Limitations of Metazoan 18S rRNA Sequence Data: Implications for Reconstructing a Phylogeny of the Animal Kingdom and Inferring the Reality of the Cambrian Explosion |
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Authors: | Ehab Abouheif Rafael Zardoya Axel Meyer |
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Institution: | (1) Department of Ecology and Evolution, State University of New York, Stony Brook, New York 11794-5245, USA, US |
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Abstract: | We document the phylogenetic behavior of the 18S rRNA molecule in 67 taxa from 28 metazoan phyla and assess the effects of
among-site rate variation on reconstructing phylogenies of the animal kingdom. This empirical assessment was undertaken to
clarify further the limits of resolution of the 18S rRNA gene as a phylogenetic marker and to address the question of whether
18S rRNA phylogenies can be used as a source of evidence to infer the reality of a Cambrian explosion. A notable degree of
among-site rate variation exists between different regions of the 18S rRNA molecule, as well as within all classes of secondary
structure. There is a significant negative correlation between inferred number of nucleotide substitutions and phylogenetic
information, as well as with the degree of substitutional saturation within the molecule. Base compositional differences both
within and between taxa exist and, in certain lineages, may be associated with long branches and phylogenetic position. Importantly,
excluding sites with different degrees of nucleotide substitution significantly influences the topology and degree of resolution
of maximum-parsimony phylogenies as well as neighbor-joining phylogenies (corrected and uncorrected for among-site rate variation)
reconstructed at the metazoan scale. Together, these data indicate that the 18S rRNA molecule is an unsuitable candidate for
reconstructing the evolutionary history of all metazoan phyla, and that the polytomies, i.e., unresolved nodes within 18S
rRNA phylogenies, cannot be used as a single or reliable source of evidence to support the hypothesis of a Cambrian explosion.
Received: 9 December 1997 / Accepted: 23 March 1998 |
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Keywords: | : 18S rRNA — Molecular phylogeny — Metazoan phylogeny — Cambrian explosion — Among-site rate variation |
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