marker-assisted selection in wheat - ictsd

Chapter 15 – Marker-assisted selection in forestry species 295or family relationships. The possibility oftransferring SNP markers among specieshas already been demonstrated in eucalypts(Thumma et al., 2005).Forest trees, including many species indeveloping countries, are near their wildstate, and significant improvement can stillbe made quite rapidly based on selectionamong existing genotypes (FAO, 2003).Association studies are ideally suited toexploiting variation in natural populationsand do not rely on the existence of extensivepedigrees from controlled crosses.Suitable populations should include a largenumber of unrelated individuals of thesame age growing on the one site. It hasbeen estimated that 500 individuals wouldbe necessary to detect association betweena quantitative nucleotide responsiblefor 5 percent or more of the phenotypicvariance (Long and Langley, 1999). Thedevelopment of such populations wouldprovide a good foundation for future GASresearch in developing countries. While themarkers developed using this approach arelikely to be more easily transferred betweenbreeding programmes, the application ofGAS would require the subsequent developmentof advanced breeding programmeswhere the selection of superior alleles couldtake place. However, publicly fundedforestry research is suboptimal in manydeveloping countries and development prioritiesdo not necessarily include geneticimprovement programmes (FAO, 2003).The major costs of GAS are associatedwith identifying candidate genes potentiallylinked to the relevant traits, and discoveringSNPs. In some cases, public databases maycontain large numbers of genes from thetarget or closely related species but, if not,there would be additional costs associatedwith EST sequencing of genes fromthe relevant tissue (i.e. xylem genes forwood traits). These additional costs may beoffset partially by EST sequencing clonesfrom mixed cDNA libraries derived from anumber of unrelated trees from the associationpopulation.Previously, the cost of genotyping SNPswas prohibitive, but this has fallen dramaticallyin recent years as high-throughputtechnologies have been developed for thehuman HapMap project (InternationalHapMap Consortium, 2003). The IlluminaBeadstation technology (www.illumina.com) is particularly suited to smaller-scalegenotyping projects such as those beingundertaken in forest trees. Cost is certainlya limitation in many developing countriesincluding much of Africa and someSouth American countries; however, inmost Asian countries and countries suchas Brazil where molecular genetic laboratoriesare already well established, costswould not be prohibitive. The full benefitsof GAS would require development of efficientclonal propagation and deploymentsystems before it was applied routinely.Less stringent regulatory approval processesand greater public acceptance ofgenetically modified plants have allowedBrazil and China to take a lead role in commercializingtransgenic tree technology.China is the only country to announce thecommercial release of transgenics (poplar)with 300–500 hectares being planted in2002 (Wang, 2004). Regulatory approvalfor the release of Bacillus thuriengensis(Bt) insect resistant eucalypts in Brazil ispending (Sedjo, 2004). Given the difficultyof carrying out long-term transgenic fieldtrials with long rotation conifers, transgenicapproaches are likely to be restrictedto modification of high-value traits such aswood fibre properties in short rotation speciesgrown on a large scale. Conventionalbreeding, using either open or controlled