Other authors subsume Acanthocephala within extended Rotifera . Leaving Support for a potential sister group relationship of Acanthocephala and .. 2), monophyletic Gnathifera (Syndermata + Gnathostomulida) were. ). Ax (, ) forcefully argued for a sistergroup relationship with the Sister-group relationships of Gnathostomulida and Rotifera-Acanthocephala. Sister-group relationship of Gnathostomulida and Rotifera-Acanthocephala. Reinhard M. Riegerl and Seth Tyler2. 'Zoologisches Institut, Universitat Innsbruck .
In recent phylogenomic studies only one or at best a few putative platyzoan subtaxa were represented, most of them by only one or two species e. The resulting trees revealed unstable and contradictory positions for individual taxa and did not produce reliable support for or against the Platyzoa hypothesis see e. Possible reasons included long-branch attraction LBA phenomena  and significantly deviating amino acid compositions leading to poor leaf stability indices  for the corresponding taxa.
These shortcomings can hopefully be overcome by increasing taxon and sequence data coverage  for putative platyzoan animals, many of which are notoriously difficult to obtain in sufficient amounts.
As a further step towards addressing these superordinate phylogenetic problems, the current study has its focus on resolving the debated internal phylogeny of the taxon Syndermata, which is particularly interesting as a model for the evolution of parasitism.
Other authors subsume Acanthocephala within extended Rotifera .
A close relation between these taxa was originally inferred from shared morphological traits such as a syncytial epidermis with an electron dense internal layer and sperm morphology  — . Additional support for a close phylogenetic relationship of acanthocephalans and at least some of the traditional rotiferan taxa came from analyses of single- and multigene molecular data  —  —  as well as from analyses of combined molecular and morphological data .
While syndermatan monophyly has received strong support, the internal syndermatan phylogeny is still not fully resolved. Morphological characters uniting the three traditional rotiferan taxa Seisonidea, Monogononta, and Bdelloidea are scarce. This is mainly due to their differences in lifestyle, morphology, and reproduction.
The four presently described species of Seisonidea live epizoically, partly even ectoparasitically, on marine crustaceans of the leptostracan genus Nebalia  — . In contrast, most bdelloids and monogononts are free-living aquatic animals capable of active swimming, employing a ciliated apical structure named the corona or wheel-organ.
The wheel-organ in seisonids, however, is reduced or rudimentary .
The evolutionary relationships of rotifers and acanthocephalans | Thomas Near - posavski-obzor.info
Seisonids are additionally distinguished from monogononts and bdelloids by a specific mastax and trophi structure as well as a protrusible neck all of which might have emerged in the context of nourishment in their epizoic, potentially ectoparasitic lifestyle. While seisonids are strictly bisexual with well-developed males that co-occur with females throughout the year, bdelloids and monogononts are capable of parthenogenesis and have dwarf Monogononta or even no males Bdelloidea .
The fourth syndermatan taxon, Acanthocephala, comprises worm-like endoparasites with a complicated life cycle including an arthropod intermediate host and a vertebrate definite host. Sexes are separate, with adult females being larger than males, and reproduction is exclusively sexual. As in other parasites, the morphology of acanthocephalans is highly modified. The many resemblances with Rotifera corroborate their close relationship, and shed more light on the evolution of Gnathifera.
Recent phylogenomic studies show that Gnathifera is probably the sister group of remaining spiralians, and therefore is of crucial importance to understand animal evolution [ 23 ].
However, studies on the different organ systems of Gnathifera are still warranted. The deep interrelationships between these three lineages is now resolved with both phylogenomics [ 3 ] and morphology [ 145 ], supporting a sister group relationship between Micrognathozoa and Rotifera, and Gnathostomulida being sister group to this clade. Although rotifers are relatively well studied in many aspects, most of their internal morphology still needs further investigation, as it is the case for the internal organization in Gnathostomulida and Micrognathozoa.
Several confocal laser scanning microscopy CLSM studies have been conducted on gnathiferans, but most of them have focused on the musculature, e.
Most studied rotifers show the presence of a brain, a mastax ganglion, a pair of ventro-lateral nerve cords as well as various head and peripheral nerves innervating the muscles and the sensory organs. However, extensive studies of the nervous system of rotifers are rare, and most recent publications focused on specific immunoreactivity IR [ 12 — 14 ]. Micrognathozoa were first collected from a cold freshwater spring in in Greenland [ 1 ], and thereafter reported from sub-antarctic islands [ 21 ], and the United Kingdom [ 11 ].
