Data from: Testing hypotheses of chaetognath origins: long branches revealed by 18S ribosomal DNA

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 deuterostome 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.

学界已针对毛颚动物门（Chaetognatha，俗称箭虫）的系统发育位置提出诸多假说；该类群的分类学定位颇具争议，核心症结在于其形态学特征难以提供可明确其与其他类群亲缘关系的确凿分类学性状。早期研究者曾提出大量毛颚动物的系统发育定位方案，将其与棘头动物（acanthocephalans）、软体动物（mollusks）等差异显著的类群归为一类；但更具传统性的假说认为，毛颚动物实则为后口动物（deuterostomes）。近期，Telford与Holland（1993，《分子生物学与进化》10:660–676）以及Wada与Satoh（1994，《美国国家科学院院刊》91:1801–1804）基于18S核糖体核RNA（18S nuclear ribosomal RNA，rDNA）基因序列数据，对毛颚动物隶属于后口动物的亲缘关系假说提出了质疑。本研究利用已发表的18S rDNA基因序列数据，针对毛颚动物与线虫（nematodes）、软体动物、棘头动物及后口动物的亲缘关系特定假说开展检验，拓展了此前的分析工作。简约法（parsimony）与邻接法（neighbor-joining）分析均支持毛颚动物与线虫构成单系群（chaetognath–nematode clade）。本研究采用Faith's T-PTP检验与获胜位点分析（winning-sites analyses），对相互竞争的各类假说进行甄别。本研究通过分析不同的四分类群树状结构，探讨了长枝吸引效应（long-branch attraction）是否为毛颚动物与线虫之间亲缘关系的成因。本研究还提出了毛颚动物类群从蠕虫状底栖生物演化起源的演化场景。