THE PHYLOGENETIC AFFINITIES OF THE CHAETOGNATHS - A MOLECULAR ANALYSIS

The chaetognaths, or arrowworms, constitute a small and enigmatic phyl um of marine invertebrates whose phylogenetic affinities have long bee n uncertain. A popular hypothesis is that the chaetognaths are the sis ter group of the major deuterostome phyla: chordates, hemichordates, a nd echinoderms. Here we attempt to determine the affinities of the cha etognaths by using molecular sequence data. We describe the isolation and nucleotide sequence determination of 18S ribosomal DNA from one sp ecies of chaetognath and one acanthocephalan. Extensive phylogenetic a nalyses employing a suite of phylogenetic reconstruction methods (maxi mum parsimony, maximum likelihood, evolutionary parsimony, and two dis tance methods) suggest that the hypothesized relationship between chae tognaths and the deuterostomes is incorrect. In contrast, we propose t hat the lineage leading to the chaetognaths arose prior to the advent of the coelomate metazoa.