Analysis of ribosomal RNA genes in somatic hybrids between wild and cultivated Solanum species

Ribosomal RNA genes were exploited as markers to identify somatic hybrids b etween Solanum tuberosom cv. Brodick and wild diploid Solanum species, S. m egistacrolobum, S. sanctae-rosae and S. sparsipilum and DNA methylation as a possible regulatory factor in gene expression was investigated. Specific restriction enzyme/probe combinations revealed useful polymorphisms in the conserved coding and variable intergenic spacer regions of the ribosomal RN A genes. Some intermediate ribosomal RNA gene profiles indicate hybridity w hereas others were characteristic of S. tuberosum cv. Brodick. This evidenc e is suggestive of somatic exchange/re-arrangement between the NOR region o f S. sanctae-rosae and S. tuberosum cv. Brodick. Ribosomal RNA gene copy nu mber analysis of the somatic hybrids did not reveal hexaploid values sugges ting that these products are not symmetric hybrids derived from the parenta l diploid and tetraploid plants. The results indicate site-specific methyla tion of ribosomal RNA gene sequences for the parental plants; while some so matic hybrids display a reduction, others show an increase. The significanc e of the findings for somatic cell genetics and plant breeding studies is d iscussed.