IDENTIFICATION OF A HOMEOLOGOUS CHROMOSOME PAIR BY IN-SITU DNA HYBRIDIZATION TO RIBOSOMAL-RNA LOCI IN MEIOTIC CHROMOSOMES OF COTTON (GOSSYPIUM-HIRSUTUM)

In situ DNA hybridization with 18S-28S and 5S ribosomal DNA probes was used to map 18S-28S nucleolar organizers and tandem 5S repeats to mei otic chromosomes of cotton (Gossypium hirsutum L.). Mapping was perfor med by correlating hybridization sites to particular positions in tran slocation quadrivalents. Arm assignment required translocation quadriv alents with at least one interstitial chiasma and sufficient distance between the hybridization site and the centromere. We had previously l ocalized a major 18S-28S site to the short arm of chromosome 9; here w e mapped two additional major 18S-28S sites to the short arm of chromo some 16 and the left arm of chromosome 23. We also identified and mapp ed a minor 18S-28S site to the short arm of chromosome 7. Two 5S sites of unequal size were identified, the larger one near the centromere o f chromosome 9 and the smaller one near the centromere of chromosome 2 3. Synteny of 5S and 18S-28S sites indicated homeology of chromosomes 9 and 23, while positions of the other two 18S-28S sites supplement ge netic evidence that chromosomes 7 and 16 are homeologous.