Fluorescent in situ hybridization to soybean metaphase chromosomes

Repetitive DNA sequences were detected directly on somatic metaphase chromosome spreads from soybean root tips using fluorescent in situ hybridization. Methods to spread the forty small metaphase chromosomes substantially free of cellular material were developed using protoplasts. The specific DNA probe was a 1.05 kb internal fragment of a soybean gene encoding the 18S ribosomal RNA subunit. Two methods of incorporating biotin residues into the probe were compared and detection was accomplished with fluorescein-labeled avidin. The rDNA probe exhibits distinct yellow fluorescent signals on only two of the forty metaphase chromosomes that have been counterstained with propidium iodide. This result agrees with our previous analyses of soybean pachytene chromosomes showing that only chromosome 13 is closely associated with the nucleolus organizer region. Fluorescent in situ hybridization with the rDNA probe was detected on three of the forty-one metaphase chromosomes in plants that are trisomic for chromosome 13.