Molecular evidence for phylogenetic affinities of <i>Rhabdochona coronacauda</i> (Thelazioidea: Rhabdochonidae) with other spiruromorph nematodes

The nematodes isolated from three species of Chanodichthys were completely consistent with the morphological description of Rhabdochona coronacauda. We provide new morphometric data for R. coronacauda, which slightly differ from those of the original description. For the first time, we performed a phylogenetic analysis by using both primary (linear) and secondary (folded structures) sequences of the complete 18S rRNA gene for three superfamilies of Spiruromorpha. The interspecific genetic distances within the genus Rhabdochona were 0.13%-3.06% between 18 species. Rhabdochona coronacauda was sister to a sub-group consisting of the type species R. denudata, R. hospeti, R. hellichi, and R. turkestanica. The secondary structures reconstructed for 35 species of three superfamilies from the order Spiruromorpha comprised 16 conformations of the region including helix 39 and expansion segment 9, and two conformations of helix 17. Helix 39 can be used to differentiate single species or separate species groups of Rhabdochona. The structures of helix 39 in both Rhabdochonidae and Cystidicolidae with Salmonema had an identical 39a domain but differed in the 39b domain and expansion segment 9. In addition, the structure of the helix 39-expansion segment 9 domain within and between families of different superfamilies and their resolution on the phylogenetic tree in combination did not correspond to the accepted classification of spiruromorph nematodes. Helix 17 did not differ within Rhabdochona, or between Rhabdochonidae and Cystidicolidae. At the superfamily level, helix 17 can distinguish Thelazioidea, Spiruroidea, and Habronematoidea from Rhabdochonidae, and Cystidicolidae from Spirocercidae, Thelaziidae, and Pneumospiruridae.