First Record of Two Pseudacrobeles Species (Nematoda: Rhabditida: Cephalobidae) in South Korea

Abstract

Pseudacrobeles(Pseudacrobeles) variabilis(Steiner, 1936) Steiner, 1938 and P. (Bunobus) pseudolatus($Hern{\acute{a}}ndez$, 1990) De Ley, Siddiqi and $Bostr{\ddot{o}}m$, 1993 belonging to the family Cephalobidae Filipjev, 1934 are newly reported from South Korea. Pseudacrobeles (Pseudacrobeles) variabilis is distinguished from its congeners by having distinctly setiform cephalic probolae, three rounded or knob-shaped labial probolae and longer female tail. Pseudacrobeles (Bunobus) pseudolatus is distinguished from its congeners by having visibly lateral lips, a short post-uterine sac, elongated conoid tail in the female, and acute mucro on the tails of both sexes. In this study, details of the morphological characters and morphometrics of Korean populations of P. (P.) variabilis and P. (B.) pseudolatus are described and illustrated based on optical microscopy.

INTRODUCTION

The genus Pseudacrobeles was established by Steiner in 1938. During a taxonomic revision of the genus, De Ley et al. (1993a, 1993b) classified it into the subgenera Pseudacrobeles Steiner, 1938 and Bunobus De Ley, Siddiqi and Boström, 1993 based on morphological characters of the lip region. Pseudacrobeles species have triradiate lips(very rarely with hexaradiate or bilateral symmetry) and labial probolae (if absent the lips are low and amalgamated), whereas Bunobus species have bilaterally symmetric lips, no labial or cephalic probolae, and smaller lateral lips than subdorsal and subventral lips. Although Andrássy (2005) elevated the subgenus Bunobus to genus level separate from the subgenus Pseudacrobeles, we here treat each as a subgenus until further confirmation.

Pseudacrobeles species are found on every continent in the world except Antarctica. Records exist for South America (Loof, 1964; Andrássy, 1968; Zell, 1987; De Ley et al., 1993a, 1993b), Europe (Zell, 1987; Hernández, 1990; Holovachov and De Ley, 2001; Abolafia et al., 2002; Abolafia and Peña-Santiago, 2005; Holovachov and Boström, 2006; Abolafia and Peña-Santiago, 2013), Asia (De Ley et al., 1993a, 1993b; Shokoohi and Abolafia, 2012; Kim et al., 2017), and Africa (Schuurmans-Stekhoven, 1951; Loof, 1964; De Ley et al., 1993a, 1993b). To date, only one Pseudacrobeles species, P. (P.) curvatus, has been reported in South Korea (Kim et al., 2017).

During a survey of several plots of farmland, P. (P.) variabilis (Steiner, 1936) Steiner, 1938 and P. (B.) pseudolatus (Hernández, 1990) De Ley, Siddiqi and Boström, 1993 were collected and isolated from soil samples from pear and potato farms. In this paper, we provide detailed descriptions of the morphological characters and morphometrics of P. (P.) variabilis and P.(B.) pseudolatus from South Korea.

MATERIALS AND METHODS

Nematode isolation

Soil samples were collected from soil below pear trees (Gongdo-eup, Anseong-si, Gyeonggi-do, South Korea [coordinates: 37°01ʹ01.1ʺN, 127°10ʹ23.4ʺE]) and potato farm soil (Buk-myeon, Uichang-gu, Changwon-si, Gyeongsangnam-do, South Korea [coordinates: 35°22ʹ22.5ʺN, 128° 36ʹ47.3ʺE]). Nematode specimens were extracted by sieving and the Baermann funnel method (Baermann, 1917).

Fixation and morphological observations

Each nematode specimen was transferred to 2 mL of water in a 15 mL tube, to which was quickly added 4 mL of 80°C TAF (2% triethanolamine and 7% formaldehyde) for fixation. The fixed nematodes were processed to dehydrated glycerin as described by Seinhorst (1959) and mounted in pure glycerin on HS slides (Shirayama et al., 1993). Nematode morphological characters were observed under an optical microscope (BX-51; Olympus, Tokyo, Japan) equipped with differential interference contrast. Morphometric characters were measured using a CoolSnap Photometrics color CCD digital camera (MP5.0-RTV-R-CLR-10; Photometrics, Tucson, AZ, USA) and the program QCapture Pro 5 (QImaging, Surrey, Canada).

