Enchodelus signyensis Loof, 1975

Elshishka, Milka, Lazarova, Stela, Radoslavov, Georgi, Hristov, Peter & Peneva, Vlada K., 2017, Biogeography and phylogenetic position of Enchodeloidessignyensis (Loof, 1975), gen. n., comb. n. from Maritime Antarctic (Nematoda, Nordiidae), ZooKeys 697, pp. 37-58 : 38-40

Enchodelus signyensis Loof, 1975 Figs 1, 2, 3, 4, 5, 6

Material examined.

Twenty-eight females and twenty-one juveniles (J1-J4) from Livingston and King George Islands (Table 1).

Description.

Measurements. See Table 2-4.

Female. Habitus curved ventrally after fixation, adopting a C-shape. Cuticle consisting of four layers with different refraction, the outer two layers thinner, the second outer with stronger refraction, the inner layers thicker, especially at tail region. Cuticle 2-3 µm thick at postlabial region at the level of the guiding ring, 2-4 µm at mid-body and 4-6 µm on tail; outer layer with very fine transverse striations, innermost layer coarsely striated (Figs 1, 2). Lip region 4-5 μm high angular (following terminology adopted by Peña-Santiago (2006)), offset from the adjoining body by a constriction; about 3 times as wide as high. Based on SEM photographs (Fig. 3), perioral area high, disc-like structure with apparently four elevations surrounding oral aperture, oral aperture appearing cross-like in shape in frontal view. Labial and cephalic papillae prominent; labial papillae button-like, each surrounded by a small ring, their openings pore-like. Inner labial papillae located at distinct elevations; separated from each other, and far from oral aperture and outer labial papillae; divided from the outer labial and cephalic papillae by a circular striation (Fig. 3). Cephalic papillae button-like; outer labial and cephalic papillae below the margin of oral field. Six radial striations beginning from the oral field interrupted by inner and ending at outer labial papillae. Amphidial fovea cup-shaped, its aperture approximately half of lip region diameter, its margin curved; under SEM, the amphidial aperture with an operculum, however the presence of this structure should be confirmed with further studies. Cheilostom a truncate cone with weakly developed walls, its anteriormost part representing a moderately cuticularised perioral ring, appearing as small perioral refractive dots. Odontostyle short and slender, straight, 18-20 times as long as wide, 1.2-1.6 times lip region diameter, aperture 14-16% of its length, 1.2-1.7% of body length. Odontophore 1.2-1.6 times as long as odontostyle, with small swellings at its base. Guiding ring double, located at 0.8-1.0 times lip region diameter from anterior end. Anterior region of pharynx enlarging gradually; pharyngeal expansion 112.5-134 µm, occupying 37-45% of total pharynx length. Location of pharyngeal gland nuclei and their orifices is presented in Table 3. Distance DO-DN 14-19 μm, nuclei of dorsal and second ventrosublateral glands clearly visible, nuclei of first ventrosublateral glands in most specimens indistinct, located slightly behind the middle of the distance DN-S2N (n = 1). Nuclei of dorsal glands 3.5-5 μm diameter, first and second pair ventrosublateral 1 μm and 2-3 μm, respectively. Excretory pore opposite the nerve ring with slightly cuticularised canal clearly visible at 100-112 μm from the anterior end. Cardia rounded conoid. Prerectum 1.7-4.8, rectum 0.9-1.4 times anal body diameter long. Tail bluntly conoid, 2-3% of body length, with numerous saccate bodies. Hyaline part 4-8 μm wide or 12-25% of tail length. Two pairs of caudal pores present. Both branches of female genital system equally and well-developed (in specimens of Livingston Island shorter: anterior 236.2 ± 23.3 (186-275) µm and posterior 208.2 ± 34.4 (143-259) µm long, in specimens from King George Island anterior 298.3 ± 31.9 (245-330) µm and posterior 323.1 ± 46.4 (243-361) µm long). Ovaries short, rarely reaching sphincter level; oviduct with well-developed pars dilatata. Sphincter well developed. Uteri tubular, thick walled, surrounded by hyaline cells along almost the whole length, anterior uterus 104-152 µm long, posterior 105-156 µm long, 2-3 times correspond ing body diameter, not differentiated. Vulva a transverse slit. Vagina extending inwards for 54-76% of body diameter; pars proximalis 19.5 –25×12– 15 μm, pars refringens with two drop shaped sclerotised pieces, with combined width of 11-13 μm, pars distalis 4-5 μm long.

