Delsuc Frédéric, Brinkmann Henner, Chourrout Daniel, Philippe Hervé
Département de Biochimie, Centre Robert-Cedergren, Université de Montréal, Succursale Centre-Ville, Montréal, Québec H3C3J7, Canada.
Nature. 2006 Feb 23;439(7079):965-8. doi: 10.1038/nature04336.
Tunicates or urochordates (appendicularians, salps and sea squirts), cephalochordates (lancelets) and vertebrates (including lamprey and hagfish) constitute the three extant groups of chordate animals. Traditionally, cephalochordates are considered as the closest living relatives of vertebrates, with tunicates representing the earliest chordate lineage. This view is mainly justified by overall morphological similarities and an apparently increased complexity in cephalochordates and vertebrates relative to tunicates. Despite their critical importance for understanding the origins of vertebrates, phylogenetic studies of chordate relationships have provided equivocal results. Taking advantage of the genome sequencing of the appendicularian Oikopleura dioica, we assembled a phylogenomic data set of 146 nuclear genes (33,800 unambiguously aligned amino acids) from 14 deuterostomes and 24 other slowly evolving species as an outgroup. Here we show that phylogenetic analyses of this data set provide compelling evidence that tunicates, and not cephalochordates, represent the closest living relatives of vertebrates. Chordate monophyly remains uncertain because cephalochordates, albeit with a non-significant statistical support, surprisingly grouped with echinoderms, a hypothesis that needs to be tested with additional data. This new phylogenetic scheme prompts a reappraisal of both morphological and palaeontological data and has important implications for the interpretation of developmental and genomic studies in which tunicates and cephalochordates are used as model animals.
被囊动物或尾索动物（住囊虫、樽海鞘和海鞘）、头索动物（文昌鱼）和脊椎动物（包括七鳃鳗和盲鳗）构成了现存的三类脊索动物。传统上，头索动物被认为是脊椎动物现存的最亲近的亲属，而被囊动物代表了最早的脊索动物谱系。这一观点主要基于整体形态上的相似性，以及相对于被囊动物，头索动物和脊椎动物明显增加的复杂性。尽管对于理解脊椎动物的起源至关重要，但关于脊索动物关系的系统发育研究结果却模棱两可。利用住囊虫Oikopleura dioica的基因组测序结果，我们组装了一个系统发育基因组数据集，该数据集包含来自14种后口动物的146个核基因（33800个明确比对的氨基酸），并将24个其他进化缓慢的物种作为外类群。在这里，我们表明对该数据集的系统发育分析提供了令人信服的证据，即脊椎动物现存的最亲近的亲属是被囊动物，而非头索动物。脊索动物的单系性仍然不确定，因为尽管统计支持不显著，但令人惊讶的是，头索动物与棘皮动物归为一类，这一假设需要用更多数据进行检验。这种新的系统发育方案促使人们重新评估形态学和古生物学数据，并对以被囊动物和头索动物为模式动物的发育和基因组研究的解释具有重要意义。
