QTL Mapping Analysis Of Seed Density Per Silique And Related Traits In Spring Brassica Napus L.

Rapeseed is an important source of edible oil supply in the world.The factors affecting yield component of rapeseed include:effective silique number per plant（ESP）,thousand-seed weight（TSW）,number of seeds per silique（SPS）,etc..Seed density（SD）of rapeseed is the number of seeds per unit silique length,which is highly correlated with SPS,and is one of the important agronomic traits that have a positive impact on yield in Brassica napus L.However,the current research on this trait using molecular biotechnology is still very limited.In order to explore the favorable loci for SD traits in spring Brassica napus L.,in this study,two elite lines with significant SD differences,No.935 and No.3641 were used as parents to conduct microspores culture on F₁ hybrids,and 213 DH populations were obtained;phenotypic identification and correlation analysis of six related traits（ESP,TSW,SPS,SD,SL and YP）were conducted under four environmental condition for two year;a high-density genetic linkage map was constructed,QTL mapping of SD and related traits SL,SPS was conducted,differentially expressed genes for SD were predicted in candidate interval through RNA-Seq analysis.Overall,these results laid a foundation for fine mapping and gene cloning for SD in B.napus.The main research results are as follows:1、Phenotypic identification results indicate that:DH lines exhibited significant SD variation and high heritability,and showed continuous variation and normal distribution in different environments;The SD value showed a very significant positive correlation under different environmental conditions,it indicated that the trait exhibited stable performance and was less affected by the environment.The correlation between SD and other four yield related traits（ESP,SPS,TSW,YP）was analyzed,the results showed that:SD has significant positive correlations with YP and SPS,and negative correlation with TSW.The yield related agronomic traits（ESP,SL,SPS,SD,TSW,and SYP）of parents were evaluated,and the results showed that:The ESP and SL of No.935 were significantly higher than those of No.3641,respectively;the SPS and SD of No.935 were significantly lower than those of No.3641,respectively,while there was no significant difference in TSW and SYP between No.935 and No.3641.The above results indicated that the increase of SD could effectively compensate for the impact of low ESP on yield when there was no difference in TSW.2、A high-density genetic linkage map of spring B.napus.was constructed using213 DH lines.The map contained 2,101 recombination bins and 1,098,259 single nucleotide polymorphism（SNP）markers,distributed across 19 linkage groups,with a total length of 2115.39 c M.The genetic distance of the linkage group was42.495-188.675 c M.The average distance between adjacent markers and between adjacent recombination bins was 0.003 c M and 1.058 c M,respectively.The maximum distance between adjacent markers was 5.389 c M.The A genome has a coverage length of 1076.162 c M,including 644,991 markers and 1,161 recombination bins,while the C genome has a coverage length of 1039.228 c M,including 453,268markers and 940 recombination bins.3、The results of QTL analysis showed that a total of 28 SD-related QTLs were detected on chromosomes A02,A04,A05,A09,C02,C03,C06,and C09.Through QTL-Meta analysis,an SD consistent QTLcq SD-A9a was obtained on chromosome A09,which could explain 10.68%of phenotypic variation.The QTL analysis of SL showed that cq SL-A6a and cq SL-A6b located on chromosome A06,were detected in 4environments,explaining 9.86%and 7.88%phenotypic variation respectively;The SPS-related QTL cq SPS-A5a and cq SPS-A5b were detected at 2020HZ and 2020XN,explaining 8.26%of the phenotypic variation,and cq SPS-A5c was detected at2019HZ and 2019XN,explaining 8.52%of the phenotypic variation.The results of epistatic analysis showed that QTLs for SD and related traits detected in the DH population have epistasis interactions,but almost no interaction effects with the environment,indicating that these traits are less affected by the environment and have higher heritability.4、A candidate interval（6.04-11.21Mb）for QTL of SD traits was obtained on chromosome A09,through QTL-Seq analysis of SD traits,including a candidate interval（7.82-10.43Mb）for consistent QTLcq SD-A9a of SD traits;In addition,18closely linked SSR markers were developed within the consistent QTLcq SD-A9a candidate interval,and ultimately SDQTLcq SD-A9a was located within the 1.86 Mb（7.80-9.66 Mb）region on chromosome A09.5、A total of 13 differentially expressed genes（DEGs）were screened in the candidate region of SDQTLcq SD-A9a by RNA-seq analysis,which were differentially expressed in buds,leaves and siliques between parents and between two pools of extremely high-SD and low-SD lines in the DH population.Based on the functional annotation of Arabidopsis homologous genes,three candidate genes that may control SD were screened:Bna A09g14070D,Bna A09g14800D and Bna A09g18250D.