Sporonchulus ibitiensis (Carvalho, 1951) Andrassy , 1958

Vu, Tam T. T., Nguyen, Anh D., Le, Thi Mai Linh & Pena-Santiago, Reyes, 2024, Updated taxonomy and new insights into the evolutionary relationships of the genus Sporonchulus Cobb, 1917 (Nematoda, Mononchida) after the study of two Vietnamese species, Zoosystematics and Evolution 100 (1), pp. 155-166 : 155

Sporonchulus ibitiensis (Carvalho, 1951) Andrassy, 1958

Mononchus ibitiensis Carvalho, 1951. Syn.

Sporonchuloides ibitiensis (Carvalho, 1951) Mohandas & Prabhoo, 1982.

Material examined.

Four females from one location, in good state of preservation.

Morphometrics.

See Table 1 .

Description.

Female. Moderately slender to slender (a = 28-34) nematodes of medium size, 1.09-1.37 mm long. Body cylindrical, slightly tapering towards the anterior end and more appreciably towards the posterior extremity as the tail is conical. Upon fixation, habitus strongly curved ventrad, C- to G-shaped. Cuticle smooth when observed with LM, but showing very fine transverse striation under SEM, two-layered, 1 µm thick at anterior region, 1.5 µm in midbody and 1.5-2 µm on tail. Lip region almost continuous with the adjoining body, 2.0-2.2 times as wide as high and one-half to two-thirds (53-67%) of body diameter at neck base, with totally fused lips and weakly protruding papillae; SEM observations (Fig. 1D ): oral field comparatively small, with almost hexagonal oral aperture surrounded by six perioral liplets, labial papillae button-like, prominent, cephalic papillae also button-like, but visibly smaller than labial ones. Amphid fovea small, goblet-like, located at 10-11 µm from the anterior end, its aperture a short transverse slit 3-3.5 µm long, occupying up to one sixth (12-16%) of lip region diameter. Vestibulum 4.5-5.5 µm long. Buccal cavity up to twice (1.8-1.9 times) as long as wide, 1.1-1.3 times longer than lip region diameter: vertical (anterior) plates somewhat convergent at their anterior and posterior ends, their walls 1-1.5 µm thick, horizontal (posterior or basal) plates visibly oblique, with foramina, dorsal tooth apex situated at anterior third of buccal cavity (68-74% from the base), anterior subventral plates bearing each two irregular rows of small teeth with decreasing size from the base till the level of dorsal tooth. Anterior end of pharynx embracing the basal part of buccal cavity, gland nuclei obscure except S2N = 81-85%. Nerve ring located at 92-101 µm or ca one-third (31-36%) of the total neck length. Pharyngo-intestinal junction non-tuberculate, 14-18 × 8.5-10 µm. Genital system diovarian, with small and equally developed branches occupying 6-13% of body length: ovaries comparatively large, 50-146 µm long, with oocytes first arranged in several rows and then in only one row; genital tract very short and poorly differentiated, oviduct 35-48 µm long or 1.0-1.2 body diameters, pars dilatata oviductus not enlarged, uterus 17-33 µm long or 0.5-0.7 body diameters, both separated by an indistinct weak sphincter; vagina 11 µm long, extending inwards to one-third (34%) of body diameter, pars proximalis 6.5 × 1.5 µm, pars refringens with two drop-shaped or somewhat trapezoidal sclerotized pieces 2.5 × 2.5 µm and a combined width of 5 µm, pars distalis 1.5 µm, vulva a transverse slit. Rectum 0.8-1.0 times the anal body diameter long. Tail conical with finely rounded tip, regularly curved ventrad, with poorly developed glands and lacking a terminal spinneret (Figs 1 , 2 ).

Male. Not found.

Molecular characterization.

After sequencing and editing, two sequences were obtained for phylogenetic analyses: one full length 18S rDNA with 1591 bp length (GenBank accession N° OQ377123) and one nearly 745 bp length D2D3 of LSU rRNA (28S) (GenBank accession OQ377128).

