3.1. Phenotypic variation and trait correlation analysis
Five fiber quality traits, including FL, FS, FU, MC, and FE, were investigated. Significant differences were found between the parents, especially FL, FS, and MC (
Table 1). Jifeng 173 showed longer FL, stronger FS, and smaller MC in all four years. Both the maximum and minimum values in the offspring populations exceeded the parents, indicating the transgressive segregation of the five observed traits, and minor effect alleles controlling these traits existed in both parents (Additional file 1). The absolute values of skew and kurt in the tested populations were smaller than one except for FE in 2022, which indicates that most of the observed traits presented nearly normal distributions. MC has the largest coefficient variations (CVs) over four years, followed by FS, FL, FU, and FE. Thus, this population is appropriate for fiber quality-related QTLs mapping.
Complex correlations were observed among FL, FS, FU, MC, and FE over four years (Additional file 2). Significant positive correlations were observed within the trait pairs, such as FL-FS, FL-FE, FS-FE, MC-FU, and FE-FU. Significant negative correlations were observed between FL and MC, between FS and MC, between FE and MC. No consistent correlation was observed between FL and FU, between FS and FU. The complex phenotypic correlations reflected the complex genetic interactions underlying these fiber quality traits.
3.2. QTL mapping for fiber quality traits
In this study, a total of 108 QTLs were found, including 27 for FL, 16 for FS, 24 for MC, 17 for FU, and 24 for FE (Additional file 3). Chromosome A7 contained the highest number of QTLs with 12. No QTL was found on chromosomes D1 and D11. Each QTL contributed 1.98%–21.45% to the PV, 13 QTLs contributed more than 10% to the PV and 9 QTLs could be mapped repeatedly (
Figure 1). Jifeng 914 conferred favorable alleles for 49 QTLs, and Jifeng 173 conferred another 59 favorable alleles.
Fiber length. Twenty-seven QTLs were mapped on 18 chromosomes, including three major effect QTLs and four stable QTLs. The favorable alleles for 13 and 14 QTLs were conferred by Jifeng 914 and Jifeng 173, respectively. The most outstanding QTL was qFL-D3-1 conferred by Jifeng 914, which was repeatedly mapped in the F2, F2:4, and F2:5 populations and contributed 9.18%–21.45% to the PV. qFL-A12-3 was mapped in the F2:3 and F2:4 populations and contributed 10.18%–13.80% to the PV, and was conferred by Jifeng 173. In addition, qFL-A6-1, qFL-A12-1, and qFL-D9-1 were mapped in two populations with favorable alleles conferred by Jifeng 914, qFL-A11-1 contributed 10.00% to the PV with a favorable allele conferred by Jifeng 173.
Fiber strength. Sixteen QTLs were mapped on ten chromosomes, including only two stable and major effect QTLs. Jifeng 914 and Jifeng 173 conferred favorable alleles for nine and seven QTLs, respectively. qFS-A11-1 was mapped in the F2:3, F2:4 and F2:5 populations and contributed 6.05%–10.41% to the PV, and was conferred by Jifeng 173. qFS-D10-2 was mapped in the F2 and F2:4 populations and contributed 10.54%–17.51% to the PV, and was conferred by Jifeng 914.
Micronaire. Twenty-four QTLs were mapped on seventeen chromosomes, and five major effect and two stable QTLs were found. Jifeng 914 and Jifeng 173 conferred favorable alleles for nine and fifteen QTLs, respectively. qMC-D6-2 was mapped in the F2 and F2:3 populations and contributed 10.15%–10.91% to the PV, and was conferred by Jifeng 173. qMC-D8-1 was mapped in the F2:3 and F2:4 populations and contributed 6.39%–10.97% to the PV, and was conferred by Jifeng 914. In addition, qMC-D6-1, qMC-D7-1, and qMC-D10-1 contributed 10.81%, 13.16%, and 12.90% to the PV, respectively.
Fiber uniformity. Seventeen QTLs were mapped on 13 chromosomes. Jifeng 914 and Jifeng 173 conferred favorable alleles for eight and nine QTLs, respectively. While only two major QTLs were found, including qFU-D7-1 and qFU-D10-1, which contributed 14.77% and 10.19% to the PV, respectively. No stable QTL was found over four years, indicating a significant environmental effect on FU.
Fiber elongation. Twenty-four QTLs were mapped on 13 chromosomes. Jifeng 914 and Jifeng 173 conferred favorable alleles for 10 and 14 QTL, respectively. In addition, six QTLs were mapped on chromosome D5 and Jifeng 173 conferred favorable alleles, which might reflect the importance of D5 from Jifeng 173 in regulating FE. Only one major effect QTL was found. qFE-D6-1 contributed 10.72% to the PV and was conferred by Jifeng 914. No stable QTL was found over the four years, which indicated a significant environmental effect on FE.
