Sempere Lorenzo F, Martinez Pedro, Cole Charles, Baguñà Jaume, Peterson Kevin J
Department of Biochemistry, Dartmouth Medical School, Hanover, NH 03755, USA.
Evol Dev. 2007 Sep-Oct;9(5):409-15. doi: 10.1111/j.1525-142X.2007.00180.x.
Phylogenetic analyses based on gene sequences suggest that acoel flatworms are not members of the phylum Platyhelminthes, but instead are the most basal branch of triploblastic bilaterians. Nonetheless, this result has been called into question. An alternative test is to use qualitative molecular markers that should, in principle, exclude the possibility of convergent (homoplastic) evolution in unrelated groups. microRNAs (miRNAs), noncoding regulatory RNA molecules that are under intense stabilizing selection, are a newly discovered set of phylogenetic markers that can resolve such taxonomic disputes. The acoel Childia sp. has recently been shown to possess a subset of the conserved core of miRNAs found across deuterostomes and protostomes, whereas a polyclad flatworm-in addition to this core subset-possesses miRNAs restricted to just protostomes. Here, we examine another acoel, Symsagittifera roscoffensis, and three other platyhelminths. Our results show that the distribution of miRNAs in S. roscoffensis parallels that of Childia. In addition, two of 13 new miRNAs cloned from a triclad flatworm are also found in other lophotrochozoan protostomes, but not in ecdysozoans, deuterostomes, or in basal metazoans including acoels. The limited set of miRNAs found in acoels, intermediate between the even more reduced set in cnidarians and the larger and expanding set in the rest of bilaterians, is compelling evidence for the basal position of acoel flatworms and the polyphyly of Platyhelminthes.
基于基因序列的系统发育分析表明，无肠扁虫并非扁形动物门的成员，而是三胚层两侧对称动物中最基部的分支。尽管如此，这一结果仍受到质疑。另一种检验方法是使用定性分子标记，原则上应能排除不相关类群中趋同（同塑性）进化的可能性。微小RNA（miRNA）是一类在强烈的稳定选择作用下的非编码调控RNA分子，是一组新发现的能够解决此类分类学争议的系统发育标记。最近研究表明，无肠扁虫Childia sp.拥有在后口动物和原口动物中发现的保守核心miRNA的一个子集，而一种多肠扁虫除了这个核心子集外，还拥有仅在原口动物中存在的miRNA。在此，我们研究了另一种无肠扁虫罗氏对称虫（Symsagittifera roscoffensis）以及其他三种扁形动物。我们的结果表明，罗氏对称虫中miRNA的分布与Childia的相似。此外，从一种三肠扁虫中克隆出的13种新miRNA中的两种，也存在于其他冠轮动物原口动物中，但不存在于蜕皮动物、后口动物或包括无肠扁虫在内的基部后生动物中。在无肠扁虫中发现的有限miRNA集，介于刺胞动物中更少的miRNA集和其他两侧对称动物中更大且不断增加的miRNA集之间，这是无肠扁虫基部位置以及扁形动物多系起源的有力证据。
