Genome differentiation between Lycopersicon esculentum and L. pennellii as revealed by genomic in situ hybridization

Abstract

Using an F₁ hybrid between Lycopersicon esculentum and L. pennellii (2n = 2x = 24) and its BC₁ progeny, an attempt was made to cytologically differentiate the genomes through genomic in situ hybridisation (GISH). When total genomic DNA of L. pennellii was used as a probe with a concentration of 2 μg/μl and the stringency of hybridisation being 50% to 60%, it was possible to distinguish the chromosomes of L. pennellii from those of L. esculentum, cv Money Maker. The differentiation was observed in both somatic metaphase as well as in pollen mother cells. Whereas late prophase I and metaphase I stages were helpful to distinguish the homoeologous chromosomes, late anaphase I and telophase I stages were useful for the analysis of half-bivalents. Because the chromosome pairing and chiasma formation was completely normal in the F₁ hybrid, the half-bivalents did carry recombinant chromatids but GISH procedure was not sufficient to resolve the recombinant fragments. The reason for the lack of resolution of recombination segments was assumed to be 1) the small genome size; 2)non-uniform distribution of repetitive DNA on the chromosomes in which the proximal (centromeric heterochromatin) part was fairly amenable for differentiation whereas the distal (euchromatin) part was poorly differentiated and 3) the close taxonomic relationship of the two species was another factor for the lack of clear differentiation. The first two reasons are unlikely and the third explanation was the probable explanation. In BC₁progeny the chromosomes with L. pennellii centromeric regions were lower than those of Money Maker but higher than the expected mean value of 5–7. The level of differentiation between the chromosomes of the two genomes was similar as in the F₁ hybrids. In view of the failure of cytological detection of homoeologous recombination, the potential of the bridging effect of L. pennellii in interspecific hybrids between potato and tomato can be better assessed through genotyping, using RFLP and AFLP analysis.