QTL-overlapping regions. A total of 14 QTL-overlapping regions were found on 12 chromosomes (
Figure 1). The additive effect directions of the QTL in 10 out of these 14 regions are different, which might explain the complex correlations among the five phenotypic traits. Four regions on chromosome A11, D3, D6, and D10 covered by both stable and major effect QTLs are promising for further fine mapping. The 0-4.5 cM region on A11 was covered by 3 QTLs (
qMC-A11-1,
qFL-A11-1 and
qFS-A11-1).
qFL-A11-1 is a major effect QTL, and
qFS-A11-1 is a stable QTL with 10.41% contribution ratio to the PV. The 49.5-57.5 cM region on D3 was covered by 2 QTLs (
qFL-D3-2 and
qFU-D3-1).
qFL-D3-2 is a stable QTL with 9.18-21.45% of contribution ratios to the PV. The 165.5-170 cM region on D6 was covered by 2 major QTLs (
qFE-D6-1 and
qMC-D6-2) and
qMC-D6-2 could be repeatedly mapped. The 99.5-100.5 cM on D10 was covered by 4 QTLs (
qFL-D10-2,
qFS-D10-2,
qFU-D10-1 and
qMC-D10-1), of which, 3 are major effect QTLs and 1 is stable QTL.
3.3. Candidate gene analysis
There are six major effect QTLs that could be repeatedly mapped in at least two populations, including qFL-D3-2 and qFS-A11-1 mapped in three populations, qFL-A12-3, qFS-D10-2, qMC-D6-2, and qMC-D8-1 mapped in two populations. There are no genes in qFS-D10-2 (16596338 bp-16630630 bp), qMC-D6-2 (1054134 bp-1066524 bp), and qMC-D8-1 (9129475 bp-9137306 bp), while 810 genes were annotated in qFS-A11-1, qFL-A12-3, and qFL-D3-2 including 6 genes in qFS-A11-1 (119649722 bp-119686364 bp), 6 genes in qFL-A12-3 (10414004 bp-10555066 bp), and 799 genes in qFL-D3-2 (6090201 bp-41836768 bp) (Additional file 4). Upon using GO and KEGG analysis, 392 GO terms and 62 KEGG pathways were mapped under the corrected p value < 0.05 (Additional file 5). The most significant GO term is protein binding, and the most significant KEGG pathway is metabolic pathways. Among the 265 genes in the mapped protein binding term and metabolic pathways, 15 genes located in the qFL-D3-2 region are highly expressed at different fiber development stages ( Additional file 6). Thirteen genes are highly expressed during the fiber elongation stage (5-25DPA fiber), which determines FL. Five genes Ghir_D03G010890, Ghir_D03G010910, Ghir_D03G006470, Ghir_D03G010430, and Ghir_D03G010470, highly expressed during the lint fiber initiation stage (−3-3DPA ovule). As FU-related QTL qFU-D3-1 overlapped with qFL-D3-2, these five genes may affect FL and FU by regulating fiber initiation. Based on the present sequencing depth, a total of 116 SNPs and 40 InDels markers were found in the qFL-D3-2 region, and none of these markers located in protein coding regions of the above mentioned genes (Additional file 7). And based on the 8 mapped SNP markers in the qFL-D3-2 region, it is difficult to narrow down the interval length to fine map qFL-D3-2 (Additional file 8).
Ovules from the fiber initiation stage and fibers from the fiber elongation stage were used for qRT-PCR. The expression levels of the above mentioned 15 genes in the ovules were higher in Jifeng 914 than in Jifeng 173 (
Figure 2). Nine genes showed continuously increased expression trend from -1DPA to 1DPA ovules, and 2 genes (
Ghir_D03G005440.1 and
Ghir_D03G011310.1) showed significantly high expression levels in 1DPA ovules. There are 2 genes (
Ghir_D03G005100.1 and
Ghir_D03G010910.1) showed continuously decreased expression trend from -1DPA to 1DPA ovules. Gene expression patterns in fibers were more complex and most of the gene expression patterns between JF4 and JF4x were different (
Figure 3). Two genes (
Ghir_D03G006470.1 and
Ghir_D03G007410.1) showed continuously increased expression trend from 5DPA to 15DPA fiber. The expression level of
Ghir_D03G006470.1 in 15DPA fiber is fifty times higher than that in 5DPA fiber, and this gene had higher expression level in 5DPA and 10DPA fiber in JF4 and higher expression level in 15DPA fiber in JF4x. It is predicted that
Ghir_D03G006470.1 has a more prominent role in promoting fiber elongation.