Differential behavior of Solanum brevidens ribosomal DNA loci in a somatic hybrid and its progeny with potato

Fluorescence in situ hybridization was used to characterize loci encoding ribosomal RNA (rDNA) among parents and progeny of a somatic fusion between tetraploid Solanum tuberosum (potato) and diploid Solanum brevidens. As expected, four major sites of hybridization to rDNA loci were evident in the tetraploid parent species, two in the diploid parent species, and six in the hexaploid somatic fusion plant. Two of the loci in the somatic fusion plant showed differential signals relative to the other four, which were interpreted as a delayed condensation of sites harboring the rDNA loci. This delayed condensation was heritable to the first backcross generation of the somatic hybrid crossed with potato. In the second backcross generation, differential condensation was not evident. However, a heterochromatic isochromosome was observed whose presence was correlated with a S. brevidens specific marker linked with the rDNA locus. It is suggested that the S. brevidens rDNA loci are preferentially affected in the somatic hybrid and its progeny, and that the delayed condensation may have contributed to the formation of the isochromosome.