marker-assisted selection in wheat - ictsd

Chapter 14 – Marker-assisted selection in Eucalyptus 273It is frequently stated that MAS for treeswould be most useful for volume growth asthis is a universal trait of interest and typicallyof low heritability at the individualtree level. However, in tropical conditions,it is most likely this will not be the targettrait of first choice for MAS. Broad senseheritability at the clone mean level, whichis the typical selection unit, is frequentlyabove 0.8, allowing an almost perfectranking and selection of clones even atvery early ages (less than two years) undertropical conditions (Rezende, Bertolucciand Ramalho, 1994). Molecular markersfor volume growth in these conditionswill hardly make a significant contributionto increasing gain per unit time. Thecost of scoring molecular markers dictatesthat the most likely application of MAS inEucalyptus will be for traits that providesignificant added value to the final productsuch as branching habit (for solid wood)and wood chemical traits, or allow clonaldeployment such as adventitious rooting orsomatic embryogenesis response. Within allpossible quality traits, the option would befor those that display medium to high heritabilitiesbut where phenotype assessment isdifficult, expensive or requires waiting untilthe tree reaches maturity. Wood qualitytraits typically require the tree to startaccumulating late wood and involve relativelylengthy procedures for phenotypicevaluation in the laboratory. These kindsof traits could be interesting targets forMAS in Eucalyptus, given that the costsof genotyping are sufficiently competitiveand precision is high when comparedwith direct phenotype measurements. It isimportant to point out, however, that withthe recent developments of fast samplingand indirect wood chemistry measurementsbased on NIRS (Schimleck, Michell andVinden, 1996), the potential gain will onlybe realized on the basis of the time savingsprovided by very early selection. Selectedindividuals could be recombined more rapidlyfollowing flower induction (Griffin etal., 1993) to produce the next generation,potentially increasing the genetic gain perunit time.MAS for multiple traits will face manyof the same difficulties faced by conventionalmultiple trait selection. Very largeprogeny sizes would have to be deployed tohave a reasonable probability of recoveringgenotypes with a combination of favourablealleles at many QTL for many traits.When using MAS, priorities will have to beestablished not only for traits but also forspecific QTL. This will require a very goodunderstanding of the relative magnitude ofeach QTL, potential QTL x backgroundinteractions and pleiotropic effects of QTL.Linkage mapping, however, will allow thebreeder to understand the basis of negativecorrelation between traits and possibly tobreak unwanted linkages by selecting specificrecombinant genotypes.Once the challenging issues relatedto the discovery of robust marker-traitassociations, either within family (LEmarkers) or at the population level (LD ordirect markers), are dealt with, a realisticstrategy for the implementation of MAS inEucalyptus might be to tackle only a fewmajor QTL for a quality trait of significantadded value. Theoretically, when the totalproportion of the additive genetic varianceexplained by the marker loci exceeds theheritability of the character, selection on thebasis of the markers alone is more efficientthan selection on the individual phenotype.Such a goal might be achieved for a specifictrait with just a few QTL alleles responsiblefor large effects. On the other hand, if nomajor gene is detected in an experiment ofreasonable size, it might be wiser to dismiss