Submitted to: Chromosoma
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/2/2008
Publication Date: 6/3/2008
Citation: Liu, Z., Yue, W., Li, D., Wang, R., Kong, X.Y., Lu, K., Wang, G.X., Dong, Y., Jin, W.W., Zhang, X. 2008. Structure and Dynamics of Retrotransposons at Wheat Centromeres and Pericentromeres. Chromosoma DOI 10.1007/s00412-008-0161-9, 2008 Interpretive Summary: All living organisms are controlled by their own heritable genes located on chromosomes in the nucleus of cells. Chromosomes are composed of active and inactive genes, unique sequences, and repetitive sequences. These DNA sequences are capable of undergoing changes, through various mechanisms, which are the basis of genome and species evolution. On the other hand, chromosomes possess characteristics to ensure orderly behaviors to maintain genome stability from generation to generation. The genetic basis of these chromosomal characteristics and the roles of different chromosomal structures are not fully understood. We isolated a large stretch of DNA sequence from the centromeric region of chromosomes in one of the three diploid progenitors of bread wheat. It was determined that this part of chromosomes contains several kinds of retrotransposons - sequences capable of jumping from one place to another on the chromosomes. We showed that certain DNA sequences have been stably passed from diploid onto tetraploid and then onto hexaploid wheats, whereas others have undergone some changes during the formation of durum and bread wheats. Thus, we demonstrated the pivotal roles of retrotransposons or dispersed repetitive sequences in the centromeric and pericentromeric regions of chromosomes in genome evolution.
Technical Abstract: A centromere-associated bacterial artificial chromosome (BAC) clone TbBAC5, containing a 90 kb insert of Triticum boeoticum Boiss, genomic DNA, was fully sequenced. The Erika, Sukkula and Wgel-2 retrotransposons identified in TbBAC5 were A-genome rich in annual Triticeae. Southern analysis revealed that large amounts of retrotransposons had invaded into the centromeric and pericentromeric regions during the divergence of diploid wheats (T. boeoticum, T. monoccocum L. and T. urartu Thumanian) from Aegilops speltoides Tausch and Ae. tauschii Cosson. They were conserved during allopolyploidization. Two intact, one autonomous and one non-autonomous, centromeric retrotransposons of wheat (CRWs) were identified. Both of them resulted in FISH signals that were tightly associated with wheat centromeres and stronger in intensity on A-genome chromosomes than on B- or D-genome chromosomes in hexaploid wheat. Many known centromeric repeats in wheat, except the 570 bp TaiI sequence, were mapped in this autonomous CRW. The CRWs were highly diverged during wheat speciation. Therefore, the conserved and dynamic characteristics of retrotransposons in centromeres and pericentromeres suggest they were actively involed in wheat genome evolution and polyploidization. The shared CRWs may play a role to unify the sub-genomes in polyploid wheat, but the genome specific/rich retrotransposons may account for the relative independent activity of the sub-genomes.