Rhyssocolpus iuventutis Andrássy, 1971

Peña-Santiago, Reyes, Guerrero, Pablo, Liébanas, Gracia, García, María del Carmen, Palomeque, Teresa & Lorite, Pedro, 2015, Characterisation of an Iberian population of Rhyssocolpus iuventutis Andrássy, 1971 (Dorylaimida: Nordiidae), with a revised taxonomy of the genus, Nematology 17 (2), pp. 139-153 : 141-146

Rhyssocolpus iuventutis Andrássy, 1971

( Figs 1-4 )

MATERIAL EXAMINED

An Iberian population of 31 females and 11 males was studied, the nematodes being in variable, but often acceptable, states of preservation .

MORPHOMETRICS

See Table 1.

DESCRIPTION

Adult

Moderately slender nematodes of medium size, 1.31- 1.66 mm long. Body cylindrical, tapering towards both extremities, but more so towards posterior end due to conical tail. Habitus curved ventrad upon fixation, C-shaped in female, G-shaped in male. Cuticle two-layered, 1.5-2.5 µ m thick in anterior region, 2.0-5.0 µ m at mid-body and 3.0- 5.0 µ m on tail, outer layer bearing fine transverse striation, almost indistinguishable under LM. Lateral chord occupying 7-27% of mid-body diam., two lateral cervical pores present, arranged close together at ca one lip region diam. posterior to amphidial aperture. Lip region weakly angular, truncate anteriad, nearly continuous with adjacent body, 2.8-4.2 times as broad as high and onefifth to one-third (20-30%) of body diam. at neck base. With SEM, lips amalgamated, oral field circular, separated from remaining lip region by an annular incisure and containing six inner labial papillae, perioral area apparently consisting of four liplets (one dorsal, one ventral and two lateral) covering oral aperture, papillae button-like, each surrounded by a small, ring-like annulus. Amphidial fovea stirrup-shaped, its aperture occupying 54-70% of lip region diam. Cheilostom a truncate cone, with slightly thickened walls. Odontostyle short, attenuated, 7-10 times as long as wide and 0.7-0.8 lip region diam. long, aperture very short, difficult to observe. Guiding ring double, with fixed ring at 6.0-7.0 µ m or 0.5-0.7 lip region diam. from anterior end. Odontophore rod-like, 1.8-2.3 odontostyle lengths long. Pharynx distinctly muscular, enlarging very gradually, basal expansion occupying up to one-third (27-33%) of total neck length, pharyngeal gland nuclei located as follows: DN = 76-81, AS indistinct, PS 1 = 43-61, PS 2 = 45-62. Cardia short, rounded, (5-10) × (8- 13) µ m. Caudal region conical, regularly curved ventrad, with more or less acute tip, its inner core not reaching tail end to form a well defined hyaline portion occupying onefourth to one-half (23-47%) of tail length.

Female

Genital system didelphic-amphidelphic, both branches equally and well developed. Ovaries large, anterior 103- 236 µ m, posterior 108-297 µ m long, often reaching and surpassing sphincter level, oocytes first in two or more rows, then in single row. Oviduct 73-145 µ m long or 1.2-2.9 times corresponding body diam., consisting of a slender part with prismatic cells and a moderately developed pars dilatata often containing sperm cells. Weak sphincter present at oviduct-uterus junction. Uterus bipartite, i.e., consisting of a wider and shorter proximal region with visible lumen and a narrower and longer distal portion with narrow lumen, its length 138-228 µ m or 2.1-3.7 corresponding body diam. long. Vagina extending inwards for ca one-half (40-60%) of body diam., pars proximalis (22-31) × (12-17) µ m in size, with slightly sigmoid walls surrounded by weak musculature, pars refringens with two refractive triangular pieces, each measuring 2-5 µ m long, and with a combined width of 15- 20 µ m, pars distalis 1.5-2.5 µ m long. Vulva a transverse slit ca 10 µ m long. Abundant and very distinct cuticular irregularities (folds, wrinkles, holes, etc.) in perivulval area forming a strip or band around entire body. Prerectum 1.2-2.1 anal body diam. long, usually with transverse, septum-like structures. Rectum 0.7-1.3 anal body diam. long.

Male

Prerectum 2.4-3.3 corresponding body diam. long. Genital system diorchic, testes opposed. In addition to adcloacal pair situated at 14-17 µ m from cloacal aperture, a series of 6-8 irregularly spaced, ventromedian supplements, posteriormost of which lying at 47-72 µ m from adcloacal pair and outside range of spicules. Spicules strong, curved ventrad, 4.6-5.4 times as long as wide. Lateral guiding pieces 10-25 µ m long, ca four times as long as wide.

