Abstract
We report on a major update to the animal rDNA loci database, which now contains cytogenetic information for 45S and 5S rDNA loci in more than 2600 and 1000 species, respectively. The data analyses show the following: (i) A high variability in 5S and 45S loci numbers, with both showing 50-fold or higher variability. However, karyotypes with an extremely high number of loci were rare, and medians generally converged to two 5S sites and two 45S rDNA sites per diploid genome. No relationship was observed between the number of 5S and 45S loci. (ii) The position of 45S rDNA on sex chromosomes was relatively frequent in some groups, particularly in arthropods (14% of karyotypes). Furthermore, 45S rDNA was almost exclusively located in microchromosomes when these were present (in birds and reptiles). (iii) The proportion of active NORs (positively stained with silver staining methods) progressively decreased with an increasing number of 45S rDNA loci, and karyotypes with more than 12 loci showed, on average, less than 40% of active loci. In conclusion, the updated version of the database provides some new insights into the organization of rRNA genes in chromosomes. We expect that its updated content will be useful for taxonomists, comparative cytogeneticists, and evolutionary biologists.
Highlights
Ribosomal DNA loci encoding 5S and 45S (18S-5.8S-28S) rRNA genes are vital components of eukaryotic chromosomes [1]
The relationship between nucleolar organizer regions (NOR) and distinct chromosome segments had already been proposed by Barbara McClintock [4], the composition of NORs and Ribosomal DNA (rDNA) loci in general were only deciphered with the aid of in situ hybridization experiments in the 1970s [5,6,7]
Many protocols are available for fluorescence in situ hybridization (FISH) of rDNA
Summary
Ribosomal DNA (rDNA) loci encoding 5S and 45S (18S-5.8S-28S) rRNA genes are vital components of eukaryotic chromosomes [1]. Ribosomal DNA loci, together with centromeres and telomeres, are distinguished features in eukaryotic chromosomes. While telomeres and centromeres tend to occupy specific chromosome regions, rDNA loci are often found at variable positions and numbers. These features make rDNAs important karyotypic and phylogenetic markers. The relationship between nucleolar organizer regions (NOR) and distinct chromosome segments had already been proposed by Barbara McClintock [4], the composition of NORs and rDNA loci in general were only deciphered with the aid of in situ hybridization experiments in the 1970s [5,6,7]. Aside from its standard position on autosomes, there are examples of rDNA occurrences on sex chromosomes [15,16] and
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
