Description of Skrjabinema longicaudatum sp. n.
Small-sized, whitish nematodes. Body cylindrical, maximum width at slightly posterior to mid-body. Cephalic vesicle indistinct in both sexes (Fig. 1a, c). Lateral alae present in both sexes (Figs. 1c and 2a, c, d). Sexual dimorphism prominent in cephalic structure (Figs. 1b and 3a, b, d). Cuticle with remarkable transverse annulations in anterior part of body (Fig. 2d, e). Buccal cavity very small, without cuticular tooth or other ornamentation. Oesophagus divided into short pharynx, cylindrical corpus, indistinct isthmus and ovoid posterior bulb with valves (Fig. 1a). Nerve ring situated at about 1/4 of total oesophageal length (Fig. 1a). Excretory pore located in body wall depression, posterior to oesophago-intestinal junction (Figs. 1a and 2a, b). Deirids not observed.
Male (based on 3 mature specimens): Body 1.92–2.85 (2.38) mm long; maximum width 141–151 (146). Oral aperture simple, triradiate, surrounded by three small, more or less triangular lips with small apical median notch (Fig. 3d). Interlabia or interlabial projections absent (Fig. 3d). Oesophagus 400–454 (427) in total length, representing 15.7–20.9 (17.6)% of body length; pharynx + corpus + isthmus 255‒308 (280) long, size of bulb 143–146 (145) × 131–132 (132). Nerve ring at 170–200 (185) and excretory pore at 760–890 (828) from cephalic extremity, respectively. Lateral alae narrow, extending from about level of nerve ring to anterior region of cloaca (Fig. 1c). Posterior end of body distinctly curved ventrally (Fig. 1h). Spicule single, pointed at distal end, 74–81 (76.7) long, representing 3.15–3.85 (3.43) % of body length (Fig. 1e, g, h). Gubernaculum small, well sclerotized, about 48 long (Fig. 1g, h). Caudal papillae large, three pairs in total, arranged as follows: one pair precloacal, one pair paracloacal and one pair postcloacal (Fig. 1g). Preventral, median finger-like protuberance present (Fig. 1h). Tail 33 long, ending in short finger-like tip (Fig. 1g, h). Phasmids present slightly posterior to cloaca.
Female (based on 10 mature specimens): Body 9.92‒12.1 (11.1) mm long; maximum width 396‒574 (475). Cephalic extremity with three anchor-shaped lips, each lip with two triangular lateral lobes not attached to cephalic rim (Figs. 1b and 3a‒c). Interlabia digitiform, between lateral lobes of lips (Figs. 1b and 3a, b). Four large cephalic papillae and two small amphidial pores present (Figs. 1b and 3a, b). Oesophagus 832‒881 (853) in total length, representing 6.98‒8.78 (7.75) % of body length; pharynx + corpus + isthmus 634‒703 (671) long, size of bulb 168‒188 (181) × 139‒188 (161). Nerve ring at 198‒248 (225) and excretory pore at 1.24‒1.92 (1.77) mm from cephalic extremity, respectively. Lateral alae extending from long distance posterior to base of cephalic extremity and ending at about middle of tail (Fig. 2a, c, d, f, h). Vulva transverse slit, very small, with rudimentary lips observed under SEM, located at 2.97‒3.37 (3.16) mm from cephalic extremity, representing 25.1‒31.1% (28.7%) of body length (Figs. 1d and 3e). Egg asymmetrical, flattened at one side, embryonated or nonembryonated, thick-shelled, with smooth surface, 40‒59 (51) × 20‒40 (30) (n=20) (Figs. 1i; 2i and 3f). Anus with small pre anal lip (Fig. 2g). Tail slender, very long, 2.63‒3.13 (2.90) mm, with pointed tip, representing 23.0‒27.7 (26.3) % of body length (Figs. 1f and 3f). Phasmids not observed.
Taxonomic summary
Type host: Tibetan antelope Pantholops hodgsonii (Abel) (Artiodactyla: Bovidae: Caprinae).
Type locality: Hoh Xil Nature Reserve near Wudaoliang (35°26′N, 93°17′E), Qinghai Province, China.
