Physical mapping of 45S and 5S rDNA on maize metaphase and sorted chromosomes by FISH

Physical locations of 45S and 5S rDNA were detected in maize by fluorescenc e in situ hybridization (FISH). The FISH results on metaphase chromosome sp reads showed the 45S rDNA was located just at the nucleolus organizer regio n (NOR) on 6S (the short arm of chromosome 6) as expected, and 5S rDNA at t he distal region of 2L (the long arm of chromosome 2). No signals were dete cted on the other maize chromosomes for these two probes. The precision of the chromosomal position of a hybridization site is related to the number o f measurements. We also mapped the location of 5S rDNA at the same sites on 2L by FISH on sorted chromosomes. We could calculate more precisely the pe rcentage distance of hybridization signals since we had large numbers of ta rget chromosomes on a small spot on the slide by flow sorting. The percenta ge distance from centromere to the hybridization site was 85 % for 5S rDNA on 2L. The physical location of 5S rDNA was inconsistent with its genetic s ite, which was positioned in the middle of genetic linkage group 2. A FISH procedure for mapping genes on sorted plant chromosomes is described and oc currence of only one 45S or 5S rDNA site on maize genome is discussed.