Ribosomal RNA gene loci and silver-stained nucleolar organizer regions associated with heterochromatin in Alaskan char Salvelinus malma and chum salmon Oncorhynchus keta

Twenty years of physical mapping of major ribosomal RNA genes across the teleosts: A review of research

Molecular cytogenetic data on the number and position of 45S ribosomal RNA genes (rDNA; located in nucleolus organizing regions, NORs) detected by FISH in 330 species of 77 families and 22 orders of bony fishes (Teleostei) and, additionally, 11 species of basal ray-finned fishes are compiled and analyzed. The portion of species with single rDNA sites in the sample amounts to 72%. The percentage of species with multiple NORs decreases with increasing numbers of rDNA loci per genome, i.e. scarcely 3% of species carry 4 or more rDNA-bearing chromosome pairs. 43% of all rDNA sites analyzed occur terminally on the short arms of chromosomes or constitute them. In general, terminal rDNA sites account for 87% of all examined cases. Interspecific variation in the location of single rDNA sites among related taxa, polymorphisms of multiple NORs in some groups of teleosts and analytical outcomes on the subject are reviewed.

Comparative chromosomal mapping in Triportheus fish species. Analysis of synteny between ribosomal genes

All Triportheus species show the conserved diploid number of 52 chromosomes and a ZZ/ZW sex chromosome system. Previous studies conducted on Triportheus nematurus reported a syntenical location of 18S and 5S sites on this species, in addition to some indications that this condition could be shared by other Triportheus species, possibly constituting a synapomorphy for this genus. In the present study, fluorescence in situ hybridization (FISH) experiments were performed in seven Triportheus species in view of a comparative analysis of the distribution of the 18S and 5S ribosomal DNAs on the chromosomes. The double-FISH experiments have showed that the synteny of the 18S and 5S rDNA genes is not a synapomorphy for the genus, since it is not present in all the species investigated, although it is present in most of them. The findings suggest that the syntenical location of the ribosomal genes is an ancestral trait in Triportheus, which was changed during the course of evolution of this group.

Mapping of rRNA genes and telomeric sequences in Danube salmon (Hucho hucho) chromosomes using primed in situ labeling technique (PRINS)

In the current paper we described the application of primed in situ (PRINS) labeling approach for the chromosomal mapping of repetitive DNA sequences in Danube salmon (Hucho hucho) (2n = 82, NF = 112). PRINS was successfully performed with primers enabling amplification of 5S rRNA genes (minor rDNAs), NOR building DNA sequences (major rDNAs), and telomeric sequences. Two loci of 5S rRNA were observed on distinct chromosome pairs; the minor arrays were located interstitially on the long (q) arms of two large metacentrics (chromosomes No. 3) and the large clusters of 5S rDNAs were assigned to the short (p) arms of two subtelocentric chromosomes No. 18. Major rDNA clusters were observed on the p-arms of two submeta-subtelocentric chromosomes No. 10. These chromosomal areas were built with GC-rich chromatin what was proved in the course of chromomycin A(3) (CMA(3)) staining performed sequentially. Major and minor rDNA families were not co-localized in the Danube salmon chromosomes. The distinct hybridization signals at the ends of all the chromosomes were provided in the course of PRINS with (CCCTAA)(n) primer. The chromosomal localization of rRNA genes and telomeric DNA sequences was discussed in the context of Salmonidae karyotype evolution.