Morphology, genetic characterization and molecular phylogeny of Xystrognathus priscus (Nematoda: Rhigonematomorpha)

The infraorder Rhigonematomorpha is a unique group of zooparasitic nematodes commonly occurring in the hindgut of millipedes. Our present knowledge of the composition of the Rhigonematomorpha nematode fauna in China is far from complete. The current genetic data base for Rhigonematomorpha remains very limited, and the molecular phylogeny of Rhigonematomorpha is still in its beginning phase. In the present study, Xystrognathus priscus was redescribed using light microscopy and, for the first time, scanning electron microscopy based on the freshly collected specimens from Litostrophus scaber (Spirobolida: Pachybolidae) in China. SEM observations showed the detailed morphology of some structures for the first time, including cephalic structures bearing 1 pair of peg-like submedian cephalic papillae and 1 pair of amphidial apertures in ventral jaw, cuticular spines (microtrichs) varied from saw-tooth blade-like at anterior of body to tooth-like at posterior of body, excretory pore without cuticular spines surrounded, caudal papillae nipple-shaped, vulva without anterior vulval flap, egg with smooth surface and oval phasmid. The present study is only the second record of X. priscus, and also represents the first record of the species of the genus Xystrognathus and family Ichthyocephalidae in China. The partial 18S, 28S, ITS, cox1 and cox2 sequences of X. priscus were provided for the first time to enrich the genetic data of Rhigonematomorpha, and enabled the molecular identification of Xystrognathus nematodes. Additionally, phylogenetic results based on the partial 18S + 28S sequence data indicated that the families Ichthyocephalidae, Carnoyidae, Rhigonematidae and Xustromatidae in Rhigonematomorpha are all non-monophyletic, but strongly supported the monophyly of the genus Xystrognathus, and suggested a close affinity between Xystrognathus and the genus Obainia belonging to the Xustrostomatidae.