marker-assisted selection in wheat - ictsd

262Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishof Agriculture of Brazil based on a set ofvalidated morphological descriptors. Tothe best of the author’s knowledge, this isthe only country today that has formalizedsuch descriptors, which include 36 morphologicaltraits of leaves, flowers, barkand fruit as well as wood density. Althoughthese descriptors generally satisfy the basicrequirements of stability and low environmentalinfluence, they are still difficult toevaluate, especially those related to maturetraits in flowers and fruits. Furthermore,it is common that clones are related bycommon ancestry making their discriminationeven more difficult. The high powerof discrimination coupled with the generalacceptance of DNA technology by eucalyptbreeders in Brazil resulted in the inclusionof molecular markers as additionaldescriptors (Grattapaglia et al., 2003). Theinclusion of DNA markers represented aremarkable advance that Brazil made in theinternational landscape of varietal protectionof forest trees. Currently all requestsfor clonal protection are accompanied bya multilocus DNA profile (DNA fingerprint)of 15 to 20 microsatellite markersthat were recommended based on severalaspects such as robustness, polymorphicinformation content and general availabilityin the public domain. The perspective forthe following years points to an increasednumber of applications for clone protectionby forest companies in view of theoutstanding value of elite eucalypt clonesfor the maintenance of competitivenessof the forestry-based industry. It can beexpected that DNA markers will add asignificant power of resolution for distinctness,uniformity and stability (DUS) testsin varietal protection of eucalypt clones,especially when closely related individualsare under scrutiny in legal disputes overclonal property.Characterization of breedingpopulationsBreeding populations can be characterizedby quantifying the levels and organizationof genetic variation within and betweenbreeding groups, sublines and progenies.These data can immediately be used toimprove the structure of breeding populations,infuse new material and decideon selection, enrichment or eliminationof germplasm entries. In the incompletepedigree systems frequently used in eucalypts,marker-based systems have been usedto monitor the levels of random geneticvariation throughout the different cyclesof a breeding programme thus allowingmuch greater flexibility and control overthe rate of reduction of genetic variability.For example, RAPD markers were successfullyused to characterize the wide range ofgenetic variation in a germplasm bank of E.globulus and thereby assist in the designingof further seed collections (Nesbitt et al.,1995). Gaiotto and Grattapaglia (1997) estimatedthe distribution of genetic variabilitywithin and between open pollinated familiesof a long-term breeding population of E.urophylla, and proposed a selection strategywithin and between families for incompletepedigreed populations based on theincorporation of genetic diversity measures.Marcucci-Poltri et al. (2003) used AFLPand microsatellite markers to obtain quantitativeestimates of genetic diversity in a E.dunnii breeding population selected for fitnessto subtropical and cold environments.Molecular data were used to design a clonalseed orchard using the nine most divergentpairs of genotypes, thereby retaining95.2 percent of the total number of allelesfrom the 140 polymorphic AFLP loci andthe four microsatellite loci analysed. In a subsequentstudy, Zelener et al. (2005) selectedE. dunni trees using trait selection index