Mitotic chromosomes of the plant pathogenic filamentous fungi Botrytis cinerea and Alternaria alternata were observed. Chromosomes prepared by the germ tube burst method were stained with the fluorescent dye 4′,6-diamidino-2-phenylindole (DAPI) to yield figures with good resolution. Using this method, component chromosomes were clearly distinguished and the chromosome number could be determined. Fluorescence in situ hybridization (FISH) was also successfully applied to the specimens, revealing one ribosomal RNA gene cluster, or nucleolus organizer region (NOR) in the genome of each fungus. A long attenuated chromatid thread expanding from a condensed metaphase chromosome, which had been called a thread-like structure in B. cinerea, was proved to be an NOR. This is the first report of the successful application of FISH to the chromosomes of filamentous fungi.
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