USE OF RFLPS LARGER THAN 100-KBP TO MAP THE POSITION AND INTERNAL ORGANIZATION OF THE NUCLEOLUS ORGANIZER REGION ON CHROMOSOME-2 IN ARABIDOPSIS-THALIANA

In Arabidopsis thaliana ribosomal RNA genes (rRNA genes or rDNA) are g rouped in two nucleolus organizer regions (NORs) that together compris e approximately 6% of the genome. The map positions of the NORs relati ve to Other genetic markers are unknown. It was found that the restric tion endonuclease HindIII cuts once in some but not all rRNA genes to yield strain-specific RFLPs of 100-700 kb that could be visualized by pulsed-field gel electrophoresis and Southern blotting. The HindIII RF LPs of the A. thaliana strains Columbia and Landsberg segregated among recombinant inbred lines derived from a cross between these two strai ns. Linkage analysis placed the NOR bearing the polymorphic HindIII si tes to the top of the upper arm of chromosome 2. The name NOR2 is sugg ested for this locus. HindIII-bearing rRNA genes are interspersed with clusters of HindIII-less genes throughout NOR2. The observed clusteri ng is most consistent with unequal crossing-over, or nearest-neighbor gene conversion, as the mechanism(s) that spread rRNA gene variants th roughout an NOR. No meiotic cross-over events yielding a 'hybrid' NOR with multiple RFLPs from both parents were observed among the 47 recom binant inbred lines examined. How ever, the appearance of novel HindII I profiles in approximately 40% of the recombinant inbred lines demons trates that fluctuations in the distribution of rRNA gene variants occ ur frequently and can be readily detected on pulsed-field gels.