Broadening the genetic base of Brassica juncea by introducing genomic components from B.rapa and B.nigra via digenomic allohexaploid bridging

A narrow genetic base has hindered improvement of Brassica juncea(A^(j)A^(j)B^(j)B^(j)).In this study,large-scale genomic components were introduced from diploid ancestor species into modern B.juncea using a digenomic hexaploid strategy.The hexaploids A^(j)A^(j)A^(r)A^(r)B^(j)B^(j) and A^(j)A^(j)B^(j)B^(j)B^(n)B^(n) were first developed from B.juncea×B.rapa(A^(r)A^(r))and B.juncea×B.nigra(B^(n)B^(n)),and then crossed with dozens of B.nigra and B.rapa,respectively.Both types of hexaploid showed high pollen fertility and moderate seed set throughout the S_(1) to S_(3) generations,and could be crossed with diploid progenitor species under field conditions,in particular for the combination of A^(j)A^(j)B^(j)B^(j)B^(n)B^(n)×B.rapa.Thirty A^(j)A^(r)B^(n)B^(j)-type and 31 A^(j)A^(r)B^(n)B^(j)-type B.juncea resources were generated,of which the A^(j)A^(r)B^(n)B^(j) type showed higher fertility.Of these new-type B.juncea resources,97 individual plants were genotyped with 42 simple sequence repeat markers,together with 16 current B.juncea accessions and 30 hexaploid plants.Based on 180 polymorphic loci,the new-type B.juncea resources and current B.juncea were separated clearly into distinct groups,with large genetic distance between the new-type B.juncea resources and current B.juncea.Our study provides a novel approach to introducing large-scale genomic components from diploid ancestor species into B.juncea.