Mesodorylaimus aethiopicus, Tabolin & Kolganova & Rybalov & Migunova, 2025

Tabolin, Sergei, Kolganova, Tatyana, Rybalov, Leonid & Migunova, Varvara, 2025, Mesodorylaimus aethiopicus sp. nov. (Nematoda: Dorylaimida), a new species from soil in Bale, Ethiopia, Zootaxa 5701 (3), pp. 382-390 : 383-387

Mesodorylaimus aethiopicus sp. nov.

( Figs. 1 , 2 )

Measurements.

See Table 1 .

Description. Female: Body slightly curved ventrad upon fixation, tapering towards both extremities but more so towards the posterior end. Cuticle without transverse striations, 2–3 µm thick at mid-body and up to 6 µm on tail. Body pores indistinct. Lip region with weakly angular profile, offset by slight depression. Lips amalgamated. Labial and cephalic papillae slightly protruding above the labial contour. Amphids with stirrup-shaped fovea and slit-like aperture, about one-half corresponding body diameter wide. Odontostyle about 1.3–1.4 times the lip region width long, its aperture about two-fifth of its length. Odontophore simple, rod-like, 1.6–1.7 times the odontostyle length. Guiding ring single, at 0.7–0.8 times the lip region width from the anterior end. Nerve ring encircling the anterior slender part of the pharynx, at 33–36% of its length from the anterior end. Position of pharyngeal gland nuclei as follows: DN = 56–59%; S1N1 = 81–83.5%; S1N2 = 82–84 %; S2N = 90–92%, their outlets indistinct. Cardia elongate conoid, about two-thirds of the corresponding body width long. Genital system didelphic-amphidelphic, both branches almost equally developed. Ovaries reflexed, short, not reaching the oviduct-uterus junction, measuring 80–127.5 µm (anterior) and 80–142.5 µm (posterior), oocytes arranged in several rows. Oviduct joining the ovary subterminally, measuring 120–152.5 µm (anterior) and 120–150 µm (posterior), consisting of a slender part and a well-developed pars dilatata oviductus. Sphincter distinct at oviduct-uterus junction, its inner part more refractive, being encircled by a muscular ring. Sperm cells were observed only in the uterus. Vulva longitudinal, slightly sunken. Vagina extending inwards for about two-fifth of the corresponding body width, its back wall sclerotised, pars proximalis vaginae 12–17 µm long, with slightly concave walls, encircled by circular musculature, pars refringens vaginae absent; pars distalis vaginae short, 7–8.5 µm long. Prerectum 2.8–3.3 anal body widths long. Rectum about 1.3–1.7 times the anal body width long. Tail conical at about its anterior 20–25%, its dorsal side slightly more convex, joining the elongate narrower subcylindrical part with rounded tip.

Male: General morphology similar to that of female, body J- or C-shaped after fixation. Genital system diorchic, testes opposed, well developed. Supplements an adcloacal pair and 12–14 regularly spaced ventromedians. Spicules dorylaimoid, 1.35–1.75 times the anal body widths long; lateral guiding pieces rod-like, about one-sixth of spicules length. Mature sperm cells spindle-shaped. Prerectum 3.0–4.3 times the anal body widths long, terminating within the range of supplements. Tail short, convex-conoid with rounded terminus. Caudal pores not observed.

Type habitat and locality. Soil in a forest with the dominance of Hagenia abyssinica Willd. in the Bale Mountains National Park , Ethiopia (GPS coordinates: 6°44′35′′N, 39°42′58′′E).

Type specimens. Slides containing holotype and paratypes of Mesodorylaimus aethiopicus sp. nov. have been deposited in the Nematode Collection of the A.N. Severtsov Institute of Ecology and Evolution, RAS, Moscow (Russia), with the following registration numbers: 51/24 ( holotype) and 51/25–51/27 ( paratypes).

Diagnosis and relationships. M. aethiopicus sp. nov. is characterised by having a medium-sized body 1483– 1858 μm long, the lip region weakly angular and offset by a slight depression, a medium-sized odontostyle (14–17 μm long), a longitudinal vulva slit positioned at mid-body, absence of pars refringens vaginae, a well-developed pars dilatata oviductus, a relatively long female tail (133.5–162.5 μm), male spicules 56.0–63.5 μm long and regularly spaced 12–14 ventromedian supplements. The main distinguishing feature of the new species—among other species of the genus Mesodorylaimus —is the structure of its reproductive system.

Species with similar body length and tail length values were selected for differential diagnosis.

From Mesodorylaimus aberrans Loof, 1969 , the new species differs in having a shorter odontostyle (14–17 vs 18–20 µm), a longitudinal vulval slit and longer spicules (56–63 vs 46–54 µm).

From M. ghanae Andrássy, 1965 , it differs in having a lower ‘V’ ratio (48.0–51.2 vs 53–55%), a longitudinal vulval slit, longer spicules (56–63 vs 40 µm) and a higher number of ventromedian supplements (12–14 vs 8).

From M. litoralis Loof, 1969 , it differs in having a longer odontostyle (14–17 vs 11–12 µm), a lower ‘b’ ratio (4.3–5.1 vs 4.8–6.1 for females and 4.05–4.8 vs 5.0–5.6 for males), a lower number of ventromedian supplements (12–14 vs 15–18) and longer spicules (56.0–63.5 vs 43–48 µm).

From M. macrospiculum Zullini, 1988 , it differs in having a longitudinal vulva, a lower ‘b’ ratio (4.3–5.1 vs 5.5–6.8 for females and 4.05–4.8 vs 5.3–6.2 for males), a higher ‘V’ ratio (48.0–51.2 vs 40–45%) and a lower number of ventromedian supplements (12–14 vs 18–20).

