Cytological And Genetic Analysis Of The Variation Of Seed Number Per Pod In Brassica Napus

Seed number per pod displays a large variation in different cultivars, lines or other germplasm resources of Brassica napus. Studies on this trait have significant influence on the using of fine variation to boost rapeseed production. Currently, the research about seed number per pod is mainly focused on QTL mapping, and study on the forming process of seed number per pod, including the developmental process and modulating network, is rarely involved. Based on the field experiment across multiple locations and seasons, we selected some sets of lines and two varieties from a DH population and conventional cultivars, among which exists a large variation of seed number per pod. Applying paraffin section and DIC method, we systematically observed the successive process from the initiation of ovule primordia to seed maturation, and ascertained the main developmental stage, during which the variation of seed number per pod happened. Further, using gene information in Arabidopsis thaliana, a gene expression analysis of certain organs were carried out in some tested lines, and results presented preliminarily the possible pathway that control seed number per pod in Brassica napus. The experiment results are as follows:1The variation of seed number per pod between different lines was mainly generated during bud stage. Ovule primordia initiation and ovule development, the two typical processes of bud stage, both may display variation in different materials. Correlation analysis manifested that the correlation coefficient between the number of ovule initiation, the percentage of well developed ovules and seed number per pod are0.539(insignificant) and0.913(very significant) in tested materials, and the correlation coefficient between the two processes is-0.038(insignificant). Indicating that ovule development, rather than ovule initiation, is a more significant factor in the formation of seed number per pod, and the two processes are two independent processes.2As to the DH population, the first set of materials with equal growth period, pod length, yield per plant and varied seed weight and seed number per pod had identical ovule number but different proportion of normal developed ovules, while the second set of materials with equal growth period, seed weight, yield per plant and varied seed number per pod had different ovule number and proportion of normal developed ovules.3Using paraffin section, we observed the whole process of ovule development in four DH lines, named C4-146, C4-58B, C18-43C and C4-247. Results showed that besides some abnormal ovules with stagnated embryo sac in one or two-nuclear stage, most abnormal ovules has a normal functional megaspore but without transition into one-nuclear gamete, suggesting that the barrier of the initiation of female gametegenesis is the main reason that obstacle the embryo sac development, and finally the seed number per pod.4Expression analysis of homology sequence in Brassica napus of genes related to the initiation of female gametegenesis in Arabidopsis thaliana was carried out, and results showed that the expression level of a C-6oxidase gene in Brassinosteroids biosynthesis, termed BnCYP85A2, was significantly reduced in lines with low seed number per pod. Although the reduction is steady and universal in carpel, the reduction is not detected in seedling. We speculate that the mutant of a flower organ-specific transcription factor or DNA binding protein gene may be a key inducement of the reduction of BnCYP85A2, resulting to a low level of brassinosteroids, barrier of the initiation of female gametegenesis, and finally the reduction of seed number per pod.