marker-assisted selection in wheat

256Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishas China, Ethiopia, India, Peru and VietNam. However, it is the increasing globaldemand for short fibre pulp that has driventhe massive expansion of eucalypt plantationsand accompanying breeding practicesthroughout the world during the twentiethcentury (Turnbull, 1999). Their highfibre content relative to other wood components,coupled with the uniformity offibres relative to other angiosperm species,has led to high demand for eucalypt pulpfor coated and uncoated free-sheet paper,bleach board, sanitary products (fluff pulp),and to a lesser extent for top liners on cardboardboxes, corrugating medium, and asa filler in long fibre conifer products suchas newsprint and containerboard (Kellison,2001). In the last ten years, the developmentof new wood drying and sawingtechnologies has also increased interest inusing plantation eucalypts for sawn wood,veneer, medium density fibreboard and asextenders in plastic and moulded timber(Kellison, 2001).An FAO report estimated a total of17.9 million ha of planted Eucalyptus worldwidewith India as the largest planter withover 8 million ha followed by Brazil with3 million (FAO, 2001). The majority ofplantations consists of only a few eucalyptspecies and hybrids. The most importantare E. grandis, E. globulus, E. urophyllaand E. camaldulensis, which together withtheir hybrids account for about 80 percentof the plantation area, followed byE. nitens, E. saligna, E. deglupta, E. pilularis,Corymbia citriodora and E. teriticornis(Eldridge et al., 1993; Waugh, 2004). Marketfavourites for pulpwood are E. grandis, E.urophylla and their hybrids in tropical andsubtropical regions and E. globulus in temperateregions.Although eucalypt breeding is currentlya very dynamic and technically advancedoperation carried out mainly by severalprivate companies, eucalypts should beseen as still in their domestication infancywhen compared with crop species, withmost breeding programmes only one ortwo generations removed from the wild.However, with the combination of amplegenetic variation both at the intra and interspecificlevels and the ability to clone elitegenotypes, eucalypts have quickly becomeamong the most advanced genetic materialin forestry. Breeding of eucalypts has movedfaster in countries such as Brazil, Chile,Portugal and South Africa that adoptedEucalyptus for industrial plantation forestry.Most eucalypt breeding programmesworldwide are focused on geneticallyimproving trees for industrial pulpwoodproduction (Borralho, 2001; Kanowski andBorralho, 2004). The target traits of mostbreeding programmes include volumetricgrowth per hectare, wood density and pulpyield (Borralho, Cotterill and Kanowski,1993). Traits such as pest and disease resistanceand adaptability to abiotic stressessuch as frost, drought or wind are usuallysecondary targets that become importantwhen they have an impact on one or moreof the main traits. Following the standardconcepts in tree breeding, large geneticgains have been obtained in the early stagesof eucalypt domestication, simply throughspecies and provenance selection followedby individual selection and establishmentof clonal or seedling seed orchards orclonal propagation of elite selections fordirect deployment (Eldridge et al., 1993;Kanowski and Borralho, 2004; Potts, 2004).Subsequent population improvement hasalso demonstrated significant genetic gainthrough recurrent selection in an open-pollinatedbreeding population coupled withopen or controlled pollinated populationsof the most elite selections or specialized