Epidermal ultrastructure of Seison nebaliae and Seison annulatus, and a comparison of epidermal structures within the Gnathifera

Abstract

The epidermis of both species of Seison is syncytial and has a characteristic internal layer divided into two distinct sublayers. Sublayer I is very thin (0.03 µm) and bounded to the outer cell membrane of the epidermis. Sublayer II is 0.5 µm thick and separated from sublayer I by a thin layer of cytoplasm. Intrusions of the outer cell membrane of the epidermis perforate the internal layer, before terminating within the cytoplasm.

A modern approach to rotiferan phylogeny: Combining morphological and molecular data

“…In spite of the incongruity between the parsimony and Bayesian analyses, the present results still allow us to evaluate the proposed synapomorphies for both Seisonidea + Acanthocephala (Ahlrichs, 1995a(Ahlrichs, , 1997 and Eurotatoria + Acanthocephala , as discussed above. In all analyses that included molecular data, the reduction of a sperm acrosome would have been homoplastic, which supports the questionable nature of the character.…”

“…6 and 7 in Dunagan and Miller (1991) with Fig. 2 in Ahlrichs (1997)], hence we prefer to treat the homology between Wbers in acanthocephalans and seisonids as uncertain until more data become available.…”

“…Morphological data have suggested various combinations of sister-group relationships between Acanthocephala and the three main rotifer clades, Seisonidea, Bdelloidea, and Monogononta Abbreviations: fu, fulcrum; ma, manubrium; im, intramalleus; pc, posterior manubrial chamber; ra, ramus; sm, supramanubrium; un, uncus. (Lorenzen, 1985;Ahlrichs, 1995aAhlrichs, , 1997Ahlrichs, , 1998Ferraguti and Melone, 1999;Sørensen et al, 2000), but all these have been based on dubious homology statements or contradictory details in sperm anatomy (Melone et al, 1998a;Ricci, 1998b;Funch et al, 2005). Hence, morphological data cannot at present provide critical support for Acanthocephala as a rotifer ingroup, although a steadily growing amount of molecular data strongly support its inclusion (Garey et al, 1996Near et al, 1998;Giribet et al, 2000Giribet et al, , 2004Miquelis et al, 2000;Near, 2002).…”

The interrelationships of metazoan parasites: a review of phylum- and higher-level hypotheses from recent morphological and molecular phylogenetic analyses

“…It is long acknowledged that Acanthocephala and Rotifera are closely related to each other (Lorenzen 1985, Nielsen 1995, Wallace et al 1996, forming the clade of Syndermata (Ahlrichs 1997(Ahlrichs , 1998=Trochata;Cavalier-Smith 1998a). Monophyly of the Syndermata (Fig.…”

“…However, the sequence of the third rotifer group, the Seisonida, was lacking at that time, and the remaining two groups, Monogononta and Bdelloida, were represented by a single species each. The phylogenetic position of the Seisonida became more interesting after detailed ultrastructural studies by Ahlrichs (1993Ahlrichs ( , 1997Ahlrichs ( , 1998 that revealed obvious acanthocephalan-like features in Seison spermatozoa and integument (see Ahlrichs 1998, Ferraguti an Melone 1999. So far, the seisonids were regarded either as basalmost rotifers (Pararotatoria vs. Eurotatoria, the latter group including monogononts and bdelloids), because of their poorly developed rotatory organs, spermatozoa with acrosomes, and a weak sexual dimorphism (see also Funch 2000, Sørensen et al 2000), or as aberrant monogononts (Clément 1993).…”