Isolation And Evolutional Mechanism Analysis Of One Triticeae St-Genome Pericentromeric Retrotransposon

Thinopyrum ponticum (2n=10x=70) in Triticeae is one of important sources of pathogen resistant genes in wheat breeding. Though it has attaracted mang concerns of wheat breeders, but its genome components is still in dispute. In order to characterize its chromosomes, according to the sequence of CRW2-BAC5 from Triticum boeoticum, six pair of specific primers were designed and several DNA fragments were amplified by PCR with the genome DNA of diploid Pseudoroegneria as the template. By purifying PCR products, sequencing and sequence alignment we screened a 1755bp DNA fragment originated from St Centromere-specific retrotransposon, we named it pStC1 (St genome centromeric associated sequence). There is a 800bp fragment of this sequence has highly homology with the LTR region of CRW2-BAC5, a less fragment share partial homology to the gag region, and also we have founded a AGCAAC-rich tandem repeat in this fragment. However, sequence comparison revealed that the primer binding site which is necessary for the activity of transposition was not found in pStC1. Therefore, this sequence might be a segment of an inactivating retrotransposon. Chinese Spring and several Elytrigia species were analysed by Fluorescent in situ hybridization (FISH) with this sequence as probe. Significant hybridization signals on the centromere related regions were observed among all the 42 chromosomes of Chinese Spring. The 70 chromosomes of Th. ponticum were definitely divided into two groups, 28 chromosomes were strongly hybridized at their centromeric regions, and the other 42 chromosome had nearly no signals, so we deduced the genomic compositions of Th. ponticum are two St genomes and three E genomes (St1St2EeEbEx). In Th. intermedium, strongest FISH signals existed on St genome chromosomes, but some E genome chromosomes also have strong signals at their pericentromeric regions. Also FISH of the 900bp downstream sequence of pStC1 was carried out in Chinese Spring, obvious signals were obversed on B genome chromosome arms. These results indicated that during the process of specation and genome evolution in allopolyploids of Thinopyrum genus, unbalanced coordination was existed between St genome and E genome at there centromeric and pericentromeric regions, and also these unbalanced coordination maybe existed among A, B and D genomes during the evolution of hexaploid wheat.