Studies On The Genomic Evolution Of Allopolyploids In Brassica

Polyploidy has played an important role in the evolution of higher plants. In order to understand how polyploid genomes have evolved after formation, we used the model system, the Brassica triangle, to study genomic evolution after the formation of polyploids.The Brassica triangle is a useful system for addressing questions of genomic evolution in polyploids. It is composed of three diploid species, Brassica nigra, B. rapa, and B. oleracea, and three allotetraploid (amphidiploid) species, B. carinata, B. juncea, and B. napus, which are produced by the crosses among the three diploid species and the following process of chromosome doubling.Random amplified polymorphic DNA (RAPD) analysis was performed on the diploids and allopolyploids in Brassica triangle with 38 random 10-base primers. A total of 273 bands were detected, among which 264 bands were polymorphic. Based on those RAPD data, genetic distances among the samples were calculated and two dendrogram was constructed by using UPGMA' method. The results revealed that B. rapa (AA) is more closely related to B. oleracea (CC) than to B. nigra (BB). B. napus (AACC) grouped with its diploid progenitors, B. rapa (AA) and B. oleracea (CC), in one cluster. The allopolyploids carrying B genome grouped with B. nigra (BB) in other cluster. The results suggested that both genome A and C were not modified very much after the formation of B. napus (AACC) . Genome B was modified less thangenome A or C after B and A, or B and C merged into a common nucleus, respectively.Based on the inter-simple sequence repeat (ISSR) analysis, the different degree of A, B or C genomic modifications were observed in the three Brassica allopolyploids. In B-contained allopolyploids, B genome always altered less than the other genome (A or C) , showing that B genome was relatively conserved in the evolution of Brassica allopolyploids. ISSR data supported that a higher degree of ancestral genomic divergence gave rise to a greater frequency of genomic change of polyploids. The possible mechanisms for the genomic changes and the reason for the relatively conserved B genome were discussed.65 Au elements (a new kind of SINEs) were amplified and sequenced from 6 species of the Brassica triangle. Like the Au elements found in other plants, all sequences possessed a tRNA-related region, which included two internal promoters of RNA polymerase III (box A and B) .There were no completely same sequences between polyploids and its ancestral diploids. This indicated the Au elements had been modified, or in other words, Au elements were active during the formation of Brassica polyploids. This was supported by the further analysis of the genetic distance among Au sequences and the consensus sequence, which was made from the alignment of 65 elements. Additionally, the Au sequences in Brassica showed no species-specific clusters on the dendrogram. These results indicated the Au elements were amplified in a common ancestor of these species but not independently in these species. Thus, the amplification of Au elements would seem to fit the transposon model more closely than the master gene model. This study suggested that replicative transposition should be an important mechanism in genomic evolution of polyploids. When Au elements were compared with SI elements, another kind of SINEs found in Brassica, they showed a different manner of amplification.The sequences of the internal transcribed spacer region (ITS) of the 18S-26S nuclear ribosomal DNA were sequenced in three Brassica allopolyploids and three diploids. The results showed that Brassica allopolyploids to a greet extent retained the ancestral ITS sequences. That meant every Brassica allopolyploid contained twotypes of 18S-26S nuclear rDNA. Although some sequences of ITS1 and ITS2 of 18S-26S nuclear rDNA in Brassica allopolyploids were completely as same as those in their ancestral diploids, some sequences of them had been modified a litter in Brassica allopolyploids. This result indicated the degree of concerted evolution in Brassica allopolyploids was low and ITS1 and ITS2 in Brassica allopolyploids were not unaffected by polyploidization.