Cross-species BAC-FISH painting of the tomato and potato chromosome 6 reveals undescribed chromosomal rearrangements

Ongoing genomics projects of tomato (Solanum lycopersicum ) and potato (Solanum tuberosum) are providing unique tools for comparative mapping studies in Solanaceae. At the chromosomal level, BACs can be positioned on pachytene comple-ments by fluorescent in situ hybridization (FISH) on homoeologous chromosomes of related species. Here we present the first results of such a cross-species multi-colour cytogenetic mapping of tomato BACs on potato chromosome 6 and vice versa. The experiments were performed under low hybridization stringency, while blocking with Cot-100 was essential in suppressing excessive hybridization of repeat signals in both within-species FISH and cross-species FISH of tomato BACs. In the short arm we detected a large paracentric inversion that covers the whole euchromatin part with breakpoints close to the telomeric heterochromatin and at the border of the short arm pericentromere. The long arm BACs revealed no deviation in the co-linearity between tomato and potato. Further comparison between tomato cultivars Cherry VFNT and Heinz 1706 revealed co-linearity of the tested tomato BACs, whereas one of the six potato clones (RH98-856-18) showed minor putative rearrangements within the inversion. Our results present cross-species multi-colour BAC-FISH as a unique tool for comparative genetic studies across Solanum species