Juveniles. Based on morphometrics of juvenile specimens and the relationships between the lengths of their functional and replacement odontostyles and body lengths, four juvenile stages were identified (Figs 4-7). Habitus in first juvenile stage slightly ventrally curved, lip region flat, continuous with the body, genital primordium 11-12 μm long, tail conical elongated with long central peg (Figs 4-6). Tail in J2 and J3 conoid elongated in J4 bluntly conoid as in females with numerous saccate bodies on tail, c’ decreasing during the successive stages to J4 and females.

Sequences and phylogenetic analyses. The phylogenies based on both gene regions showed that Enchodelus sp. and E. signyensis are parts of two distantly related and well-supported groups (I and II), and in both analyses, they revealed similar relationships with other dorylaimid species (Figs 8, 9). With one exception (AY593052, E. macrodorus (de Man, 1880) from The Netherlands), E. signyensis , was evolutionary close to Pungentus spp. (AY593050, AY593052-53 for D2-D3 28S, and AJ966501 and AY284788 for 18S rDNA) while, Enchodelus sp. from Bulgaria clustered with other Enchodelus spp. from the Netherlands and Iran being a part of well-supported clade including species of various genera ( Eudorylaimus Andrássy, 1959, Epidorylaimus Andrássy, 1986, Prodorylaimus Andrássy, 1959 and Crassolabium Yeates, 1967).

The estimates of evolutionary divergences (p-distances) between D2-D3 28S rDNA sequences within and between both groups are presented in Table 5. The dissimilarity between E. signyensis and other Enchodelus spp. is very high, varying from 16.6% to 17.1% while within group II the distances between sequences are between 0.8-7.1%. The dissimilarity within group I varies from 0.1% to 7.6% with the highest values (7.4-7.6%) estimated from pair-wise comparison of E. signyensis to other sequences within the group. A similar pattern was observed when 18S rDNA evolutionary divergences were analysed. Although having much lower resolution, the 18S rDNA distance of E. signyensis to other Enchodelus species available at NCBI was 2.6-2.8% (or 44-47 nucleotides). This species was the most closely related to two Pungentus spp. from Europe (AJ966501 and AY284788) showing 1.4-1.6% dissimilarity (or 24-26 nucleotides difference). The SNPs analyses of the parsimony-informative sites between sequences for Enchodeloides gen. n., Enchodelus and Pungentus Thorne & Swanger, 1936 and for both genes are given as Suppl. materials 1 and 2.

Discussion.

Based on the main morphological characters, the studied populations are very similar, but specimens from King George Island differ by a somewhat longer (average 1.47-1.66 vs 1.20-1.51 mm), and wider body (55.2 ± 3.5 (50-60) µm vs 48.0 ± 3.9 (39-54) µm), longer female genital branches (anterior 298.3 ± 31.9 (245-330) µm and posterior 323.1 ± 46.4 (243-361) µm vs 236.2 ± 23.3 (186-275) µm and 208.2 ± 34.4 (143-259) µm, respectively, vulva position (V=50.4 ± 0.7 (49.5-51.5)% vs V=54.1 ± 1.3 (51-56)%), and tail (32-39 vs 25-35 µm). The specimens examined generally agree well with data previously reported for this species ( Loof 1975; Andrássy 1998a; Peneva et al. 2002), although some minor differences occurred: our populations have somewhat shorter body length (1.20-1.66 vs 1.37-1.88 mm) and the presence of a moderately developed cuticularised ring around the oral aperture has not been described in those studies. Although E. signyensis resembles members of the genus Enchodelus in many respects, this structure has not been reported for any of its species. The number of morphological characters (see below), as well as molecular data, do not support the current taxonomic position of this species as a member of the genus Enchodelus and therefore a new genus Enchodeloides gen. n. is proposed.