Relative rates of divergence of spacer and gene sequences within the rDNA region of species in the Triticeae: Implications for the maintenance of homogeneity of a repeated gene family.

The relative rates of divergence of 11 regions of the wheat rDNA cloned in pTA250 were estimated by measuring sequence change in 6 Triticum species. The Tm analysis of (32)P probes synthesized from the pTA250 regions and hybridized to DNA from the Triticum species provided an estimate of sequence change relative to T. aestivum. The results revealed a region of 1.2 kb preceding the 18S rRNA gene which was more conserved than the rest of the spacer. In addition the transcribed spacer between the 18S and 26S rRNA genes was shown to be poorly-conserved; the genes, as expected, were highly conserved. A model which proposes RNA as a co-factor in gene conversion is suggested to account for the observations.