From M. meyli ( Andrássy, 1958) Andrássy, 1959 , the new species differs in having a shorter odontostyle (14–17 vs 17.6–19.9 µm), a longitudinal vulval slit and a higher number of ventromedian supplements (12–14 vs 9–11).

From M. noreasus Uzma & Ahmad, 2010 , it differs in having a longitudinal vulva, a lower ‘a’ ratio (26.5–36 vs 40.2–43.4 in females), longer spicules (56–63 vs 42–43 µm) and a higher number of ventromedian supplements (12–14 vs 9).

From M. imperator Loof, 1975 , it differs in having a longer body (1483–1858 vs 1190–1500 µm for females and 1409.4–1744.2 vs 1130–1490 µm for males), the absence of pores in the spear area and cuticular folds in the vulval region.

From M. szechenyii Andrássy, 1961 , it differs in having a greater ‘V’ ratio (48.0–51.2 vs 43.9–46.5%), an absence of pars refringens vaginae, longer spicules (56–63 vs 41 µm) and a lower number of ventromedian supplements (12–14 vs 15).

In addition to the species listed above, Mesodorylaimus aethiopicus sp. nov. is similar to different species of the genus Calcaridorylaimus Andrássy, 1986 , especially C. signatus ( Loof, 1975) Andrássy, 1986 and C. andrassyi Ahmad & Shaheen, 2004 . From the first, it differs by the structure of the female reproductive system (an absence of pars refringens vaginae, well-developed pars dilatata oviductus) and the shape of male spicules. From the latter, it differs in having a lower ‘a’ ratio (26.5–36 vs 42–46 in females), a longitudinal vulval slit ( vs porelike), an absence of pars refringens vaginae and a higher number of ventromedian supplements (12–14 vs 10). However, in our opinion, the new species should belong to the genus Mesodorylaimus . The shape of spicules of Mesodorylaimus aethiopicus sp. nov. is similar to those of the type species Mesodorylaimus mesonyctius ( Kreis, 1930) Andrássy, 1986 and some other species of the genus Mesodorylaimus (e.g., M. aequatorialis Andrássy, 1986 , M. arcuatus Andrássy, 1986 , M. aduncus Andrássy, 1986 ) except for longer humped part. Also, the posterior end of male body is not strongly twisted.

Molecular characterisation. High-quality sequences of the 18S rRNA gene and the D2–D3 expansion segments of the 28S rRNA gene were obtained in all three samples. The sequences obtained from different individuals in this study were identical within each marker. After removing low-quality segments at the 5ʼ and 3ʼ ends, a partial sequence of the 18S rRNA gene consisting of 1673 bp and a partial sequence of the 28S rRNA gene consisting of 752 bp were obtained.

The 18S rRNA gene sequence obtained in this study was the most similar to Calcaridorylaimus sp. from China ( MG921242 ), with 98.68% identity, and Mesodorylaimus sp. also from China ( MG921251 ), with 98.63% identity. Other 18S rRNA gene sequences deposited in GenBank with similarity above 98% belonged to representatives of the genera: Amblydorylaimus , Allodorylaimus , Aporcelaimellus , Aporcelinus , Ecumenicus , Eudorylaimus , Rhyssocolpus . The identity with the sequences of morphologically similar species C. signatus was only 98%.

In the 18S rRNA gene phylogenetic tree ( Fig. 3 ), the sequence of M. aethiopicus clustered together with the sequences of Calcaridorylaimus spp. from China ( MG921242 and MG921244 ), Calcaridorylaimus signatus from Signy Island ( LC457653 and LC457654 ) and Mesodorylaimus cf. nigritulus from Belgium ( AJ966490 ).

The 28S rRNA gene sequence of Mesodorylaimus aethiopicus n.sp. was most similar to the sequences of Mesodorylaimus subtilis from China ( MG921249 and MG921250 ), with 94.97% identity, and Calcaridorylaimus heynsi from South Africa ( MH062988 ), with 94.19% identity. Also, it had a 97.56% identity with the sequences of Mesodorylaimus bastiani from Germany ( MF325233 – MF325239 ). However, the use of these sequences in the phylogenetic analysis is impossible, because they belong to the segments D3–D5, while the sequence of M. aethiopicus sp. nov. belongs to the segments D2–D3.

In the 28S rRNA gene phylogenetic tree ( Fig. 4 ), the sequence of M. aethiopicus clustered on a separate branch apart from M. pini and M. subtilis .

It should be noted that accurate phylogenetic inferences cannot be drawn with absolute certainty because a small number of sequences of the genus Mesodorylaimus are currently available in GenBank.

Figures

FIGURE 1. Mesodorylaimus aethiopicus sp. nov. A: Entire female. B: Entire male. C: Anterior region. D: Expanded part of pharynx. E: Anterior end showing amphid. F: Male posterior region. G: Vulval region (subventral view). H: Female posterior region. I: Female genital branch (posterior).

FIGURE 2. Mesodorylaimus aethiopicus sp. nov. A: Anterior region. B: Vulval region. C: Female posterior region. D: Male posterior region.

FIGURE 3. Phylogenetic relationships of Mesodorylaimus aethiopicus sp. nov. with molecularly close sequences of different representatives from the family Dorylaimidae as inferred from the ML method using the 18S rRNA gene under the GTR + I + G model.

FIGURE 4. Phylogenetic relationships of Mesodorylaimus aethiopicus sp. nov. with molecularly close sequences of different representatives from the family Dorylaimidae as inferred from the ML method using the D2–D3 expansion segments of the 28S rRNA gene under the GTR + I + G model.