Panagrellus ceylonensis Hechler, 1971

Kumar, Nitesh & Chaubey, Ashok Kumar, 2023, Redescription and molecular characterization of Panagrellus ceylonensis (Nematoda, Rhabditida, Panagrolaimidae) from India, Zootaxa 5383 (4), pp. 491-513 : 493-507

Panagrellus ceylonensis Hechler, 1971

( Fig. 1–2 )

Material examined: 15 females and 15 males obtained from culture.

Measurements

Morphometric measurements of the Indian population of Panagrellus ceylonensis are provided in Table 1 .

Description

Adult

Body cylindrical, small size, 1.25–1.48 mm long in females and 0.95–1.30 mm long and tapered at both ends. Habitus almost straight or slightly curved ventrally upon fixation in females while in males it is ventrally arcuate or curved ventrad in posterior body region. Cuticle 1–2µm thick with fine transverse striations. Lateral fields distinct with four longitudinal incisures or three ridges (alae). Lip region 8–11µm wide, slightly offset with the body contour. Oral opening large, circular, and surrounded by lips. Lips six, rounded, radially arranged (two lateral, two subventral and two subdorsal), separated and with protruding labial sensilla. Four cephalic sensilla bilaterally arranged and submedian in position. Amphids small, oval, located at lateral lips. Stoma panagrolamoid: cheilostom large with wide lumen posteriorly, without refringent rhabdia; gymnostom shorter than cheilostom, with conspicuous small rhabdia; stegostom funnel-shaped, narrower posteriorly, with poorly refringent rhabdia.Pharynx also panagrolamoid: pharyngeal corpus robust, cylindrical, 65–90 µm long, 1.8–3.0 times the isthmus length; isthmus distinct, slender, comparatively thin, 26–44 µm long, forming 20–28% of pharynx length; basal bulb ovoid, with valvular apparatus and well-developed posterior duplex haustrulum. Esophago-intestinal junction or cardia short, well-developed, surrounded by intestinal tissue. Nerve ring surrounding the mid-part of isthmus, situated at 59–72% of the neck length from the anterior end. Excretory pore ventrally located at level of basal bulb or at 82–95% of neck length. Hemizonid located at level of bulbus. Deirids situated at 80–86% of neck length, at level of isthmus-bulbus junction. Intestine with slightly thinner walls at cardiac (anterior) and rectal (posterior) regions.

Female

Reproductive system mono-prodelphic. Ovary very long, antidromously reflexed dorsally, with post-rectal extension mostly with double flexure at its distal end, positioned on the right side of intestine, occupying about 67–86% of body length. Oviduct short. Uterus long, measuring 588–738 µm, 6.6–9.6 times the body diameter, filled with intrauterine eggs of 34–40 × 21–26 µm dimensions in single row or in double row having different developmental stages. Post-vulval uterine sac (PUS) short, well developed, thin walled, wide at posterior part, connected to the vagina and uterus by a short, narrow, tubular part, 1.7–2.6 times vulval body diameter long. Vagina anteriorly directed, extending inwards obliquely, about 40–60% of body diameter and surrounded by heavily muscled wall. Vulva a transverse slit, slightly protruding, located nearly two-third of the body length. Distance from vulva to anus 1.3–2.0 times the tail length. Rectum distinct, 0.8–1.5 times anal body diameter long, with three glands at its junction with the intestine. Tail elongate-conoid, with acute terminus, 4.7–6.7 times the anal body diameter long. Phasmids at 36–50% of tail length.

