Abstract
Ribosomal DNA (rDNA) is a repetitive DNA which encodes for ribosomal RNA, essential for cell function. Nuclear rDNA is organized in two families.
One family consists of genes for the 5S (minor) and 45S (major) subunits. The second family comprises the 18S, 5.8S and 28S genes corresponding to the Nucleolar Organizer Regions (NORs).
Data derived from rDNA gene mapping have been used as markers for comparative cytogenetic studies. Their number may vary among species, thus facilitating studies of evolutionary relationships.
Different numbers and locations of rDNA genes are hypothesized to result from chromosomal rearrangements or transposition events. We mapped, using fluorescence in situ hybridization (FISH), 18S and 28S probes on karyotypes of 14 species representing the main groups of Primates as well as in Tupaia minor (Scandentia), which often serves as an outgroup for primate phylogenetic reconstructions.
The aim of our study was to establish the chromosomal distribution of rDNA gene sequences in primate genomes. In the species analysed, we found that the rDNA probe provided signals on 1 to 6 pairs of chromosomes, including acrocentric and metacentric elements.
For each species, inverted DAPI banding allowed us to identify the chromosomes hybridized by the rDNA probe and, on the basis of previous painting data, to analyse human syntenic homologies. Our results showed rDNA probes provided signals on both homologous and non-homologous chromosome segments.
From an evolutionary perspective, our results support the hypothesis that different mechanisms are responsible for rDNA distribution in primates. Further, the many synapomorphies found could be useful phylogenetic markers.
Future research on more species, coupling classic and molecular cytogenetics approaches, could provide useful information on chromosomal and species evolution.