Background: 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. Results: 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. Conclusion: 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.
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We are indebted to two anonymous reviewers and Steven Nadler for their insightful comments that helped to improve this manuscript. We also thank Mi-Hyun Park, Jina Baek (Inha University), Seokha Kang (Chungbuk National University) for their assistance in performing laboratory works. This work was supported by National Research Foundation of Korea Grant funded by the Korean Government (KRF-2008-313-C00813) and the research grant of the Chungbuk National University in 2008.