5S ribosomal RNA genes in six species of Mediterranean grey mullets: genomic organization and phylogenetic inference

This paper describes a study of the 5S ribosomal RNA genes (5S rDNA) in a group of 6 species belonging to 4 genera of Mugilidae. In these 6 species, the relatively short 5S rDNA repeat units, generated by PCR and ranging in size from 219 to 257 bp, show a high level of intragenomic homogeneity of both coding and spacer regions (NTS-I). Phylogenetic reconstructions based on this data set highlight the greater phylogenetic and genetic diversity of Mugil cephalus and Oedalechilus labeo compared with the genera Liza and Chelon. Comparative sequence analysis revealed significant conservation of the short 5S rDNA repeat units across Chelon and Liza. Moreover, a second size class of 5S rDNA repeat units, ranging from roughly 800 to 1100 bp, was produced in the Liza and Chelon samples. Only short 5S rDNA repeat units were found in M. cephalus and O. labeo. The sequences of the long 5S rDNA repeat units, obtained in Chelon labrosus and Liza ramada , differ owing to the presence of 2 large insertion/deletions (indels) in the spacers (NTS-II) and show considerable sequence identity with NTS-I spacers. Interspecific sequence variation of NTS-II spacers, excluding the indels, is low. Southern-blot hybridization patterns suggest an intermixed arrangement of short and long repeat units within a single chromosome locus.