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Vol. 45 No. 3, 2006
Analyses of the Ribosomal Internal Transcribed Spacers (ITS)
and the 5.8S Gene Indicate that Extremely High rDNA Heterogeneity is a
Unique Feature in the Scleractinian Coral Genus Acropora (Scleractinia;
Acroporidae)
Nu-Wei
Vivian Wei1,2, Carden C. Wallace3, Chang-Feng Dai2, Kamla Ruby Moothien
Pillay4, and Chaolun Allen Chen1,2*
1Research
Center for Biodiversity, Academia Sinica, Nankang, Taipei, Taiwan 115
2Institute of Oceanography, National Taiwan
University, Taipei, Taiwan 106
3Museum of Tropical Queensland, Townsville, Queensland
Q4810, Australia
4Mauritius Oceanography Institute, France Centre,
Victoria Avenue, Quatre-Bornes, Mauritius
Nu-Wei
Vivian Wei, Carden C. Wallace, Chang-Feng Dai, Kamla Ruby Moothien
Pillay, and Chaolun Allen Chen (2006) One characteristic of
ribosomal DNA (rDNA) sequences in staghorn corals, Acropora spp., is
the extremely high levels of intragenomic heterogeneity and
interspecific variation. This high genomic diversity is ascribed to
incomplete lineage sorting that predated the divergence of species or
to recent introgressive hybridization. In order to elucidate whether
the high heterogeneity of rDNA is a unique feature of Acropora or a
general pattern applicable to scleractinian corals, we examined the
molecular evolution of the internal transcribed spacers (ITS) and 5.8S
rDNA sequences from 78 species, representing 28 genera, and 12 families
of scleractinian corals. Genetic distances (measured by p-distances)
and frequency distribution analyses revealed that both extremely high
intra- and interspecific heterogeneities of the ITS-5.8S rDNA are
specific to the genus Acropora. The 5.8S rDNA phylogeny clearly showed
a significantly long branch length leading to the cluster containing
the genus Acropora. The molecular-clock hypothesis tested using the
likelihood ratio test indicated a highly significant difference in the
global evolutionary rate of scleractinian 5.8S rDNA. The relative rate
tests showed that the rDNA of Isopora, Caribbean Acropora, and
Indo-Pacific Acropora all evolved at constant tempos, indicating that
the highly divergent rDNA was present in Acropora before it split into
these three lineages. In contrast, rate constancy was rejected for most
comparisons between Acropora/Isopora and other coral genera, suggesting
that the rates of evolution of 5.8S differed between Acropora/Isopora
and the other lineages, and that the evolutionary rate of
Acropora/Isopora has accelerated since divergence from the common
ancestor of scleractinian corals.
Key words: Acropora,
Hybridization, Ribosomal DNA, Concerted evolution, Ancient divergence.
*Correspondence: Tel: 886-2-27899549. Fax: 886-2-27858059.
E-mail: cac@gate.sinica.edu.tw
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