DIAGNOSIS

Rhyssocolpus iuventutis is characterised by its 1.30- 1.66 mm long body, lip region nearly continuous and 11- 13 µ m broad, odontostyle 7-10 µ m long with very small aperture, odontophore rod-like, guiding ring double, neck 232-307 µ m long, pharyngeal expansion 67-98 µ m long or occupying 24-33% of total neck length, female genital system amphidelphic, pars refringens vaginae with two well developed separate triangular pieces, V = 45-51, very abundant and distinct cuticular irregularities near vulva, tail conical and curved ventrad (31-52 µ m, c = 30- 55, c′ = 1.0-1.5), spicules 50-60 µ m long and 5-8 spaced ventromedian supplements.

DISTRIBUTION

South-eastern Iberian Peninsula, province of Granada, Sierra Nevada National Park, where the specimens were collected near a rivulet leading to the ‘Laguna de las Yeguas’, associated with highland meadows, locally termed ‘borreguiles’ (plant series Genisto baeticae-Festuceto clementei sigmetum), latitude 37.16612°N, longitude 3.585743°E.

REMARKS

This is the first record of the species since its original description, which was based on material from northern Hungary ( Andrássy, 1971) and new details are herein provided for its better characterisation, especially those referring to the morphology of the lip region and the nature of the female genital system. The Iberian specimens perfectly fit the type population in their general morphology and morphometrics with the exception of their less slender body (a = 21-28 vs 27-32 in females; 24-30 vs 30-34 in males), longer tail (35-52 vs 31-35 µ m; c = 30-44 vs 43-55) and longer spicules (54-60 vs 50-54 µ m). These differences are regarded as geographical variation since other key features are identical.

Compared with other Rhyssocolpus species, R. iuventutis is very similar to R. aquilonius Andrássy, 2003 , R. arcticus Ebsary, 1984 , R. vinciguerrae Pedram, Pourjam, Robbins, Ye & Peña-Santiago, 2011 and R. vulvostriatus . It can be distinguished from R. arcticus and R. vulvostriatus by the absence vs presence of well developed cuticular flaps in the vulval region (but see below the morphological comparative analysis under the next section); from R. aquilonius by its more anterior vulva (V = 45- 51 vs 52-54), larger spicules (54-60 vs 41 µ m) and fewer (5-8 vs 11) ventromedian supplements; and from R. vinciguerrae by its larger general size (female body = 1.41- 1.66 vs always under 1.4 mm long), absence vs presence of a dorsal cell mass between the cardia and the end of the anterior ovary/testis, transverse vs longitudinal vulval aperture, comparatively shorter female tail (c′ = 1.0-1.5 vs 1.4-1.9) with more acute vs finely rounded tip, and larger spicules (54-60 vs 43-51 µ m).

The evolutionary relationships of R. iuventutis , as revealed by a phylogenetic tree derived from partial 18S rDNA analysis ( Fig. 5 ), shows it is most closely related to R. vinciguerrae , the only other species of the genus sequenced so far. Both Rhyssocolpus species form part of a moderately supported clade that also includes representatives of Heterodorus , along with several members of the Qudsianematidae and one sequence of Longidorella parva , another member of the Nordiidae .

Figures

Fig. 1. Rhyssocolpus iuventutis Andrássy, 1971 (female, LM). A: Entire; B-D: Anterior region in lateral median view; E: Anterior genital branch; F: Lip region in lateral surface view; G-I: Vagina and vulva, showing cuticular irregularities in vulval area; J: Posterior body region; K-M: Caudal region. (Scale bars: A = 200 µm; B, G-I, K-M = 10 µm; C, D, F = 5 µm; E = 50 µm; J = 20 µm.)

Fig. 2. Rhyssocolpus iuventutis Andrássy, 1971 (male, LM). A: Entire; B-D: Pharyngeal expansion, showing location of DN (B) and S2N (C, D); E: Neck region; F: Posterior body region; G: Caudal region; H, I: Spicules. (Scale bars: A = 200 µm; B-D = 20 µm; E, F = 50 µm; G-I = 10 µm.)

Fig. 3. Rhyssocolpus iuventutis Andrássy, 1971 (male, SEM). A: Anterior region in sublateral view; B: Lip region in face view; C: Caudal region in subventral view; D: Posterior body region; E: Caudal region in ventral view. (Scale bars: A, B = 5 µm; C, E = 20 µm; D = 50 µm.)

Fig. 4. Rhyssocolpus iuventutis Andrássy, 1971 (female, SEM). A: Anterior region in lateral view; B: Same in sublateral view; C: Lip region in face view; D-F: Vulval region; G-I: Caudal region. (Scale bars: A-C = 5 µm; D-F = 20 µm; G-I = 10 µm.)

Fig. 5. Maximum-likelihood tree generated with the K2 + G + I model from the SSU rDNA sequence alignment. The scale bar represents the number of substitutions per site units. In clades the first value corresponds to the bootstrap value calculated using the K2 + G + I model and the second value corresponds to the bootstrap value calculated using the GTR + G + I model. Newly obtained sequences are indicated in boldface.