Abstract
The many reports of phenotypic variation among plants regenerated from tissue culture suggest underlying alterations at the DNA level. This hypothesis was tested with protoplast-derived Solanum tuberosum plants. Random potato-DNA clones were used to probe the genome of individual plants at specific sites. Two out of twelve plants were shown to be variant by Southern-hybridisation with one of the tester-clones. As this clone turned out to represent 25S-rDNA, both somaclonal variants can be regarded as mutants deficient in ribosomal RNA-genes.