Evolution of parental ITS regions of nuclear rDNA in allopolyploid Aegilops (Poaceae) species

The genus Aegilops comprises approximately 25 diploid. tetraploid and hexap loid species. in which the genome types of all allopolyploids involve eithe r U or D genome, or both of them. The internal transcribed spacer (ITS) reg ion of 18S-265 nuclear ribosomal DNA (rDNA) from 11 allopolyploid species a nd 7 related diploid species in the genus were directly sequenced by pooled PCR products. Phylogenetic analyses for tracing evolutionary patterns of p arental rDNA in allopolyploid species were performed using the neighbor-joi ning method. The D genome involved tree included three clades (CC-DDCC. DDM M-DDMMSS-DDMMUU, and MM-(MMh)-M-h-DDNN). but did not include Ar. squarrosu (DD). It indicated that the rDNA of ancestral D genome had been somewhat di fferentiated in allopolyploids. The U genome involved tree showed that the allopolyploids and their common ancestor. Ae. umbellulatu, formed a clade. suggesting that rDNA in UUMM and UUSS genomes has been homogenizing toward that of ancestral U genome. The phylogenetic pattern of U genome based on I TS sequences also supported the "pivotal-differential" hypothesis.