marker-assisted selection in wheat - ictsd

Chapter 14 – Marker-assisted selection in Eucalyptus 271directly applicable in breeding. However,they are the most difficult to detect becausecausality is very difficult to prove unlessvery high penetrance Mendelian inheritancesare tackled.Prospects for using MAS in EucalyptusEucalyptus breeding programmes varybroadly according to several aspectsincluding the target species or hybrid,the possibility of deploying clones andthe amount of resources available to thebreeder. However, from the standpoint ofintegrating MAS, a reasonable premise isthat this will only be a justifiable optionwhen the breeding programme has alreadyreached a relatively high level of sophistication,fully exploiting all the accessiblebreeding and propagation tools. Advancedbreeding programmes that aim at elite cloneselection involve a significant amount oftime and effort being devoted to clonaltesting before effective recommendationscan be made concerning new clones foroperational plantations. Small sublinebreeding for hybrid performance combinedwith clonal propagation of selected individualsis being used increasingly for extractingnew elite clones (Potts, 2004). The recombinationstep of a breeding cycle involvesthe generation of several segregating progeniesfrom selected parents derived fromrecurrent selection programmes for generalcombining ability, or reciprocal recurrentselection programmes for hybrid combiningability. This latter strategy has been adoptedin tropical countries where the two reciprocalpopulations are actually two differentspecies such as E. grandis and E. urophylla.Controlled crosses that were oncean important obstacle for implementingpedigreed selection methods are now usedroutinely after the relatively recent advancesmade in controlled pollination methods forEucalyptus (Harbard, Griffin and Espejo,1999; de Assis, Warburton and Harwood,2005) (see Figure 2). Progeny trials, togetherwith expanded single family plots wherelarger numbers of full-sibs per family aredeployed, are used to allow very intensivewithin-family selection based on all theavailable information, both at the familyand individual level using BLUP-basedselection indices. This selection is generallycarried out at half-rotation age based ongrowth performance and on a preliminaryassessment of wood specific gravity usingindirect non-destructive techniques suchas pilodyn penetration and/or NIRS andRaman spectroscopy (Schimleck, Michelland Vinden, 1996). Vegetative propagulesare then rescued from selected trees eitherby coppicing, sequential grafting or in vitrotechniques, multiplied and then used forthe establishment of clonal tests.This breeding scheme generates largeamounts of linkage disequilibrium byhybridization and substantial amounts ofnon-additive genetic variation can be capturedby vegetative propagation. These arefavourable conditions for MAS in foresttrees (Strauss, Lande and Namkoong,1992). Favourable alleles at QTL segregatingwithin-families could be efficientlytagged with microsatellite markers in linkageequilibrium with the actual functional polymorphismsand used for marker-assistedwithin-family selection for superior individuals.QTL linked markers could be usedto carry out early selection thus reducing thetime necessary to carry out the first selectionespecially for traits related to woodproperties, and at the same time reducingthe number of trees to be selected, propagatedand advanced all the way to clonaltrials (Figure 7). Therefore, in the contextof molecular breeding, given their relativelyshort rotations and the possibility of