Genome differentiation by GISH in interspecific and intergeneric hybrids of tomato and related nightshades.

We employed genomic in situ hybridization to analyze the chromosomal constitution and pairing of interspecific and intergeneric hybrids involving cultivated tomato (Lycopersicon esculentum) and two related wild nightshade species, Solanum lycopersicoides and S. sitiens. Using standard stringency conditions, the tomato genome was readily distinguished from that of the two nightshades, whereas the latter were only distinguishable under increased stringency. These observations indicate a more distant phylogenetic relationship between L. esculentum and the Solanum group, and suggest S. lycopersicoides and S. sitiens share a high degree of sequence homology. Chromosomal associations during meiosis of interspecific and intergeneric hybrids were consistent with these relationships: chromosomes of F1 L. esculentum x S. lycopersicoides and F1 L. esculentum x S. sitiens hybrids frequently formed univalents during diakinesis. In contrast, F1 S. lycopersicoides x S. sitiens hybrids showed complete bivalent formation. L. esculentum x S. sitiens hybrids, including the F1 plants, a monosomic addition, and an allotetraploid, showed lower frequencies of pairing between homeologous chromosomes than the corresponding L. esculentum x S. lycopersicoides genotypes. A trigenomic 2n + 14 hybrid, with 12 extra chromosomes from S. sitiens and 2 from S. lycopersicoides, displayed mostly homologous chromosome associations. The distribution of rDNA genes appeared similar in the three genomes.