Correlating the Genetic and Physical Map of Barley Chromosome 3H Revealed Limitations of the FISH-Based Mapping of Nearby Single-Copy Probes Caused by the Dynamic Structure of Metaphase Chromosomes.

Genetic maps are based on the recombination frequency of molecular markers which often show different positions in comparison to the corresponding physical maps. To decipher the position and order of DNA sequences genetically mapped to terminal and interstitial regions of barley (Hordeum vulgare) chromosome 3H, fluorescence in situ hybridization (FISH) on mitotic metaphase chromosomes was performed with 16 genomic single-copy probes derived from fingerprinted BAC contigs. Long genetic distances at subterminal regions translated into short physical distances, confirming that recombination events occur more often at distal regions of chromosome 3H. Nonoverlapping FISH signals were frequently obtained for probes with a physical distance of at least 30-60 kb. Only 8% of the analyzed chromosomes showed a symmetric order of FISH signals on both sister chromatids. Due to the dynamic packing of metaphase chromatin, the order of 2 adjacent single-copy signals along the chromosome arms outside the (peri)centromeric region can only reliably be determined if the cytological distance is approximately 3%, corresponding to 21.6 Mb.