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 between Solanum tuberosum cv. Brodick and wild diploid Solanum species, S. megistacrolobum, 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 RNA genes. Some intermediate ribosomal RNA gene profiles indicate hybridity whereas others were characteristic of S. tuberosum cv. Brodick. This evidence is suggestive of somatic exchange/re-arrangement between the NOR region of S. sanctae-rosae and S. tuberosum cv. Brodick. Ribosomal RNA gene copy number analysis of the somatic hybrids did not reveal hexaploid values suggesting that these products are not symmetric hybrids derived from the parental diploid and tetraploid plants. The results indicate site-specific methylation of ribosomal RNA gene sequences for the parental plants; while some somatic hybrids display a reduction, others show an increase. The significance of the findings for somatic cell genetics and plant breeding studies is discussed.