Male

Reproductive system monorchic with testis reflexed ventrad anteriorly. Spicules paired, of equal length, separate, ventrally curved and 2.1–3.4 times as long as body diameter at level of cloacal aperture. Manubrium (head) hook-shaped, curved ventrad anteriorly, with a large latero-dorsal cytoplasmic core opening on its outer lower part; calamus (shaft) very short; lamina (blade) ventrally curved, lacking dorsal hump, with a dorsal deflection at about 60% of spicule length, small fork-like bifurcated tip (7–11% of spicule length) and ventral velum, arising from shoulder level and extending nearly to terminus. Gubernaculum well developed, slightly curved ventrad, with thin corpus, about 0.4 times of the spicule length. Tail elongate-conoid, with acute terminus, 3.5–5.7 times the cloacal body diameter long. Seven pairs of genital papillae (GP) in 1+1+1/1+2+1 configuration with three pre-cloacal (GP1–GP3) and four post-cloacal (GP4–GP7) pairs: four pairs subventral (GP1, GP2, GP5 and GP6), one pair lateral (GP3), two pairs subdorsal (GP4 and GP7). Of the genital papillae, GP1 at the level of manubrium; GP2 at the level of dorsal deflection of lamina; GP3 at level of cloaca, appear close to gubernaculum; GP4 forms a group with GP5 and GP6; GP5 close to GP6; GP7 at beginning of filiform part of tail. One single mid-ventral papilla (MP) present at adcloacal level, close to GP2. Phasmids located close to GP7, at 34–46% of tail length from cloacal aperture.

Taxonomical Remarks

The material examined in this study represents the first report of Panagrellus ceylonensis from India and agrees perfectly with the type material as described by Hechler (1971) from Sri Lanka, especially in the morphology of spicule (chief diagnostic characteristic to identify this species) and also slightly with population described as Anguillula rediviva by Rühm (1956) from Germany, in spicule length and presence of excretory pore at bulbus level. According to Abolafia & Vecchi (2021), the German population of Anguillula rediviva described by Rühm (1956) is very similar to P. ceylonensis and probably the Rühm’s material is its synonym. Hechler (1971) described the species on the basis of morphology in the original description but unfortunately, most of the measurements were not provided in the study. Also , the post-vulval uterine sac (PUS) and genital papillae (GP) illustrated but not described by the author. Later , Stock and Nadler (2006) characterized this species from Vietnam, morphologically and molecularly using 28S rDNA only, but the study lacks the morphometrics of female specimens and drawing illustrations. Also , with respect to the morphology of the spicules illustrated in their study, Abolafia & Vecchi (2021) observed some inconsistencies regarding the identity of spicules, where the spicule named as P. ceylonensis ( Fig. 3B ) is more similar to P. redivivus (slightly bent manubrium, wide lamina bearing slightly developed dorsal hump and long bifurcated terminus) and the spicule named as P. redivivus ( Fig. 3D ) agrees with P. ceylonensis (ventrally curved hook-shaped manubrium, ventrally bent slender lamina without dorsal hump and short bifurcated terminus). Furthermore , the morphometric data ( Table 2 ) is also contradictory. Because of this contradiction, the illustration of spicule ( Fig. 3D ) is measured again to obtain correct morphometry (71 µm long spicule and 5.7 µm spicule bifurcation length) for precise identification of the species. However , the genetic make up of Vietnam isolate is similar to the Indian population of P. ceylonensis . Compared to P. redivivus described by Rühm (1956) from Germany, it differs by only in length of male tail (102–140 µm vs 140–154 µm).

Diagnosis of Indian Population

The material examined of P. ceylonensis from India is characterized by having 1.25–1.48 mm long body in females, 0.95–1.30 mm long body in males, cuticle with fine transverse striations, lateral field with four incisures, lip region slightly offset with body contour, stoma panagrolamoid with wider cheilostom, reduced gymnostom and funnel-shaped stegostom, pharyngeal corpus cylindrical with slightly swollen posterior part, excretory pore at basal bulb level, female reproductive system mono-prodelphic with antidromously reflexed ovary and well developed post vulval uterine sac, female tail conoid-elongate (129–181 µm long, c = 7.8–11, c’= 4.6–6.6), male tail conoid-elongate (102–140 µm long, c = 7.2–9.9, c’ = 3.5–5.7), spicule 71–91µm long, with ventrally curved hooked manubrium having a large latero-dorsal cytoplasmic core opening on its outer lower part, lamina ventrally curved with dorsal deflexion at about 60% of spicule length and short bifurcated lamina tip, gubernaculum 26–31 µm long, genital papillae seven pairs: two subventral precloacal, one lateral precloacal, two subventral postcloacal and two subdorsal postcloacal.

