Introduction to the animal database of ribosomal DNA loci

Abstract

Eukaryotic ribosomal DNA (rDNA) loci vary both in positions and numbers. rDNA encodes the four critical genes needed for ribosome function: the 5S, 5.8S, 18S and 26S rRNA. In animals, the 18S-5.8S-28S genes (45S) are always linked in a single 45S rDNA unit. The 5S genes occur as separate tandems or linked to the 45S units. Here, we describe a database containing information about the chromosomal position, number of 5S and 45S and their relative position (linked /co-localized/ separated) in more than 790 animal species. The data based on in situ hybridization of metaphase chromosomes have been collected from more than 320 research articles. Species representation is descending in the following order: Fishes (45% of species), arthropods (31%), mammals, mollusks, reptiles, amphibians, birds, annelids, cartilaginous fish and flatworm. Fishes also have the greatest variability in the number of rDNA loci (1 to 21 5S sites and 1 to 23 45S sites per haploid genome) which may be explained by overrepresentation of these genera in the database, but other factors such as ploidy, karyotypic divergence and genome plasticity should also be considered. Average numbers of rDNA loci are 2.6 5S and 2.7 45S per haploid genome. The 5S and 45S loci are located on different chromosomes in most of the genomes (81%). Thirty five percent of species have 5S rDNA in a single locus, whereas 51% of species have 45S rDNA in a single locus. The animal database will soon become accessible via a web-page as a mirror to recently issued Plant rDNA database