DNA Complementary to Ribosomal RNA: Relation between Genomic Proportion and Ploidy.

Ten Nicotiana species were assayed for the proportion of DNA that is complementary to ribosomal RNA. This proportion varies from 0.27 to 0.9 percent, with tetraploid species having lower values than the diploid species. The tetraploid species have about twice as much DNA per cell as do diploid species. Thus, the absolute number of genes for ribosomal RNA varies less than the proportion of complementary DNA. Further, the number of genes for the RNA in 80S ribosomes varies less among species than does that for the RNA in 70S ribosomes. The data indicate that loss of DNA complementary to ribosomal RNA is associated with tetraploidy in the genus Nicotiana.