Abstract
BackgroundThe 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.ResultsWe 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.ConclusionComparisons 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.
Highlights
IntroductionThe 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)
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, Bdelloidea, Seisonidea, and Acanthocephala
Protein-coding genes Twelve protein-coding genes were identified using NCBI open reading frame (ORF) Finder and by comparing their sequence with those of homologous genes reported from the acanthocephalan L. thecatus [42] and monogonont B. plicatilis [43]
Summary
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). The Rotifera ( called Rotatoria) is a group of aquatic micrometazoans, mostly less than a millimeter in size It includes more than 2,000 described species that usually occur in freshwater and marine environments throughout the world, but some species are found in wet terrestrial habitats, such as moist soil, or mosses and lichens living on fallen trees and rocks [1,2,3]. This group is usually distinguished from other metazoans by presence of the corona ("wheel organ" = the crown of cilia) located in the cephalic region, which is used for locomotion and food gathering. The class Seisonidea consists of three epizoic species (belong to two genera, Seison and Paraseison [11]) on marine crustaceans (Nebalia species) and all of these are known to reproduce by amphimixis [12]
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