Linkage Map Construction And QTL Analyses For Important Agronomic Traits In Chinese Cabbage

Chinese cabbage (Brassica campestris L. ssp. pekinensis) is one of the most important vegetable crops in China. Genetic improvement of Chinese cabbage has been achieved mainly by conventional plant breeding methods. But recent advances in marker-assisted breeding have opened new avenues for crop improvement. Identification of QTLs for a particular trait can contribute to crop improvement strategies through marker-assisted selection (MAS), especially where the traits are of high value In the present study, a molecular genetic map for Chinese cabbage was constructed based on AFLP, RAPD, SSR and Isozyme markers. Marker analysis was performed on 100 randomly chosen DH lines obtained by microspore culture from the F₁ between two homozygous parents: 91-112 and T12-19 and QTL analyses for 24 agronomic traits were also carried out.Isozymes including AAT, MDH, ME, PGM, LDH, FDH and GDH were investigated and 14 polymorphic bands were obtained between two parents. Among these markers, 10 markers are observed to be codominant, occupying 71.4%. Other markers including 263 AFLP markers, 150 RAPDs, 17 SSRs, 1 SCAR and 1 morphological maker obtained by the former researchers in our Lab. were used for map construction with software JoinMap 3.0.The linkage map constructed in this study contained 406 markers including 246 AFLP markers, 135 RAPD markers, 11 SSR markers 12 Isozyme marker 1 SCAR and 1 morphological maker and integrated into 10 groups. A total of 45.0% distorted markers distributed in the map. Among 406 markers used in map construction, 222(54.7%) markers derived from parent 91-112, and 184(45.3%) from another parent T12-19. The map covered 826.3 cM with a mean marker interval of 2.0 cM. Number of markers included in linkage groups varied from 7 to 111, mean marker interval distance from 1.0 cM to 3.8 cM and the length of linkage groups from 26.4 cM to 156.1 cM individually. The molecular genetic map constructed would be useful in QTL mapping of important agronomic traits for Chinese cabbage.QTLs controlling 24 agronomic traits were mapped on 8 linkage groups with software MapQTL5.225 QTLs controlling 24 agronomic traits investigated in 2003 were mapped on 8 linkage groups. These QTLs included 10 for plant expansion, 23 for head height, 1 for head diameter, 12 for number of wrapper leaf, 11 for number of non-wrapper leaves, 5 for length of internal stem, 11 for length of midrib, 5 for thickness of midrib, 10 for width of midrib, 10 for leaf length, 8 for leaf width, 15 for plant height, 10 for head weight, 22 for percentage of wrapper leaf blade weight, 14 for plant attitude, 12 for leaf color, 8 for serration of leaf margin, 3 for crimple of leaf, 7 for undulation of leaf margin, 6 for color of midrib, 4 for head internal color, 5 for shape of internal stem, 7 for leaf shape and 6 for shape of midrib. Unequal gene effects were observed on the expression of these agronomic traits, and the variation explained ranged from 8.90%-88.90%. 28 major QTLs were identified.233 QTLs controlling 22 agronomic traits investigated in 2004 were mapped on 8 linkage groups. These QTLs included 16 for plant expansion, 19 for head height, 11 for head diameter, 19 for number of wrapper leaf, 19 for number of non-wrapper leaves, 9 for length of internal stem, 7 for length of midrib, 7 for thickness of midrib, 14 for width of midrib, 3 for leaf length, 6 for leaf width, 14 for plant height, 19 for head weight, 24 for percentage of wrapper leaf blade weight, 3 for leaf color, 3 for serration of leaf margin, 8 for crimple of leaf, 5 for color of midrib, 4 for head internal color, 5 for shape of internal stem, 6 for leaf shape and 12 for shape of midrib. The variation explained ranged from 9.40%-96.20%.40 major QTLs were identified in 2004.5 QTLs controlling plant expansion, 11 QTLs controlling head height, 6 QTLs controlling number of wrapper leaf, 3 QTLs controlling number of non-wrapper leaves, 3 QTLs controlling length of internal stem, 1 QTLs controlling length of midrib, 3 QTLs controlling thickness of midrib, 1 QTLs controlling width of midr...