Twenty Years of Physical Mapping of Major Ribosomal RNA Genes across the Teleosts: A Review of Research

Abstract

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.

Differentiation of Sex Chromosomes and Karyotype Characterisation in the Dragonsnake <b><i>Xenodermus javanicus</i></b> (Squamata: Xenodermatidae)

“…The variation in NORs size and/or location has been frequently reported in various vertebrates [e.g. Gornung et al, 2011;Britton-Davidian et al, 2012;Gornung, 2013]. Therefore, we suggest that the polymorphism of the XJA pair can be attributed to the variation in the numbers of copies of the rDNA gene.…”

Karyotype and Mapping of Repetitive DNAs in the African Butterfly Fish <b><i>Pantodon buchholzi, </i></b>the Sole Species of the Family Pantodontidae

“…To date, this association has been observed not only in a number of teleostean groups but also in representatives of non-teleostean lineages [Ráb et al, 1999;Gornung, 2013]. Inside Osteoglossiformes, an association of GC-rich heterochromatin with the NOR was also previously reported in A. gigas [da Rosa et al, 2009].…”

“…Although the rDNA multigene families have been reported in a small number of teleosts, their distribution in only 1 chromosome pair represents the most frequent condition [Gornung, 2013]. In A. gigas and in some Mormyridae, the major 18S rRNA genes were also confined to 1 chromosome pair, not homologous across species [Marques et al, 2006;da Rosa et al, 2009;Ozouf-Costaz et al, 2015].…”

Allopatric chromosomal variation in Nematocharax venustus Weitzman, Menezes & Britski, 1986 (Actinopterygii: Characiformes) based on mapping of repetitive sequences

“…Indeed, even though some genera of Characidae share single NORs, a basal condition in teleosteans (Gornung, 2013), as reported in Tetragonopterus Cuvier (Alberdi & Fenocchio, 1997), Moenkhausia Eigenmann (Castro & Júlio Júnior, 2002), Bryconamericus Eigenmann (Marques et al, 2003) and Piabina Reinhardt (Pazian et al, 2012), most species in this family present multiple NORs (Kavalco & Moreira Filho, 2003).…”

“…Apparently, this behavior favors the rearrangement of NORs without interfering with other genetic linkage groups (Hanson et al, 1996), besides promoting the redistribution of ribosomal genes by the proximity of telomeres in the interphasic nuclei (Gornung, 2013). Such distribution of 18S rRNA genes in N. venustus could account for the putative transposition of some copies of this rDNA in one homologous of pair 8 in the population from Jequitinhonha 2, giving rise to the bitelomeric NORs (Fig.…”