BAC-FISH MAPPING ON TOMATO CHROMOSOME 6 AT PACHYTENE

¹ Laboratory of Genetics, Wageningen University, Arboretumlaan 4, 6703 BD Wageningen, The Netherlands

² Laboratory of Plant Breeding, Wageningen University, Binnenhaven 5, 6709 PD Wageningen, The Netherlands

³ Keygene N.V., Agro Business Park 90, 6708 PW Wageningen, The Netherlands

⁴ Plant Research International, Wageningen University and Research Centre, P.O. Box 16, 6700 AA Wageningen, The Netherlands

Fluorescence in situ Hybridization (FISH) of BACs on pachytene complements in tomato is a powerful physical mapping tool. Chromosomes at this stage are about fifteen times longer than at mitotic metaphase and display a clear morphology in which centromere, nucleolar organizer (NORs), heterochromatin and euchromatin regions can be distinguished. We selected seven RFLP markers that evenly cover the linkage group of chromosome 6, and 2 AFLP markers tightly linked to genes conferring resistance to Oidium (Oi-1/3) and Meloidogyne (Mi-1). FISH with the bacterial artificial chromosome (BAC) DNAs of each of these DNA markers or genes as probes was used to map their position on the pachytene chromosome and DNA fibres. Blocking with Cot-100 genomic DNA was used to suppress FISH signals from repetitive DNA sequences. The establishment of this FISH physical map showed the different ratios of physical/genetic distance between DNA markers and Mi gene and confirms the previously described suppression of recombination in the pericentromeric region. FISH on DNA fibres allowed fine mapping of adjacent and overlapping BACs. The consequences of these results for ongoing DNA sequencing work are discussed.