Locality and habitat.

Vietnam, Yen Bai Province, Tram Tau town (coordinates 21°50'18"N, 104°44'22"E, altitude 930 m) and Ha Giang Province, Du Gia Natural Reserve, (coordinates 22°43'18"N, 105°11'38"E, altitude 780 m) where the nematodes were found in soil around the roots of forest trees.

Voucher specimens.

Permanent slides are stored at the Department of Nematology, Institute of Ecology and Biological Resources, VAST, Hanoi, Vietnam.

Remarks.

Present description provides new data and illustrations of S. ibitiensis , especially useful for comparative purposes. General morphology of Vietnamese females very well fits that of type specimens and other known populations ( Mulvey 1963; Mulvey and Jensen 1967; Lordello 1970; Chaves and Geraert 1977; Mohandas and Prabhoo 1979; Jairajpuri and Khan 1982; Chaves 1990; Loof 2006; Tahseen et al. 2013; Perichi et al. 2021) of the species. Nevertheless, their morphometrics need further analysis. Table 1 shows the most relevant measurements and ratios of females herein studied; meanwhile, Table 2 includes those available from the literature. Especially relevant is the variation observed in buccal cavity length (18-34 µm), an unusually wide range in mononchid species. Actually, Vietnamese specimens display 26-28 µm long buccal cavity, totally comparable or with appreciably overlapping ranges to those reported for Afrotropical ( Mulvey and Jensen 1967; Chaves and Geraert 1977) and Indomalayan ( Mulvey 1963; Mohandas and Prabhoo 1979; Jairajpuri and Khan 1982; Loof 2006) populations, but they differ from some South American specimens ( Carvalho 1951; Chaves 1990), 25-33 vs 18-23 µm, indeed a remarkable difference that might be the result of a biogeographical pattern with two tentative species or subspecies. Nevertheless, a doubt persists over the true identity of several of these populations, which should be resolved before proposing any taxonomical change. Thus, South American females recorded by Carvalho (1951) and Chaves (1990) were not characterised enough for a comparative analysis. The two Brazilian females studied by Lordello (1970) showed 23 and 27.5 µm long buccal cavity. Chaves and Geraert (1977) studied three females from the former Zaire characterised by bearing a terminal subdorsal pore, which might be a remarkable distinguishing trait. Besides, two (more recent) contributions ( Tahseen et al. 2013; Perichi et al. 2021) raise more uncertainties. On the one hand, Tahseen et al. (2013) studied only one Indian female with 22 µm long buccal cavity, which suggests that the species might display more variation in Indomalayan range than previously reported. On the other hand, Perichi et al. (2021) described two Venezuelan populations whose buccal cavities display very wide variation in their length (27-33 and 24-34 µm) and with appreciable difference in their lip region width (26-30 and 20-27 µm, respectively).

Figures

Figure 1. Sporonchulus ibitiensis (Carvalho, 1951) Andrassy, 1958 (Female). A. Entire; B-D. Anterior body region, lateral median (B, C) and lateral submedian (D) view; E. Neck region; F. Anterior genital branch; G. Anterior body region, lateral surface view; H. Caudal region; I. Pharyngo-intestinal junction; J. Vagina. Scale bars: 200 µm (A); 10 µm (B-D, F, H, I); 50 µm (E); 5 µm (G, J).

Figure 2. Sporonchulus ibitiensis (Carvalho, 1951) Andrassy, 1958 (Female, LM). A. Entire; B, C, F, G. Anterior body region, lateral median (B, C) and lateral submedian (F, G) view; C. Lip region (SEM), in face view; E. Neck region; H. Caudal region. I. Vagina; J. Pharyngo-intestinal junction. Scale bars: 200 µm (A); 10 µm (B, C, F, G, H, J); 5 µm (D, I); 50 µm (E).