Construction Of High Density Genetic Linkage Map Based On Bin-markers And Mapping QTL For Flowering Time In Brassica Rapa

Brassica rapa belongs to Brassicaceae, comprising several important leafy vegetables in China, including Chinese cabbage and pak choi. The happening of pre-mature flowering greatly limits both the yield and quality of Chinese cabbage. Thus, the late-flowering variety has been an important goal of Chinese cabbage. Flowering time is a complex quantitative trait controlled by multiple genes and the interaction between environmental and genetics factor. To identify the genetic locus that confers late-flowering, we utilized a DH population generated from crossing between two B. rapa DH lines with great difference of flowering to map QTL related to flowering time. A high-density linkage map was constructed based on bin-markers. Furthermore, we investigateed phenotypes under multiple environments and identified QTL for bolting time and flowering itime in B. rapa.The main results of our study are listed as follows:(1) The segregating population consisting of 172 F1 DH lines, was produced through two parents Y195(early-flowering) and Y177(late-flowering). To construct a high-density linkage map, we re-sequenced the parents and F1 DH lines at the depth of 5× and 0.5×,respectively. A total of 22,747 SNPs with high-confidence were called using the software SOAP2.0. 1,708 bin markers were derived by combining the least recombined fragments. Among them, 887 were integrated into the high-density linkage map. This linkage map has 10 linkage groups with a total length of 958.6 cM, covered ?% of B. rapa reference genome.The average interval between bin markers is 1.08 cM. The high density linkage map facilitates QTL mapping for interested traits.(2) Bolting time and flowering time for the DH population were investigated under four conditions, including long day-length(LD), short day-length(SD), vernalization before growing under LD and non-vernalization before growing under LD. A total of 12 QTLwere detected, each six for bolting time and flowering time, respectively. Amongest the detected 12 QTL, six located on the linkage group A02 and six on A08 with contribution to the phenotype variaction ranged from 8.9% to 28.7%. Most(10) of them explained more than 10% of the phenotype variation. Candidate genes were analyzed based on the annoation of the reference genome.Taken together, this study constructed a high-density linkage map which is helpful for QTL mapping and chromosome anchoring of genome sequence in B. rapa, the QTL detected under different enviroments provide clues for further study of molecular mechanism of flowering control and molecular breeding of pre-mature resistance in B. rapa.