Derived alloploidy: an unexplored avenue for augmenting genetic variation in Brassica digenomics

摘要

From empirical plant breeding perspective, alloploidy is a genetic bottle neck. This is very much evident from generally a poor response to selection for yield in alloploid Brassica species. Resynthesis of Brassica digenomics or homoeologous introgression from related digenomics has not helped greatly to enlarge the base of working germplasm in the past due to associated genetic and / or phenotypic instabilities besides poor breeding value. This is partly explained by the fact that two of the three Brassica monogenomic progenitor species (B. nigra and B. oleracea), used for resynthesis of Brassica alloploids, have not faced any significant human intervention for evolution as oilseed crop. Derived alloploidy, i.e. isolation of an amphiploid through hybridization of non parental amphiploids, is proposed as a way forward. The process involves development of F1 hybrid between two digenomics {e.g.B. juncea×B. napus → F1 (AABC)}, chromosome doubling of the F1 hybrid (AABC → AAAABBCC),selfing and selection for target non parental digenomic (BBCC) in successive generations. There is a rapid chromosome loss with preferential e limination of tetrasomic genome (AAAA). Raising of a large self population following colchiploidy and use of genome specific molecular markers may restrict the process to 1-2 generations of selfing. Derived amphiploids are expected to be superior to resynthesized ones as these involve intensely bred parents. Further, it is a one step procedure to channelize breeding gains of two digenomics into a third. Using this strategy we could develop '0' B. carinata following hybridization between elite '0' / '00' B. juncea and B. napus types. Similarly, B. juncea was extracted by hybridizing B. napus and B. carinata. Besides the breeding significance, derived alloploids will provide useful insight into evolutionary processes as these bring together genomes that have undergone varied genetic / epigenetic changes following their past tryst with natural alloploidy, cohabiting with different genomic partner(s). Molecular diversity, cytogenetic stability and breeding value of the derived B. carinata and B. juncea will be discussed.