Path Analysis, Genetic Analysis And QTL Mapping Of The Number Of Pirmary Branches In Brassica Napus L.

Plant branches not only have an effect on morphogenesis but also are regarded as an important traitto reflect productivity per plant in rapeseed (Brassica napus L.). The number of primary branches forMB01, a line selected by our laboratory can reach up to20while that of commercial varieties are8to9,indicating there is still great potential to improve the number of branches. In this study, we analysed therelationships between agronomic characters and yield per plant under different planting density toclarify the effect of branches on yield. We performed genetic analysis of major gene plus polygene forthe number of primary branches by constructing population of six generations derived from MB01and0830112. At the same time, we constructed F2:3population to map QTLs associated with the number ofprimary branches. The results are as following:1. The relation and path analysis revealed that the number of branches had a big indirect effect onyield through the number of effective siliques per plant (0.208), under the planting density of12000-18000/hm2, suggestting of the impotance of selection of effective branches with more siliques forthe varieties in this planting density. Under the density of39000/hm2, the effect of primary brancheswas decreased dramatically, which showed no significant relation with yield. However, it had a bigindirect effect through the density of siliques and branch height, indicating ideal plant selected for thisplanting density should have high-density siliques on main inflorescence and upper branches with lesslower branches, which can increase production through population effect.2. Genetic analysis of number of primary branches was performed using a joint analysis ofmuti-generations derived by MB01and0830112. We found that the the number of primary branchesdominated by two pairs of major genes and polygene with additive, dominant and epistatic effect. Theadditive effect of both genes leaded to the increased branches while the dominant effect was negativeand the degree of dominance was less than one, indicating additive effect is more important for themajor genes. In addition, a high effect by dominant and dominant interaction was deteced, which cancover the negative effects of heterozygous genes. Heritability of major genes and polygene in F2population were48.16%and23.13%, respectively, suggestting the heritability of major gene wasdominating and the over-high-parent individuals could be found in an enlarged F2population. However,considering of an impact of environment (28.71%), we should reduce selection criteria properly in F2generation and select in middle generation with a higher heritability.3. A middle-density genetic linkage map of rapeseed with19groups and142marker loci wasconstructed by SSR and SRAP, which covered1199.5cM, accounting for49.98%of the whole genomeof rapeseed and the average interval was8.45cM. QTLs associated with the number of primarybranches were detected by composite interval mapping method using the phenotype data of F2:3population. Nine QTLs were identified, of which two QTLs (qPB7and qPB9) with a high variationwere mapped on linkage groups of LG14and LG15, explaining22.27%and17.53%of the totalphenotypic variation, respectively.