EVOLUTION OF 28S RIBOSOMAL DNA IN CHAETOGNATHS - DUPLICATE GENES AND MOLECULAR PHYLOGENY

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 centuri es, 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 t he 5' end of 28S ribosomal DNA (rDNA) was amplified using the polymera se chain reaction (PCR), cloned, and sequenced from 26 chaetognath sam ples representing 18 species. An unusual finding was the presence of t wo distinct classes of 28S rDNA gene in chaetognaths; our analyses sug gest these arose by a gene (or gene cluster) duplication in a common a ncestor of extant chaetognaths. The two classes of chaetognath 28S rDN A have been subject to different rates of molecular evolution; we pres ent evidence that both are expressed and functional. In phylogenetic r econstructions, the two classes of 28S rDNA yield trees that root each other; these clearly demonstrate that the Aphragmophora and Phragmoph ora are natural groups. Within the Aphragmophora, we find good support for the groupings denoted Solidosagitta, Parasagitta, and Pseudosagit ta. The relationships between several well-supported groups within the Aphragmophora are uncertain; we suggest this reflects rapid, recent r adiation during chaetognath evolution.