marker-assisted selection in wheat - ictsd

Chapter 6 – Targeted introgression of cotton fibre quality quantitative trait loci using molecular markers 69INtroductionAmong the four species of Gossypium thatproduce seeds with spinnable fibres calledcotton, Gossypium hirsutum dominates theworld’s cotton fibre production, accountingfor approximately 90 percent of total worldproduction. The second most cultivated species,G. barbadense, includes superior extralong, strong and fine cottons. However,compared with G. hirsutum, the marketingadvantage of “high quality” G. barbadensecottons is offset by their lower productivityand a narrower adaptability to harshenvironments. Breeding approaches withinthese two species have essentially relied onhybridization and selection methods (subsequentto simple or complex crosses, apedigree system, sometimes combined withrecurrent selection, is applied). AlthoughG. hirsutum and G. barbadense displaycomplementary characteristics, attempts toutilize deliberate interspecific G. hirsutum/G. barbadense recombinations throughconventional breeding have had limitedimpact on cultivar development.In the past 10–15 years, DNA markersfor analyses of QTL and MAS have receivedconsiderable attention by plant and animalbreeders (Dekkers and Hospital, 2002).However, following an initial keen interestand promises for molecular-based breedingapproaches, the successful application ofthis technology has been shown to dependon the reliability and accuracy of the QTLanalyses, which in turn are strongly affectedby both population size and environmentalfactors (Schön et al., 2004). Examples ofapplied MAS in breeding programmes arestill scarce, particularly when complextraits (yield components, product quality)are under consideration.In the case of cotton, it is only recentlythat the results of efforts to gain a betterunderstanding of the genome and themolecular basis of fibre quality have beenpublished. Most of the earlier efforts incotton molecular breeding concentratedon interspecific hybridization, due to thefact that, intraspecifically, the major speciesG. hirsutum displayed a very lowlevel of molecular variability (Brubakerand Wendel, 2001). Based on studies ofinterspecific G. hirsutum x G. barbadensepopulations, published reports relate (i) tothe construction of high-resolution geneticmaps (Lacape et al., 2003; Rong et al.,2004); and (ii) to the identification of fibrequality-related QTL (Jiang et al., 1998;Kohel et al., 2001; Paterson et al., 2003;Lacape et al., 2005). In parallel, data haveaccumulated describing the cotton fibretranscriptome (reviewed by Wilkins andArpat, 2005). These studies confirmed thatkey fibre quality properties, such as length,fineness and strength, are controlled quantitatively,thus complicating conventionalbreeding for fibre improvement.Within the framework of a marker-assistedbackcross introgression scheme aimed attransferring fibre quality traits from a lowproductivityline of G. barbadense (donor)into a productive line of G. hirsutum (recipient),a saturated genetic map of tetraploidcotton was first developed (Lacape et al.,2003). This chapter describes how molecularmarkers were used in the early BC 1 and BC 2generations to identify QTL-rich regionsinvolved in determining fibre quality, asrecently reported by Lacape et al. (2005),and how MAS was actually implemented inthe later BC 3 and BC 4 generations.MethodologyThe major milestones (Figure 1) in themarker-assisted backcross selection processincluded the construction of two geneticmaps from the BC 1 and BC 2 populations,the detection of fibre quality QTL from