| Popis: |
The trophi of rotifers resemble the sclerotized jaws of gnathostomulids, but whether trophi are homologous to gnathostomulid jaws, and consequently show phylogenetic relationship between Gnathostomulida and Rotifera, has been unclear. We have found that the trophi of a rotifer in the genus Seison, which is ranked close to the ancestral stock of rotifers, have an ultrastructural feature similar to that reported in the literature for jaws of both scleroperalian and filospermoidean gnathostomulids, as well as for trophi in the more derived bdelloid rotifer genus Philodina. Specifically, these trophi and jaws have arrays of tube-like support rods composed of lucent material surrounding a dense core. Jaw-like structures in other small vermiform animals (certain polychaetes and molluscs) lack this special feature. We propose that jaw substructure shows a homology, and thus a sister-group relationship, between Gnathostomulida and the clade containing Rotifera plus Acanthocephala. Additional key words: jaw ultrastructure, phylogeny, aschelminth Relationships among the phyla of lower worms are problematic. The monophyletic status of the group Aschelminthes, for instance, is uncertain (Lorenzen 1985, 1995; Ruppert 1991; Garey et al. 1994); and the relationships of the acoelomate phyla-Platyhelminthes, Nemertea, and Gnathostomulida-are no more clear (Rieger & Rieger 1977, 1980; Rieger & Mainitz 1977). Gnathostomulids have been variously aligned with both the aschelminths and acoelomate phyla (Rieger & Mainitz 1977; Ax 1984, 1989; Ruppert 1991). Ax (1984, 1989) has even proposed a taxon "Plathelminthomorpha" to encompass Gnathostomulida and Platyhelminthes as sister groups. Among the aschelminths, the best candidates for sister-group status are Nematoda and Gastrotricha (see Ruppert 1991 for literature) and Rotifera and Acanthocephala (Welsch & Storch 1973; Rieger & Rieger 1977; Storch 1979; Garey et al. 1994). Ties between these two sister groups or between them and other aschelminths (Kinorhyncha, Nematomorpha, Loricifera, Priapulida) are questionable (Nebelsick 1993; Neuhaus 1993); and the most recent nucleic-acid-sequence data actually speak against monophyly of Aschelminthes (Garey et al. |