Site of infection: Caecum and colon.
Prevalence and intensity of infection: A single Tibetan antelope examined with 124 worms.
Type specimens: Holotype: male (HBNU–N-2020M001L); allotype: female (HBNU–N-2020M002L); paratypes: 9 females (HBNU–N-2020M003L) deposited in College of Life Sciences, Hebei Normal University, Hebei Province, China; paratypes: 2 males and 100 females (KLAEPB No.019001) deposited in Key Laboratory of Adaptation and Evolution of Plateau Biota, Northwest Institute of Plateau Biology, Chinese Academy of Sciences, Qinghai Province, China.
Etymology: The specific epithet is derived from a combination of the Latin words longus- (long) and caudatum- (cauda), and refers to the unusually long tail in the female of the new species.
Genetic characterization
Partial 18S region
Three 18S sequences of S. longicaudatum sp. n. obtained herein were all 678 bp in length and represent one genotype. There are two species of Skrjabinema with 18S sequence registered in GenBank, namely S. kamosika (AB699690) and Skrjabinema sp. (EF180060). Pairwise comparison of 18S sequences between S. longicaudatum sp. n. and the two species of Skrjabinema displayed 0.29‒1.18% nucleotide divergence. The 18S sequences of S. longicaudatum sp. n. are deposited in the GenBank database under following accession numbers [MW020179‒MW020181] (http://www.ncbi.nlm.nih.gov).
Partial ITS region
Three ITS sequences of S. longicaudatum sp. n. obtained herein were all 1079 bp in length and represent one genotype. There are two species of Skrjabinema with ITS sequence registered in GenBank, namely S. kamosika (AB699691) and Skrjabinema sp. (AB367796). Pairwise comparison of ITS sequences between S. longicaudatum sp. n. and the other two species of Skrjabinema displayed 20.3‒23.7% nucleotide divergence. The ITS sequences of S. longicaudatum sp. n. are deposited in the GenBank database under following accession numbers [MW020057‒MW020059] (http://www.ncbi.nlm. nih.gov).
Partial 28S region
Three 28S sequences of S. longicaudatum sp. n. obtained herein were all 819 bp in length and represent one genotype. There is only one species of Skrjabinema, namely S. ovis (KY990019) with 28S sequence registered in GenBank. Pairwise comparison of ITS sequences between S. longicaudatum sp. n. and S. ovis displayed 8.36% nucleotide divergence. The 28S sequences of S. longicaudatum sp. n. are deposited in the GenBank database under following accession numbers [MW020098‒MW020100] (http://www.ncbi.nlm. nih.gov).
Partial cox1 region
Three cox1 sequences of S. longicaudatum sp. n. obtained herein was 360 bp in length. There is no species of Skrjabinema with cox1 sequence registered in GenBank. The cox1 sequence of S. longicaudatum sp. n. are deposited in the GenBank database under following accession numbers [MW021552‒MW021554] (http://www.ncbi.nlm. nih.gov).
Phylogenetic analyses
Phylogenetic trees based on the partial 28S sequence data showed that representatives of the family Oxyuridae were divided into three monophyletic clades. Clade I included members of the genera Syphacia, Passalurus, Syphatineria, Syphabulea and Rauschtineria, representing the subfamily Syphaciinae. Clade II contained species of the genera Oxyuris and Skrjabinema, representing the subfamily Oxyurinae. Clade III included species of the genus Trypanoxyuris, representing the subfamily Enterobiinae (Fig. 4). Skrjabinema longicaudatum sp. n. displayed a sister relationship to S. ovis.
Phylogenetic tree constructed based on the 18S+28S+ cox1 sequence data had similar topology to the phylogenetic results using the partial 28S sequence data, in which representatives of the Oxyuridae also divided into three monophyletic clades (Fig. 5). Species of Trypanoxyuris and Enterobius formed clade I, representing the subfamily Enterobiinae. The members of Syphabulea and Syphacia grouped together (clade II), belonging the subfamily Enterobiinae. Clade III included representatives of Oxyuris and Skrjabinema, representing the subfamily Oxyurinae. Skrjabinema longicaudatum sp. n. clustered together with S. kamosika.