Chromosomal mapping of 18S-28S and 5S rRNA genes by two-colour fluorescent in situ hybridization in six sturgeon species.
The number and distribution of the 18S-28S and 5S ribosomal RNA (rDNA) gene sequences were examined on mitotic chromosomes of 6 sturgeon species by two-colour in situ hybridization. It was shown that 4 of the 6 species, Huso huso, Acipenser stellatus, A. sturio and A. ruthenus, with about 120 chromosomes, showed from six to eight 18S-28S rDNA signals, while 5S rDNA signals were on only one chromosome pair. The two species with 250-270 chromosomes, A. baerii and A. transmontanus, showed from 10 to 12 18S-28S sites and two chromosome pairs bearing 5S rDNA signals. In all examined species, the rather intense 5S rDNA signals apparently overlapped those of 18S-28S rDNA. These data support the diploid-tetraploid relationships between the two chromosome groups of sturgeons. The close association between the two rDNA families in species belonging to an ancestral fish order, such as Acipenseriformes, supports the hypothesis that the association represents a primitive condition.