Abstract
The genus Aegilops comprises approximately 25 diploid, tetraploid and hexaploid species, in which the genome types of all allopolyploids involve either U or D genome, or both of them. The internal transcribed spacer (ITS) region of 18S-26S nuclear ribosomal DNA (rDNA) from 11 allopolyploid species and 7 related diploid species in the genus were directly sequenced by pooled PCR products. Phylogenetic analyses for tracing evolutionary patterns of parental rDNA in allopolyploid species were performed using the neighbor-joining method. The D genome involved tree included three clades (CC-DDCC, DDMM-DDMMSS-DDMMUU, and MM-MhMh-DDNN), but did not include Ae. squarrosa (DD). It indicated that the rDNA of ancestral D genome had been somewhat differentiated in allopolyploids. The U genome involved tree showed that the allopolyploids and their common ancestor, Ae. umbellulata, 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 ITS sequences also supported the "pivotal-differential" hypothesis.
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