Integrating the genetic and physical chromosome maps of Brassica oleracea var. alboglabra

Linking genetic maps constructed with molecular markers to the corresponding physical maps is an important goal in the investigation of crops. The relationship between these maps is reviewed, and we illustrate our approach by the analysis of the short arm of chromosome 4 of the model dicotyledonous plant, Arabidopsis thaliana L. We show that it is possible to produce a detailed molecular cytogenetic map by placing multicopy and unique sequences in relation to euchromatic and heterochromatic segments. This map correlates well with the molecular physical map (sequence map) but, as expected, shows differences from the genetic map. For Brassica olerncea L., as a first step we have produced a partial karyotype based on the combination of chromosome size, arm ratios and the location of repeated 45S rDNA, 5S rDNA and pericentromeric heterochromatin sequences by fluorescence in situ hybridisation (FISH). We are currently using BACs, cosmids and RFLPs, with known genetic map positions, as FISH probes to identify additional linkage group associated markers for the chromosomes. The construction of a definitive karyotype, with all linkage groups assigned to our cytogenetic map is approaching completion.