The 18S ribosomal RNA gene of Soboliphyme baturini Petrow, 1930 (Nematoda: Dioctophymida) and its implications for phylogenetic relationships within Dorylaimia

Phylogenetic estimations from 18S rDNA sequence data reveal close relationships of dioctophymids with Trichinellida (Trichocephalida), the latter represented by the families Trichinellidae and Trichocephalidae (Trichuridae). This phylogeny is congruent with a scenario of molecular evolution deduced from conservative motifs within the V4 and V9 regions of 18S rRNA. A phylogenetic approach to analyse data containing highly unequal rates of sequence evolution is proposed. The entire gene possesses only a few conservative molecular synapomorphies of a clade consisting of Dorylaimida, Mononchida and Mermithida. Dioctophymida, together with Trichinellida, are inferred as a sister taxon to this clade which jointly constitute the Dorylaimia. Molecular data juxtaposed with morphology were used to reconstruct some of the putative features of the common ancestor of Dorylaimia which is speculated to have possessed a spear and, as found in extant Enoplia and Dioctophymida, pharyngeal gland outlets located in the stoma. Mononchids are postulated to have secondarily lost the spear contrary to all previously published phylogenies. Reduction of caudal glands and transformation of pharyngeal glands into the stichosome are not parsimonious across the tree of Dorylaimia. There are no unequivocal adult morphological synapomorphies for Dorylaimia; the only non-molecular diagnostic feature is the unique specification of the endodermal precursor in early embryogenesis.