SYSTEMATIC ACCOUNTS

Order Rhabditida Chitwood, 1993

Suborder Tylenchina Thorne, 1949

Infraorder Cephalobomorpha De Ley and Blaxter, 2002

Family Cephalobidae Filipjev, 1934

Genus Pseudacrobeles Steiner, 1938

Subgenus Pseudacrobeles Steiner, 1938

Pseudacrobeles(Pseudacrobeles) variabilis (Steiner, 1936) Steiner, 1938 (Table 1, Fig. 1)

Acrobeles variabilis Steiner, 1936: 76, fig. 23.

Pseudacrobeles variabilis Steiner, 1938: 37, fig. 13c-e.

Pseudacrobeles (Pseudacrobeles) variabilis De Ley et al., 1993a: 223, figs. 1, 2, 3b-f, 3h-l; Abolafia et al., 2002: 148, fig. 6.

Table 1. Morphometrics of Pseudacrobeles (Pseudacrobeles) variabilis and Pseudacrobeles (Bunobus) pseudolatus

Table 1. Continued.

Fig. 1. Pseudacrobeles (Pseudacrobeles) variabilis (Steiner, 1936) Steiner, 1938. A, Entire female; B, Head region; C, Female neck region; D, Female reproductive system; E, Female posterior region; F, Male posterior region; G, Entire male. am, amphid; an, anus; bb, basal bulb; ca, cardia; co, corpus; cpa, cephalic papilla; cpr, cephalic probolae; de, deirid; ep, excretory pore; gp, genital papilla; gu, gubernaculum; in, intestine; is, isthmus; lf, lateral field; lpa, labial papilla; lpr, labial probolae; mu, mucro; nr, nerve ring; ova, ovary; ovi, oviduct; ph, phasmid; pus, post-uterine sac; re, rectum; spe, spermatheca; spi, spicule; st, stoma; te, testis; ut, uterus; va, vagina; vu, vulva. Scale bars: A, G=50 μm, B=5 μm, C, D=20 μm, E, F=10 μm.

Material examined. 2♀♀ and 2♂♂, South Korea: Gyeonggi-do, Anseong-si, Gongdo-eup, Deokbongseowon-ro, 04 Oct, 2017, extracted by sieving and the Baermann funnel method from soil below pear trees. Two specimens(slide Nos. NIBRIV0000754015 [female] and NIBRIV0000754016 [male]) are deposited at the National Institute of Biological Resources, Republic of Korea and two specimens (slide Nos. 01010403001 [female] and 01010403004 [male]) are deposited in the Animal Phylogenomics Laboratory, Ewha Womans University, Republic of Korea.

Measurements. See Table 1.

Description. Adult: Body cylindrical; length 633.5-699.9 μm in females and 598.9-612.1 μm in males; ventrally curved after fixation, C-shaped in females, J-shaped in males. Cuticle annulated; annuli 1.3-1.8 μm wide and 1.2- 1.9 μm thick at mid-body. Lateral field with three incisures, occupying 18.7-21.2% of width of body at mid-body; incisures fading out at phasmid region. Head region continuous with neck. Lip region 7.3-7.6 μm in diameter, with triradiate symmetry, six cephalic probolae distinctly setiform, three labial probolae rounded or knob-shaped, with six labial and four cephalic papillae. Stoma cephaloboid, length 1.7-1.9 times the lip region diameter. Cheilorhabdions bar-shaped. Pharyngeal corpus cylindrical, 3.4-4.0 times the isthmus length. Isthmus narrower than corpus, distinctly demarcated from metacorpus. Basal bulb spheroid, with well developed valves; 1.2-1.3 times as long as its width. Cardia conoid, surrounded by intestinal tissue. Nerve ring located at posterior corpus, at 61.0-63.2% of pharynx length. Excretory pore positioned at corpus-isthmus junction or anterior isthmus, at 67.5-71.7% of pharynx length. Position of deirids in lateral field at isthmus level, at 77.3-80.9% of total neck length.

Female: Reproductive system monodelphic-prodelphic. Vagina 0.3 body-diameters long. Post-uterine sac length 0.7- 0.9 times the body width. Uterus 2.3-2.4 body-diameters long. Spermatheca length same as the body width. Oviduct short. Ovary directed posteriorly, with double flexure. Rectum length 1.3-1.4 times the anal body diameter. Tail elongated conoid. Mucro 8.7-9.2 μm long, with a ragged surface. Phasmids at 25.1-25.7% of tail length.

Male: Genital system monorchic. Testis reflexed ventrad anteriorly. Spicules curved ventrad, 23.3-24.2 μm long; manubrium rounded; calamus as wide as manubrium; without hump and velum; with one longitudinal incisure. Gubernaculum slightly curved ventrad. Two pairs of pre-cloacal and one pair of ad-cloacal subventral papillae present. Post-cloacal genital papillae in five pairs: one subventral and one lateral at mid-tail, one subdorsal near phasmid, two ventral at tail terminus region. Tail conoid, bluntly terminus, with bar-shaped mucro. Phasmid opening at 55.7-62.1% of tail length.