Relationships

The chief diagnostic characteristic of this genus is the spicule morphology. With respect to the spicule morphology, Panagrellus ceylonensis is similar to P. pycnus , P. leperisini , P. silusioides , P. redivivus and P. dubius , since they all have ventrally curved spicules with ventrally bent hook-shaped manubrium and well-developed bifurcated lamina tip. However, these species can be differentiated from each other by curvature degree of manubrium, shape of lamina, and length of bifurcation at tip. P. ceylonensis is distinguished from all these species by its ventrally curved lamina with dorsal deflexion at about 60% of spicule length ( Fig. 3 ). Altogether, it differs from all other species by having more posterior excretory pore (at bulbus level vs metacorpus level or isthmus level).

Based on morphological and morphometrical characteristics, P ceylonensis can be differentiated from P. dubius , by having longer body length (949–1305 µm vs 760–1010 µm in males, 1250–1481 µm vs 980–1200 µm in females), higher b ratio (7.2–9.2 vs 4.7–5.7 in males and 8.6–10.9 vs 5.4–6.0 in females), more posterior excretory pore position (at bulbus level vs isthmus level), longer spicule length (71–91 µm with small bifurcation vs 57–72 µm long, C-shaped with extremely large bifurcation), longer female tail length (129–181 µm vs 92 µm). From P. filiformis , by having larger male body (949–1305 µm vs 780 µm), more posterior excretory pore position (at 82–95% of neck length or at bulbus level vs 70% of neck length or at isthmus level), longer spicules (71–91 µm vs 22 µm) and gubernaculum (26–31 µm vs 14 µm). From P. leperisini , by having longer body length (949–1305 µm vs 740–920 µm in males, 1250–1481 µm vs 820–970 µm in females), more posterior excretory pore position (vs 80% of neck length or at isthmus level), longer spicule length (71–91 µm vs 60 µm), longer tail length (102–140 µm vs 91 µm in males, 129–181 µm vs 129 µm in females). From P. pycnus by having slightly narrower stoma, more posterior excretory pore position (at 82–95% of neck length or at bulbus level vs 52–75% or at metacorpus level). From P. redivivus , by having more posterior excretory pore position (at 82–95% of neck length or at bulbus level vs 70–82% or at level of isthmus), longer spicule length (71–91 µm long with small bifurcation vs 36–70 µm with large bifurcation and slightly developed dorsal hump). From P. silusioides , by having smaller body length (949–1305 µm vs 1700–2100 µm in males, 1250–1481 µm vs 2200–2600 µm in females), more posterior excretory pore position (at 82–95% of neck length or at bulbus level vs 77% of neck length or at isthmus level), longer spicule length (71–91 µm vs 53–62 µm long), longer gubernaculum length (26–31 µm vs 9–13 µm) ( Table 2 & 3 ).

Molecular characterization and phylogenetic analyses

For molecular characterization, isolate HNK14 was identified using 18S and 28S genes. For 18S, a fragment of 916 bp (Accession no. OP393928) and for 28S, a fragment of 725 bp (Accession no. OM367902) were obtained for the Indian P. ceylonensis isolate HNK14. The Indian P. ceylonensis population has G+C content of 46.6% and A+T content of 53.4% in the 18S rDNA and G+C content of 50.1% and A+T content of 49.9% in the D2D3 rDNA. The obtained sequences of the present isolate (HNK14) were compared with already available sequences of other Panagrellus species which showed a high level of rDNA similarity. For 18S rDNA, there is no record available in the GenBank for P. ceylonensis , which is now sequenced for the first time. However, for 28S rDNA, only one sequence is available in the GenBank for P. ceylonensis (DQ408251) from Vietnam. When the sequence of D2D3 region of 28S rDNA of the present isolate is compared with the Vietnam population, it shows 99.86% similarity (only one bp difference).

