Fine mapping of clustered quantitative trait loci for fiber quality on chromosome 7 using a Gossypium barbadense introgressed line

By backcrossing and self-pollinating twice, a series of improved lines (BC2F3) of cotton derived from super-quality fiber property chromosome segment introgression lines were developed by molecular marker-assisted selection (MAS). One of these improved lines, 3326-7, in the background of Xinluzao (XLZ) 41, was transferred from an introgression line (IL088-A7-3) derived from a cross between Gossypium hirsutum acc. TM-1 and G. barbadense cv. Hai7124. This line has consistently demonstrated super-quality fiber properties including fiber length, strength, and fineness. To fine-map the clustered fiber quality quantitative trait loci (QTLs), we further crossed the homozygous line 3326-7 with its recurrent parent XLZ41 to produce BC3F2, BC3F3, and BC3F4 populations. A linkage group was constructed using 1248 BC3F2 plants, and QTL analysis using the fiber qualities of 229 recombinants confirmed the existence of qFL-chr.7 for fiber length, qFS-chr.7 for fiber strength, and qFM-chr.7 for micronaire value within the introgressed region, with R ² values of 18.6, 29.4, and 26.3 %, respectively. The presence of qFL-chr.7, qFS-chr.7, and qFM-chr.7 led to an increase in fiber length and strength of approximately 2.0 mm and 3.0 cN/tex, respectively, and a decrease in micronaire of 0.8. Using substitution mapping with 229 BC3F2 recombinants and 207 BC3F3 and BC3F4 lines with informative recombination breakpoints in the target region, we anchored qFL-chr.7 for fiber length and qFS-chr.7 for strength to the same position, with a 0.36-cM interval between the two simple sequence repeat markers, NAU3735 and NAU845. Another QTL, qFM-chr.7, was mapped to the 0.44-cM interval between MGHES75 and NAU7445. These tightly linked QTLs may explain their positive relationship and have the potential to improve cotton fiber qualities through MAS.