Department of Biological Sciences, Inha University, Incheon 402-751, Republic of Korea.
BMC Genomics. 2009 Nov 17;10:533. doi: 10.1186/1471-2164-10-533.
The Syndermata (Rotifera+Acanthocephala) is one of the best model systems for studying the evolutionary origins and persistence of different life styles because it contains a series of lineage-specific life histories: Monogononta (cyclic parthenogenetic and free-living), Bdelloidea (entirely parthenogenetic and mostly benthic dweller), Seisonidea (exclusively bisexual and epizoic or ectoparasitic), and Acanthocephala (sexual and obligatory endoparasitic). Providing phylogenetic resolution to the question of Eurotatoria (Monogononta and Bdelloidea) monophyly versus paraphyly is a key factor for better understanding the evolution of different life styles, yet this matter is not clearly resolved. In this study, we revisited this issue based on comparative analysis of complete mitochondrial genome information for major groups of the Syndermata.
We determined the first complete mitochondrial genome sequences (15,319 bp) of a bdelloid rotifer, Rotaria rotatoria. In order to examine the validity of Eurotatoria (Monogononta and Bdelloidea) monophyly/paraphyly, we performed phylogenetic analysis of amino acid sequences for eleven protein-coding genes sampled from a wide variety of bilaterian representatives. The resulting mitochondrial genome trees, inferred using different algorithms, consistently failed to recover Monogononta and Bdelloidea as monophyletic, but instead identified them as a paraphyletic assemblage. Bdelloidea (as represented by R. rotatoria) shares most common ancestry with Acanthocephala (as represented by L. thecatus) rather than with monogonont B. plicatilis, the other representative of Eurotatoria.
Comparisons of inferred amino acid sequence and gene arrangement patterns with those of other metazoan mtDNAs (including those of acanthocephalan L. thecatus and monogonont B. plicatilis) support the hypothesis that Bdelloidea shares most common ancestry with Acanthocephala rather than with Monogononta. From this finding, we suggest that the obligatory asexuality of bdelloideans may have secondarily derived from some other preexisting condition in earlier lineage of rotifers. Providing a more complete assessment of phylogenetic relationships and inferring patterns of evolution of different types of life styles among Syndermata awaits comparisons requiring mitochondrial genome sequencing of Seisonidea.
Syndermata（轮虫动物门+棘头动物门）是研究不同生活方式进化起源和持续存在的最佳模式系统之一，因为它包含一系列谱系特异性的生活史：Monogononta（周期性孤雌生殖和自由生活）、Bdelloidea（完全孤雌生殖和主要底栖居住）、Seisonidea（专性两性和外寄生或外寄生虫）和棘头动物门（有性和专性内寄生）。为了更好地理解不同生活方式的进化，提供对 Eurotatoria（Monogononta 和 Bdelloidea）单系性与并系性的问题的系统发育解析是一个关键因素，但这一问题尚未得到明确解决。在这项研究中，我们基于对 Syndermata 主要类群的完整线粒体基因组信息的比较分析，重新探讨了这个问题。
我们确定了轮虫动物门的一种 Bdelloidea 轮虫 Rotaria rotatoria 的第一个完整线粒体基因组序列（15319bp）。为了检验 Eurotatoria（Monogononta 和 Bdelloidea）单系性/并系性的有效性，我们对来自广泛的两侧对称动物代表的十一个蛋白质编码基因的氨基酸序列进行了系统发育分析。使用不同算法推断的线粒体基因组树一致地未能将 Monogononta 和 Bdelloidea 恢复为单系群，而是将它们识别为并系群。Bdelloidea（以 R. rotatoria 为代表）与棘头动物门（以 L. thecatus 为代表）共享最多的共同祖先，而不是与 Eurotatoria 的另一个代表 monogonont B. plicatilis 共享最多的共同祖先。
与其他后生动物 mtDNA（包括棘头动物门的 L. thecatus 和 Monogononta 的 B. plicatilis）的推断氨基酸序列和基因排列模式的比较支持 Bdelloidea 与棘头动物门而不是与 Monogononta 具有最多共同祖先的假设。从这一发现中，我们推测 Bdelloideans 的强制性无性繁殖可能是从轮虫动物门早期谱系中某些其他预先存在的条件中次生衍生而来的。提供 Syndermata 之间不同生活方式的系统发育关系的更完整评估，并推断不同类型生活方式的进化模式，需要对 Seisonidea 的线粒体基因组进行测序。