18S rDNA and 28S rDNA sequences reveal clear genetic differences between the species of Panagrellus genus. With respect to the 18S rDNA sequence alignment, the present isolate showed 16 nucleotide difference and 3 gaps with P. pycnus (MZ656001), 16 nucleotide difference and 2 gaps with P. redivivus (AF083007), 17 nucleotide difference and 5 gaps with P. levitatus (KY126845), and 43 nucleotide difference and 7 gaps with P. redivivoides (MH608262). However, with respect to the 28S rDNA sequence alignment, present isolate share showed 42 nucleotide difference with P. pycnus (MZ656000) and P. dubius (DQ408252), 43 nucleotide difference with P. redivivus (DQ408250), and 67 nucleotide difference and 1 gap with P. redivivoides (MH608298).

The phylogenetic analysis based on 18S rDNA sequences ( Fig. 4 ) shows that the present isolate formed sister clade with other species of Panagrellus genus and the phylogenetic analysis based on 28S rDNA sequences ( Fig. 5 ) shows the Panagrellus species studied now formed a well-supported monophyletic clade with the already available sequence of P. ceylonensis from Vietnam ( Stock & Nadler, 2006) and also formed sister clade with other species of this genus.

Figures

FIGURE 1. Panagrellus ceylonensis Hechler, 1971 from India. (A) Neck. (B) Anterior end. (C) Entire male. (D) Entire female. (E) Vagina and post-vulval uterine sac. (F) Male posterior end (black arrow showing genital papillae (GP) and Mid-ventral papilla (MP) and grey arrow (Ph) showing phasmid). (G) Female posterior end. (H) Spicule.

FIGURE 2. Panagrellus ceylonensis Hechler, 1971 from India (light microscopy). (A) Neck. (B) Anterior end. (C) Entire male. (D) Entire female. (E) Vagina and post-vulval uterine sac. (F) Male posterior end (black arrow showing genital papillae (GP) and Mid-ventral papilla (MP) and white arrow showing phasmid). (G) Female posterior end. (H) Spicule.

FIGURE 3. Morphology of the spicules and gubernacula of Panagrellus species with curved or hooked shaped manubrium. (A) P. ceylonensis (redrawn from Hechler, 1971). (B) P. pycnus (redrawn from Thorne, 1938). (C) P. leperisini (redrawn from Massey, 1974). (D) P. silusioides (redrawn from Tsalolikhin, 1965). (E) P. redivivus (redrawn from Goodey, 1945). (F) P. dubius (redrawn from Sanwal, 1960).

FIGURE 4. Phylogenetic tree based on 18S rDNA sequences of Panagrellus ceylonensis isolate HNK14 and other species of Panagrellus genus. The tree was constructed by using Maximum Likelihood method based on Tamura 3-parameter model. The percentage of trees in which the associated taxa clustered together is shown next to the branches. Scale bar shows the number of substitutions per site. Steinernema feltiae was used as outgroup taxa. NCBI accession numbers of the sequences used for the analyses are shown.

FIGURE 5. Phylogenetic tree based on 28S rDNA sequences of Panagrellus ceylonensis isolate HNK14 and other species of Panagrellus genus. The tree was constructed by using Maximum Likelihood method based on Kimura 3-parameter model. The percentage of trees in which the associated taxa clustered together is shown next to the branches. Scale bar shows the number of substitutions per site. Steinernema abbasi was used as outgroup taxa. NCBI accession numbers of the sequences used for the analyses are shown.