Variation in organisation and copy number of ribosomal RNA genes in Petunia hybrida somaclones

The copy number of genes encoding 5S ribosomal RNA has been found to be constant in Petunia hybrida plants regenerated from protoplast and leaf disc-derived callus cultures. However, in one somaclone a heritable change in the length of the major 5S rDNA repeat has arisen. Despite the constant copy number of 5S rRNA genes, that of the 18S–25S rRNA genes is found to very by at least ten-fold. The relevance of these findings to ribosomal RNA gene variability and to somaclonal variation is discussed.