TY - JOUR
T1 - Unusual structure of ribosomal DNA in the copepod Tigriopus californicus
T2 - Intergenic spacer sequences lack internal subrepeats
AU - Burton, R. S.
AU - Metz, E. C.
AU - Flowers, J. M.
AU - Willett, C. S.
N1 - Funding Information:
The authors thank T. Ballard and R. Byrne for technical assistance and National Science Foundation (DEB9815424 and DEB 0236363) for financial support. Rob McAllen kindly provided the sample of Tigriopus brevicornis from Scotland. We also thank the associate editor and two anonymous reviewers for helpful comments.
PY - 2005/1/3
Y1 - 2005/1/3
N2 - Eukaryotic nuclear ribosomal DNA (rDNA) is typically arranged as a series of tandem repeats coding for 18S, 5.8S, and 28S ribosomal RNAs. Transcription of rDNA repeats is initiated in the intergenic spacer (IGS) region upstream of the 18S gene. The IGS region itself typically consists of a set of subrepeats that function as transcriptional enhancers. Two important evolutionary forces have been proposed to act on the IGS region: first, selection may favor changes in the number of subrepeats that adaptively adjust rates of rDNA transcription, and second, coevolution of IGS sequence with RNA polymerase I transcription factors may lead to species specificity of the rDNA transcription machinery. To investigate the potential role of these forces on population differentiation and hybrid breakdown in the intertidal copepod Tigriopus californicus, we have characterized the rDNA of five T. californicus populations from the Pacific Coast of North America and one sample of T. brevicornicus from Scotland. Major findings are as follows: (1) the structural genes for 18S and 28S are highly conserved across T. californicus populations, in contrast to other nuclear and mitochondrial DNA (mtDNA) genes previously studied in these populations. (2) There is extensive differentiation among populations in the IGS region; in the extreme, no homology is observed across the IGS sequences (>2 kb) from the two Tigriopus species. (3) None of the Tigriopus IGS sequences have the subrepeat structure common to other eukaryotic IGS regions. (4) Segregation of rDNA in laboratory crosses indicates that rDNA is located on at least two separate chromosomes in T. californicus. These data suggest that although IGS length polymorphism does not appear to play the adaptive role hypothesized in some other eukaryotic systems, sequence divergence in the rDNA promoter region within the IGS could lead to population specificity of transcription in hybrids.
AB - Eukaryotic nuclear ribosomal DNA (rDNA) is typically arranged as a series of tandem repeats coding for 18S, 5.8S, and 28S ribosomal RNAs. Transcription of rDNA repeats is initiated in the intergenic spacer (IGS) region upstream of the 18S gene. The IGS region itself typically consists of a set of subrepeats that function as transcriptional enhancers. Two important evolutionary forces have been proposed to act on the IGS region: first, selection may favor changes in the number of subrepeats that adaptively adjust rates of rDNA transcription, and second, coevolution of IGS sequence with RNA polymerase I transcription factors may lead to species specificity of the rDNA transcription machinery. To investigate the potential role of these forces on population differentiation and hybrid breakdown in the intertidal copepod Tigriopus californicus, we have characterized the rDNA of five T. californicus populations from the Pacific Coast of North America and one sample of T. brevicornicus from Scotland. Major findings are as follows: (1) the structural genes for 18S and 28S are highly conserved across T. californicus populations, in contrast to other nuclear and mitochondrial DNA (mtDNA) genes previously studied in these populations. (2) There is extensive differentiation among populations in the IGS region; in the extreme, no homology is observed across the IGS sequences (>2 kb) from the two Tigriopus species. (3) None of the Tigriopus IGS sequences have the subrepeat structure common to other eukaryotic IGS regions. (4) Segregation of rDNA in laboratory crosses indicates that rDNA is located on at least two separate chromosomes in T. californicus. These data suggest that although IGS length polymorphism does not appear to play the adaptive role hypothesized in some other eukaryotic systems, sequence divergence in the rDNA promoter region within the IGS could lead to population specificity of transcription in hybrids.
KW - Genetic variation
KW - Population differentiation
KW - Transcribed spacers
KW - rRNA
UR - http://www.scopus.com/inward/record.url?scp=12344295553&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=12344295553&partnerID=8YFLogxK
U2 - 10.1016/j.gene.2004.09.001
DO - 10.1016/j.gene.2004.09.001
M3 - Article
C2 - 15656977
AN - SCOPUS:12344295553
SN - 0378-1119
VL - 344
SP - 105
EP - 113
JO - Gene
JF - Gene
ER -