 percent. RAPD markers were usedto recommend more divergent crosses in areciprocal recurrent selection programmefor hybrid breeding in Brazil (Ribeiro,Bertolucci and Grattapaglia, 1997). A setwith the 20 most and 20 least divergentcrosses between populations was recommended.Matings between more divergentindividuals will potentially allow segregationto be maximized in the resultingprogenies and transgressive segregants tobe recovered and used as clones.RAPD data were used to quantifyrelatedness among elite eucalypt clonesfor deployment purposes. As the historyof selective breeding in eucalypts is veryrecent, little, if any, pedigree informationis typically available. Furthermore, clonalplantations of Eucalyptus generally involveonly a few superior genotypes of unknownorigin. Costa e Silva and Grattapaglia(1997) used RAPD markers to quantify thegenetic relatedness among a group of 15elite clones. Comparative similarity analysesshowed that there was significantlymore genomic variation in the group ofclones than both within and between unrelatedhalf-sib families from a single species.Data on genetic similarity among cloneswere also used to propose a deploymentstrategy in a “genetic mosaic”, i.e. avoidingplanting more genetically related clones sideby side in contiguous forest blocks. Thisproposed strategy was based on the premisethat related clones share a common originand ancestry, have been subject to similarevolutionary selective pressures, and thereforeshare common susceptibility/tolerancealleles at pest and pathogen defence loci.Mating system and paternity inbreeding populationsOpen pollinated breeding by controllingexclusively the maternal progenitor and