Evolution of 28S Ribosomal DNA in Chaetognaths: Duplicate Genes and Molecular Phylogeny |
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Authors: | Maximilian J Telford Peter WH Holland |
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Institution: | (1) Molecular Zoology Laboratory, Department of Zoology, Oxford University, South Parks Road, Oxford, OX1 3PS, United Kingdom, GB |
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Abstract: | The chaetognaths are an extraordinarily homogeneous phylum of animals at the morphological level, with a bauplan that can
be traced back to the Cambrian. Despite the attention of zoologists for over two centuries, there is little agreement on classification
within the phylum. We have used a molecular biological approach to investigate the phylogeny of extant chaetognaths. A rapidly
evolving expansion segment toward the 5′ end of 28S ribosomal DNA (rDNA) was amplified using the polymerase chain reaction
(PCR), cloned, and sequenced from 26 chaetognath samples representing 18 species. An unusual finding was the presence of two
distinct classes of 28S rDNA gene in chaetognaths; our analyses suggest these arose by a gene (or gene cluster) duplication
in a common ancestor of extant chaetognaths. The two classes of chaetognath 28S rDNA have been subject to different rates
of molecular evolution; we present evidence that both are expressed and functional. In phylogenetic reconstructions, the two
classes of 28S rDNA yield trees that root each other; these clearly demonstrate that the Aphragmophora and Phragmophora are
natural groups. Within the Aphragmophora, we find good support for the groupings denoted Solidosagitta, Parasagitta, and Pseudosagitta. The relationships between several well-supported groups within the Aphragmophora are uncertain; we suggest this reflects
rapid, recent radiation during chaetognath evolution.
Received: 19 March 1996 / Accepted: 5 August 1996 |
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Keywords: | : Chaetognaths — Duplicate genes — Molecular phylogeny |
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