Phylogenetic analysis of the genus Cylicocyclus (Nematoda: Strongylidae) based on nuclear ribosomal sequence data.

Seven species of Cylicocyclus Ihle, 1922 (Nematoda: Strongylidae) were collected from donkeys from Henan Province, China. Five samples of each species were selected for sequencing. Sixteen different internal transcribed spacer (ITS) sequences representing the seven species of Cylicocyclus were obtained. Sequence differences in the first internal transcribed spacer (ITS-1) among species was lower than that of the second internal transcribed spacer (ITS-2). Phylogenetic analyses were conducted using the combined ITS-1 and ITS-2 data sets from the present study and using reference sequences from the GenBank database. The MP and ML trees were similar in topology. The phylogenetic trees were divided into two clades. Clade I included 8 species of Cylicocyclus; within this group, Cylicocyclus leptostomus (Kotlan, 1920) is nested between different samples of Cylicocyclus ashworthi (LeRoux, 1924), suggesting C. ashworthi may represent a species complex. Clade II included Cylicocyclus elongatus (Looss, 1900) and Cylicocyclus ultrajectinus (Ihle, 1920); however, these two species always clustered with the comparative species (Petrovinema poculatum (Looss, 1900) and Poteriostomum imparidentatum Quiel, 1919), suggesting that C. elongatus and C. ultrajectinus represent members of other genera.