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The ploidy status of Acipenser mikadoi was examined using nuclear DNA contents, karyotypes and fluorescence in situ hybridization ( FISH) with 5.8S + 28S rDNA as a probe. In flow-cytometrically sorted specimens with 8.2-9.1 pg DNA content per somatic cell, i.e. genetic diploid, the best informative metaphase with 268 chromosomes had 80 biarmed meta- or submetacentric ( M or SM) chromosomes, 48 monoarmed telocentric ( T) chromosomes and 140 microchromosomes. In genetic triploid specimens with 12.6-13.0 pg DNA content, the best informative metaphase with 402 chromosomes showed 120 biarmed M or SM, 72 monoarmed T chromosomes and 210 microchromosomes. The rDNA FISH detected a maximum 18 and 27 signals in the diploid and triploid A. miakdoi, respectively. The obtained findings thus corroborated a clear parallel between nuclear DNA contents and karyological or FISH profiles in the genetic diploid and triploid specimens, suggesting 1.5 times chromosome complements of diploid counterparts or three sets of homologues in the triploid sturgeons. Moreover, the estimated genome size and the observed molecular cytogenetic features in the diploid A. mikadoi strongly suggest that this species is a member of a functional tetraploid group recently proposed in the literature. [ABSTRACT FROM AUTHOR]