Testing Hypotheses of Chaetognath Origins: Long Branches Revealed by 18S Ribosomal DNA

Abstract

Many hypotheses regarding the phylogenetic position of the Chaetognatha (arrow worms) have been proposed; these organisms are problematic primarily because their morphology offers few unambiguous systematic characters that ally them with other taxa. Early researchers proposed a plethora of phylogenetic placements for the Chaetognatha, grouping them with such divergent taxa as acanthocephalans and mollusks, but more traditional hypotheses posit that chaetognaths are, in fact, deuterostomes. Recently, Telford and Holland (1993, Mol. Biol. Evol. 10: 660-676) and Wada and Satoh (1994, Proc. Natl. Acad. Sci. USA 91:1801-1804) disputed the deu- terostome affinities of chaetognaths based on 18S nuclear ribosomal RNA (rDNA) gene sequence data. By employing published 18S rDNA gene sequence data, I extended these previous analyses by testing specific hypotheses of chaetognath affinities to nematodes, mollusks, acanthocephalans, and deuterostomes. Both parsimony and neighbor-joining analyses supported the monophyly of a chaetognath-nematode clade. Faith's T-PTP test and winning-sites analyses were employed to discriminate among competing hypotheses. The possibility of long-branch attraction accounting for the chaetognath-nematode relationship was explored by analyzing alternative four-taxon trees. An evolutionary scenario for the origin of the chaetognath lineage from a vermiform benthic organism is presented. (Chaetognatha; Nematoda; 18S rDNA; long branches; winning-sites test; phylogeny.)