ChIP-cloning analysis uncovers centromere-specific retrotransposons in Brassica nigra and reveals their rapid diversification in Brassica allotetraploids
Centromeres are indispensable functional units of chromosomes. The evolutionary mechanisms underlying the rapid evolution of centromeric repeats, especially those following polyploidy, remain unknown. In this study, we isolated centromeric sequences of Brassica nigra, a model diploid progenitor (B genome) of the allopolyploid species B. juncea (AB genome) and B. carinata (BC genome) by chromatin immunoprecipitation of nucleosomes containing the centromere-specific histone CENH3. Sequence analysis detected no centromeric satellite DNAs, and most B. nigra centromeric repeats were found to originate from Tyl/copia-class retrotransposons. In cytological analyses, six of the seven analyzed repeat clusters had no FISH signals in A or C genomes of the related diploid species B. rapa and B. oleracea. Notably, five repeat clusters had FISH signals in both A and B subgenomes in the tetraploid B. juncea. In the tetraploid B. carinata, only CL23 displayed three pairs of signals in terminal or interstitial regions of the C-derived chromosome, and no evidence of colonization of CLs onto C-subgenome centromeres was found in B. carinata. This observation suggests that centromeric repeats spread and proliferated between genomes after polyploidization. CL3 and CRB are likely ancient centromeric sequences arising prior to the divergence of diploid Brassica which have detected signals across the genus. And in allotetraploids B. juncea and B. carinata, the FISH signal intensity of CL3 and CRB differed among subgenomes. We discussed possible mechanisms for centromeric repeat divergence during Brassica speciation and polyploid evolution, thus providing insights into centromeric repeat establishment and targeting.
KeywordsPolyploids Brassica Centromere Chromatin immunoprecipitation
Jin conceived the research and corrected this manuscript, Wang, He, and Zhao conducted cytogenetic experiments. Cai and Guo analyzed data. Zong, Han, and Liu provide and cultivate the plant materials. Wang wrote the article.
This study was supported partially by grants from the Youth Science Research Foundation of Beijing Academy of Agriculture and Forestry Sciences (No. QNJJ2016) and the Natural Science Foundation of China (31000538).
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Conflict of interest
The authors declare that they have no conflict of interest.
- Cheung F, Trick M, Drou N, Lim YP, Park JY, Kwon SJ, Kim JA, Scott R, Pires JC, Paterson AH, Town C, Bancroft I (2009) Comparative analysis between homoeologous genome segments of Brassica napus and its progenitor species reveals extensive sequence-level divergence. Plant Cell 21:1912–1928CrossRefGoogle Scholar
- Langdon T, Seago C, Mende M, Leggett M, Thomas H, Forster JW, Jones RN, Jenkins G (2000) Retrotransposon evolution in diverse plant genomes. Genetics 156:313–325Google Scholar
- Lim KB, de Jong H, Yang TJ, Park JY, Kwon SJ, Kim JS, Lim MH, Kim JA, Jin M, Jin YM, Kim SH, Lim YP, Bang JW, Kim HI, Park BS (2005) Characterization of rDNAs and tandem repeats in the heterochromatin of Brassica rapa. Mol Cell 19:436–444Google Scholar
- Lim KB, Yang TJ, Hwang YJ, Kim JS, Park JK, Kwon SJ, Kim JA, Choi BS, Lim MH, Jin M, Kim HI, Jong H, Bancroft I, Lim YP, Park BS (2007) Characterization of the centromere and peri-centromere retrotransposons in Brassica rapa and their distribution in related Brassica species. Plant J 49:173–183CrossRefGoogle Scholar
- Nagaki K, Song J, Stupar RM, Parokonny AS, Yuan Q, Ouyang S, Liu J, Hsiao J, Jones KM, Dawe RK, Buell CR, Jiang J (2003) Molecular and cytological analyses of large tracks of centromeric DNA reveal the structure and evolutionary dynamics of maize centromeres. Genetics 163:759–770Google Scholar
- U N (1935) Genome analysis in Brassica with special reference to the experimental formation of B. napus and peculiar mode of fertilization. Jpn J Botan 7:389–452Google Scholar