Nuclear DNA content and chromosome number in somatic hybrid allopolyploids of Solanum

An attempt was made to change the proportion of the parental genomes in interspecific hybrids Solanum nigrum + S. tuberosum (ngr + tbr) by means of repeated protoplast fusion. In order to enlarge the potato input into the hybrid genome, the protoplasts of two ngr + tbr hybrids of different ploidy (7x and 8x) were fused with the protoplasts of two different diploid potato clones in three combinations. Protoclonal variability was studied in three populations of new ngr + tbr allopolyploids maintained in vitro. The absolute nuclear DNA content (2C) was measured using flow cytometry to estimate the ploidy of the hybrids. The ploidy level of the selected clones was verified by chromosome counts in root meristems. The newly synthesized allopolyploids (75 clones) showed only a small gain in nuclear DNA content above the mean value determined for the parents, instead of the expected addition of an entire diploid potato genome to the combined parental ngr + tbr genome. An increase in nuclear DNA was observed mostly in the clones having the 7x hybrid as a parent (75% of allopolyploids from two combinations). When the 8x hybrid was used as a parent, only two allopolyploids (5%) exhibited a significantly increased nuclear DNA content. The 8x level of ngr + tbr allopolyploids was shown to be stable and was only occasionally exceeded. Somatic hybrids ngr + tbr offer a model system for studying the molecular mechanism(s) and processes involved in stabilization and establishment of the synthetic Solanum allopolyploids.