Broadening the genetic base of Brassica junceaby introducing genomic components from B. rapaand B. nigravia digenomic allohexaploid bridging

A narrow genetic base has hindered improvement of Brassica juncea(AjAjBjBj). In this study, large-scale genomic components were introduced from diploid ancestor species into modern B. junceausing a digenomic hexaploid strategy. The hexaploids AjAjArArBjBjand AjAjBjBjBnBnwere first developed from B. juncea × B. rapa(ArAr) and B. juncea × B. nigra(BnBn), and then crossed with dozens of B. nigraand B. rapa, respectively. Both types of hexaploid showed high pollen fertility and moderate seed set throughout the S1to S3generations, and could be crossed with diploid progenitor species under field conditions, in particular for the combination of AjAjBjBjBnBn × B. rapa. Thirty AjArBnBj-type and 31 AjArBnBj-type B. juncearesources were generated, of which the AjArBnBjtype showed higher fertility. Of these new-type B. juncearesources, 97 individual plants were genotyped with 42 simple sequence repeat markers, together with 16 current B. junceaaccessions and 30 hexaploid plants. Based on 180 polymorphic loci, the new-type B. juncearesources and current B. junceawere separated clearly into distinct groups, with large genetic distance between the new-type B. juncearesources and current B. juncea. Our study provides a novel approach to introducing large-scale genomic components from diploid ancestor species into B. juncea.