title: Molecular organization and phylogenetic analysis of 5S rDNA in crustaceans of the genus Pollicipes reveal birth-and-death evolution and strong purifying selection.
abstract: BACKGROUND
  The 5S ribosomal DNA (5S rDNA) is organized in tandem arrays with repeat units that consist of a transcribing region (5S) and a variable nontranscribed spacer (NTS), in higher eukaryotes. Until recently the 5S rDNA was thought to be subject to concerted evolution, however, in several taxa, sequence divergence levels between the 5S and the NTS were found higher than expected under this model. So, many studies have shown that birth-and-death processes and selection can drive the evolution of 5S rDNA. In analyses of 5S rDNA evolution is found several 5S rDNA types in the genome, with low levels of nucleotide variation in the 5S and a spacer region highly divergent. Molecular organization and nucleotide sequence of the 5S ribosomal DNA multigene family (5S rDNA) were investigated in three Pollicipes species in an evolutionary context.
  
  
  RESULTS
  The nucleotide sequence variation revealed that several 5S rDNA variants occur in Pollicipes genomes. They are clustered in up to seven different types based on differences in their nontranscribed spacers (NTS). Five different units of 5S rDNA were characterized in P. pollicipes and two different units in P. elegans and P. polymerus. Analysis of these sequences showed that identical types were shared among species and that two pseudogenes were present. We predicted the secondary structure and characterized the upstream and downstream conserved elements. Phylogenetic analysis showed an among-species clustering pattern of 5S rDNA types.
  
  
  CONCLUSIONS
  These results suggest that the evolution of Pollicipes 5S rDNA is driven by birth-and-death processes with strong purifying selection.
authors: Department of Cell and Molecular Biology, Evolutionary Biology Group, Universidade da Coruña, A Fraga 10, E-15008 A Coruña, Spain.
authors: Alejandra
authors: A
authors: Perina
authors: David
authors: D
authors: Seoane
authors: Ana M
authors: AM
authors: González-Tizón
authors: Fernanda
authors: F
authors: Rodríguez-Fariña
authors: Andrés
authors: A
authors: Martínez-Lage
mesh_terms: D000818:Animals; D001483:Base Sequence; D005075:Biological Evolution; D003445:Crustacea / Q000235:genetics*; D004275:DNA, Ribosomal / Q000235:genetics*; D009690:Nucleic Acid Conformation; D010802:Phylogeny; D012341:RNA, Ribosomal, 5S / Q000235:genetics
chemical_list: D004275:DNA, Ribosomal; D012341:RNA, Ribosomal, 5S