But specimens from the United Kingdom are extremely rare, and the sub-antarctic islands as well as Greenland are remote localities, making the study of fresh material difficult. Sites with gaps were not used in the analyses. Recent morphological analyses from a Rotifera and Acanthocephala as the 18S rRNA study number of laboratories seem to be converging on the in Figures 3 and 4. While the Nemathelminthes are most prob- ation and multiple substitutions at the same site is ably the sister group of Spiralia Lophotrochozoa important for the fast evolving mitochondrial 16S within Protostomia Ehlers et al.
MP analysis does not carry out those correc- Gnathostomulida named Gnathifera have been hypo- tions, and produced a tree similar to tree B in Figure 2, thesized as the sister taxon of Platyhelminthes within with Acanthocephala as a sister taxon to Rotifera. Similarly, when ML analysis was carried out without correcting for site to site variation, Acanthocephala appeared as the sister taxon to Rotifera, but when Molecular evidence for the position of the analysis was repeated with multiple rate categor- Rotifera-Acanthocephala within the Bilateria ies four categories: It is well established that rRNA more distant taxa that have few uniting characters.
More recent studies based on the In most textbooks, rotifers are placed among the 18S rRNA gene have extended the entire clade to aschelminths e. Other views support and Halanych et al. Some cladistic studies of analysis is consistent with the 18S rRNA gene findings the entire Metazoa consider the pseudocoelom an im- Figure 5. Schram, ; Eernisse et al. It appeared basal to the bilateria, an artifact now recog- is clear that the aschelminths are polyphyletic, but nized as caused by unequal rate effects Aguinaldo the more rigorous treatments of aschelminth taxa of- et al.
Loren- apulida to include Tardigrada. Recently, Aguinaldo zen, ; Wallace et al. With careful attention to unequal rate effects, they provided evidence that the protostomes consist of two clades: Since ecdysozoans generally lack spiral cleavage which is present in spiralians, we prefer to use the term Spir- alia instead of Lophotrochozoa see Malakhov, and Nielsen, for descriptions of cleavage in nematodes.
For example, the develop- Figure 6. Proposed position of Rotifera within the Bilateria based mental pattern of growth by molting under the control on morphological and molecular data. The annelid-mollusc lineage of the steroid hormone ecdysone has been confirmed refers to the bulk of the non-ecdysozoan protostomes, but not neces- among Arthropoda, Nematoda, and Tardigrada see sarily all of them.
Only a few key characters are given. Blastopore becomes the anus. Ventral lateral nerve chord Ahlrichs, Molting by ecdysis Aguinaldo et al. Filiform sperm without accessory centriole Ahlrichs, Biciliary terminal cell in the protonephridia Ax, Jaws composed of rods imbedded in a cuticular matrix Ahlrichs, Conclusions and future directions 8: The molecular and morphological evidence is over- whelmingly in favor of a close relationship between Rotifera and Acanthocephala.
Analyses of nuclear 18S rRNA and mitochondrial 16S rRNA genes strongly favor a sister relationship between Bdelloidea and Acanthocephala, one of three possible relationships sequencing the complete mitochondrial genome of B. It is important for new molecular analyses to be versial. In this regard, the time is ripe for a series of carried out rigorously, with special attention paid to rigorous ultrastructural comparisons of the epidermis alignments, unequal rate effects, site to site variation, underlying the rotatory organ of bdelloid rotifers to the and multiple substitutions at the same site.
Similarly, ultrastruc- The position of Rotifera is becoming clearer as tural studies should be carried out to compare rotifer morphological and molecular evidence are considered adhesive glands and acanthocephalan cement glands. We propose a scheme Figure 6 that places The molecular data supporting the sister group Rotifera among the Bilateria and appears to be con- relationship between Bdelloidea and Acanthocephala sistent with many of the more recent molecular and appears very strong, but it is based on only two genes morphological studies.
Gnathostomulida because they all have jaws com- The sequences of more genes from more rotifer taxa posed of rods imbedded in a cuticular matrix Rieger should be analyzed, particularly from Seison.
We are currently which in turn is the sister group to Ecdysozoa. Molecu- Landefeld for technical assistance. Morphogenetic hormones and their glands in arthropods: Nickol for provid- Haffner, K.
Organisation und systematische Stellung der ing frozen M. We also thank Acanthocephalen. Liz Wurdak and four anonymous reviewers for helpful M. Evidence of 18S ribosomal DNA comments. Ap- References plication and accuracy of molecular phylogenies.
Evidence for a clade evolution and phylogenetic inference. An empirical test of bootstrapping Ahlrichs, W. Zur Ultrastruktur und Phylogenie von Seison as a method for assessing confidence in phylogenetic analysis.
Acanthocephala, Aschelmin- innerhalb der Bilateria. McGraw Hill, New York, pp. Epidermal ultrastructure of Seison nebaliae Kumar S, K. Multicellular Animals, a new approach to the phylo- Li, W. Springer, Berlin, pp.