Internal Transcribed Spacer Region of rDNA in Common Wheat and Its Genome Origins

Abstract

The objective of this study was to disclose the origin and evolution of the A, B, and D genomes in common wheat ( Triticum aestivum L.) with evidence on ribosomal DNA (rDNA) internal transcribed spacer (ITS) sequences. Specific primers were used to amplify the rDNA ITS sequences from common wheat, and 3 types of the amplification fragment were obtained. The sequence of wheat ITS region was 602 bp in length, of which ITS1 and ITS2 had 8 and 20 variation sites, respectively. The genetic distance among the ITS sequences ranged from 0 to 0.038 with the mean value of 0.021. Using MEGA, PAUP, and PHYLIP programs, the ITS sequences of common wheat were compared with the corresponding sequences of its close relatives in GenBank under the Kimura-2 model. A dendrogram was constructed with Bromus tectorum as the out-group. Common wheat had the ITS sequences highly similar to a few of its wild relatives, which indicated that the formation of common wheat genome was a relatively recent event and the concerted evolution in its genome is incomplete. After comparing the ITS regions of possible donors for each genome of wheat, T. urartu, T. speltoides, and T. tauschii are proposed as the most probable donors of the wheat genomes A, B, and D, respectively. The phenomenon of sequence elimination may result in the confusions when investigating the true donors of wheat genomes. In addition, the “concerted evolution” appears inconsonance at the level of whole types of gene or DNA fragment.