Genetic Improvement Of The Clubroot Resistance Of The Core Parents Y522R Et Al Of Brassica Napus L.

Clubroot is one of the most serious diseases of cruciferous crops,and also one of the main diseases of rapeseed at present in China.More than 6.7 M ha of rapeseed are grown annually in China,but about 0.677 M ha of rapeseed suffers from clubroot since the disease had spread,resulting serious yield loss.Breeding resistant varieties is one of the most effective ways to deal with the occurrence of clubroot.For developing new rapeseed varieties resistant to clubroot,two resistance loci,PbBa8.1 and CRb,were transferred into three elite parents,Y522R,6105 and 3928,using a strategy combining successive backcross and marker-assisted selection（MAS）.The main results were as follows:1.Molecular markers A08-300 and A3-50,which are closely linked to PbBa8.1 and CRb loci respectively,were used to transfer PbBa8.1 and CRb loci from“Huashuang 5R”and“Huayouza 62R”to Y522R,6105 and 3928 by the strategy of backcross breeding（backcross 3 or 4 times）combined with marker assisted selection,respectively.Then,the selected resistant plants in the BC₃ or BC₄ populations with Y522R,6105 or 3928backgrounds were crossed with each other to obtain NIL-F₁ populations with Y522R,6105 or 3928 background,respectively.2.Foreground selection was conducted in NIL-F₁progenies by using the markers A3-50 and A08-300.Under the background of Y522R,6105 or 3928,5,3 and 1heterozygous individuals with the genotype（PbBa8.1pbba8.1CRbcrb）were selected out,respectively.Then,foreground selection was conducted in NIL-F₂ progenies.Under the background of Y522R,6,5 and 5 individuals homozygous for the resistant allele of PbBa8.1（PbBa8.1PbBa8.1crbcrb）,CRb（pbba8.1pbba8.1CRb CRb）and the both loci（PbBa8.1PbBa8.1CRb CRb）were obtained,respectively.Under the background of 6105,6,4 and 5 individuals homozygous for the resistant allele of PbBa8.1,CRb and both loci were obtained,respectively.Under the background of 3928,1 and 1 individuals homozygous for the resistant allele of CRb and the both loci were obtained,respectively.3.The selected homozygous individuals from the NIL-F₂ population with Y522R background and the NIL-F_2:3 lines with 6105 or 3928 background were sequenced,and the background recovery rate of these individuals was calculated.As a result,under the background of Y522R,the background recovery rate of individuals homozygous for the resistant loci of PbBa8.1,CRb and the both loci was 85.24%-93.43%,83.11%-91.22%and 80.36%-96.70%,respectively;under the background of 6105,the background recovery rate of individuals homozygous for the resistant loci of PbBa8.1,CRb and the both loci was 95.40%,97.57%-97.88%,94.09%-95.16%,respectively;under the background of 3928,the background recovery rate of individuals homozygous for the resistant loci of CRb and the both loci was 95.51%-96.21%and 95.47%-96.06%,respectively.4.The resistance level of the improved NIL-F_2:3 lines of Y522R,6105 and 3928 was identified in greenhouse.The results indicated that the individuals homozygous for the resistant allele of CRb and the both loci showed high resistance to race 4,and no clubroot symptom was detected.Under the background of Y522R,the disease index of the individuals homozygous for the resistant allele of PbBa8.1 was 21.2.Under the background of 6105,the individuals homozygous for the resistant allele of PbBa8.1 also showed a certain degree of resistance,and the disease index was 25.9.By contrast,Y522R,6105 and 3928 were highly susceptible to race 4,the disease index of Y522R,6105 and 3928 was 89.9,72.0 and 97.4,respectively.5.The clubroot resistance level between the hybrids derived from the improved line of Y522R and RG430A（a nuclear male sterile line）and“Shengguang168”（a hybrid derived from Y522R and RG430A）was evaluated in greenhouse.The results showed that the hybrids with CRb single resistance loci and the double resistance loci showed 100%resistance to race 4,and the disease index was 0;the disease index of the hybrid with PbBa8.1 single resistance loci was 53.9;while the disease index of the control hybrid“Shengguang 168”was as high as 99.4.6.The agronomic traits of the improved lines and their recurrent parents Y522R,6105 and 3928 were investigated under the normal field condition.There was no significant difference between the improved lines and their recurrent parents.The agronomic traits of the improved hybrids of Y522R and“Shengguang168”were investigated under the normal field condition.Also,there was no significant difference between the improved hybrids and“Shengguang168”.