marker-assisted selection in wheat - ictsd

Chapter 6 – Targeted introgression of cotton fibre quality quantitative trait loci using molecular markers 73Table 1Range of parental (G. hirsutum [Gh] and G. barbadense [Gb]) values over the five sets of dataGh Gb BC 1N=75BC 2N=200BC 2S 1N=200BC 3N=43BC 4N=37Length (mm)* 27.5–31.8 39.2–43.7 33.8(27.8–38.2)Length uniformity 81.3–85.5 83.9–87.1 85.0(82.0–88.2)Strength (g/tex) 26.5–32.5 41.4–46.7 35.7(29.7–41.6)Elongation 5.1–6.4 5.5–6.0 6.3(5.7–7.4)Fineness (mtex)** 207–243 178–191 218(177–308)Colour reflectance 71.2–77.7 74.6–75.6 74.3(65.9–81.1)28.6(22.9–35.3)81.3(73.3–86.7)28.3(17.8–43.7)5.5(3.9–7.6)224(165–379)72.2(56.8–81.3)30.7(26.9–36.1)83.3(80.6–85.1)29.0(23.7–34.5)6.3(5.4–7.4)225(176–285)74.1(69.8–77.5)30.0(25.0–33.9)81.9(77.6–86.4)24.5(16.8–32.8)5.7(4.5–7.0)128***(117–148)71.5(64.7–76.6)31.7(28.1–37.1)86.1(82.5–89.5)33.8(29.7–39.1)6.3(4.9–7.4)243(192–283)75.1(67.1–82.0)* Length is upper half mean length (UHML), ** standard fineness, *** low fibre fineness values in BC 3 generation because ofpoor maturitiesNote: Mean values and range (in brackets) observed in each BC 1–4 generation (number of plants, N, indicated) of fibretechnological parameters.ness index, for example, were positivelyconsidered.Details of the plant material used andthe types of analyses undertaken duringthe different steps of the MAS process aregiven in Figure 1.ResultsPhenotypic variationThe two parents were characterized by theircontrasting fibre properties (Table 1) withsignificant advantages for the G. barbadenseparent in terms of length (+9.7 mm onaverage over all data sets), strength (+15.9 g/tex) and fineness (-38 mtex). By contrast,the G. hirsutum parent displayed better yellownessindex/colour reflectance. For eachBC population, it was observed that thedata fitted normal distributions, that transgressivesegregants were regularly in thelower range of phenotypic values and that,although progeny values rarely reachedthose of G. barbadense, high phenotypicvalues were observed, including within themost advanced BC 4 generation (Table 1).Genetic mappingThe first step in the programme involvedthe construction of two genetic maps oftetraploid cotton by combining RFLP, SSRand AFLP markers generated separatelyfrom the first two backcross generations(BC 1 and BC 2 ). The initial BC 1 map comprising888 loci grouped in 37 linkagegroups and spanning 4 400 cM (Lacape etal., 2003), benefited from the developmentand integration of new additional microsatellitemarkers (Nguyen et al., 2004). Thisupdated saturated BC1 map spans 5 500 cMand comprises a total of 1 160 loci orderedalong 26 chromosomes or linkage groups(Nguyen et al., 2004). On the other hand,the BC 2 map constructed using AFLP andSSR markers had 514 loci in total. The twomaps agreed perfectly for loci order. Theyhad 373 loci in common (between sevenand 26 per chromosome throughout the 26chromosomes), thus allowing their mergerinto a combined consensus map. The consensusframework map comprises 1 306loci and spans 5 597 cM, with an averagemarker interval of 4.3 cM.QTL detectionThe QTL analyses, conducted throughcomposite interval mapping, used twomolecular data sets (BC 1 and BC 2 ) andthree sets of fibre measurements (per plant