Wada H, Makabe K W, Nakauchi M, Satoh N
Biol Bull. 1992 Dec;183(3):448-455. doi: 10.2307/1542021.
Ascidians (tunicates) are primitive chordates. In spite of their elevated phylogenetic position in the animal kingdom, ascidians have evolved a varied reproductive repertoire; some of them live as individuals (solitary ascidians), while others form colonies (colonial ascidians). Colonial ascidians propagate asexually by budding and strobilation, and they have an extensive capacity for regeneration. However, the orthodox taxonomic classification of ascidians categorizes them into two major groups (the orders Enterogona and Pleurogona), irrespective of their solitary or colonial life style. To examine whether the orthodox classification of ascidians is substantiated by molecular phylogeny, the complete nucleotide sequence of a region of about 1000 base pairs in the central part of their respective 18S rDNAs was determined, and the sequences were compared among five solitary and three colonial ascidians. The phylogenetic tree deduced from these results suggests that the three species of Enterogona and the five species of Pleurogona examined form discrete and separate groups irrespective of their potential to form colonies. Therefore, a solitary or colonial life style is likely to have developed independently after the divergence of the two major groups of ascidians.
海鞘（被囊动物）是原始的脊索动物。尽管海鞘在动物界的系统发育地位较高，但它们进化出了多样的繁殖方式；其中一些以个体形式生活（独居海鞘），而另一些则形成群体（群体海鞘）。群体海鞘通过出芽和横裂进行无性繁殖，并且具有广泛的再生能力。然而，海鞘的传统分类学分类将它们分为两个主要类群（肠鳃纲和侧鳃纲），而不考虑它们是独居还是群体的生活方式。为了研究海鞘的传统分类是否有分子系统发育学的依据，测定了它们各自18S rDNA中部约1000个碱基对区域的完整核苷酸序列，并在5种独居海鞘和3种群体海鞘之间进行了序列比较。从这些结果推导的系统发育树表明，所检测的肠鳃纲的3个物种和侧鳃纲的5个物种形成了离散且独立的类群，而不考虑它们形成群体的潜力。因此，独居或群体的生活方式可能是在海鞘的两个主要类群分化之后独立发展起来的。
