Loss of 5S rDNA units in the evolution of Agropyron, Pseudoroegneria, and Douglasdeweya.

We have investigated relationships among the three closely related genera Agropyron, Pseudoroegneria, and Douglasdeweya. Based upon grouping of 330 5S rDNA sequences into unit classes, we found that Douglasdeweya, with the genomic constitution PPStSt, has 2 unit classes, the long P1 and short S1, and Pseudoroegneria, with the genomic constitution StSt or StStStSt, has the long S1 and short S1 unit classes. In contrast, only the long P1 unit class was found in species of the genus Agropyron (PP). Having a single unit class is unique among all the genera of the tribe Triticeae investigated so far and may reflect gene loss or lineage sorting during its genesis. The presence of the short S1 and long P1 unit classes confirms the amphiploid origin of Douglasdeweya.