Distribution. Brazil, Kenya, South Korea, Spain, Tanzania, USA.

Habitat. Soil sample from a pear farm.

Remarks. Morphological characters and measurements of the specimens described here perfectly fit the description of P. (P.) variabilis in previous studies, except for the female tail mucro (long and ragged vs. with or without acute mucro) (Steiner, 1936; De Ley et al., 1993a; Abolafia et al., 2002). Intraspecific variation in some morphologies (such as body size, tail length, and mucro shape) among populations of this species has been reported from many geographic areas: North America, Africa and Europe (Steiner, 1936; De Ley et al., 1993a; Abolafia et al., 2002). Although a long ragged mucro has not been reported from any P . ( P.) variabilis so far, the evidence does not justify classifying these specimens as a new species. Pseudacrobe ­ les (Pseudacrobeles) variabilis is reported for the first time from South Korea.

Subgenus Bunobus De Ley, Siddiqi and Boström, 1993

Pseudacrobeles(Bunobus) pseudolatus (Hernández, 1990) De Ley, Siddiqi and Bostr öm, 1993 (Table 1, Fig. 2)

Fig. 2. Pseudacrobeles (Bunobus) pseudolatus (Hernández, 1990) De Ley, Siddiqi and Boström, 1993. A, Entire female; B, Entire male; C, Female neck region; D, Head region; E, Female reproductive system; F, Female posterior region; G, Male posterior region. am, amphid; an, anus; bb, basal bulb; ca, cardia; co, corpus; cpa, cephalic papilla; de, deirid; ep, excretory pore; gp, genital papilla; gu, gubernaculum; in, intestine; is, isthmus; lf, lateral field; ll, lateral lip; lpa, labial papilla; mu, mucro; nr, nerve ring; ova, ovary; ovi, oviduct; ph, phasmid; pus, post-uterine sac; re, rectum; spe, spermatheca; spi, spicule; st, stoma; te, testis; ut, uterus; va, vagina; vu, vulva. Scale bars: A, B=50 μm, C, E-G=10 μm, D=2 μm.

Heterocephalobus pseudolatus Hernández, 1990: 102, fig. 1.

Pseudacrobeles (Bunobus) pseudolatus De Ley et al., 1993b: 290; Abolafia and Peña-Santiago, 2013: 201, figs. 1, 2.

Material examined. 2♀♀ and 2♂♂, South Korea: Gyeong - sangnam-do, Changwon-si, Uichang-gu, Buk-myeon, 25 Jan 2012, extracted by sieving and the Baermann funnel meth - od from potato farm soil. The two specimens (slide Nos. NIBRIV0000754017 [female] and NIBRIV0000754018 [male]) are deposited at the National Institute of Biologi - cal Resources, Republic of Korea and two specimens (slide Nos. 01010402001 [female] and 01010402002 [male]) are deposited in the Animal Phylogenomics Laboratory, Ewha Womans University, Republic of Korea.

Measurements. See Table 1.

Description. Adult: Body cylindrical; length 574.3 -692.2 μm in females and 576.0 -608.4 μm in males; ventrally curved after fixation, C-shaped in females, J-shaped in males. Cuticle annulated; annuli 1.7 -2.4 μm wide and 1.2 - 1.4 μm thick at mid-body. Lateral field with three incisures, occupying 19.1 -26.9% of width of body at mid-body, fad - ing out at phasmid region in female; outer two incisures extending beyond phasmids in males. Head region contin - uous with neck. Lip region 6.2 -7.1 μm in diameter, with bilateral symmetry; two subventral and two subdorsal lips separated, conoid and rounded; two lateral lips sometimes slightly reduced, but visible with a light microscope. Six labial and four cephalic papillae present. Cephalic and labi - al probolae absent. Amphid aperture small, slit-like. Stoma cephaloboid, length 1.7 -2.1 times the lip region diameter. Cheilorhabdions bar-shaped. Tiny dorsal denticle on metas - tom. Pharyngeal corpus cylindrical, 2.9 -3.2 times isthmus length. Isthmus narrower than corpus, distinctly demarcated from metacorpus. Basal bulb spheroid, with well developed valves; 1.2 -1.3 times as long as its width. Cardia conoid, surrounded by intestinal tissue. Nerve ring located at pos - terior corpus or corpus-isthmus junction, at 58.0 -68.9% of pharynx length. Excretory pore positioned at posterior corpus or corpus-isthmus junction, at 61.2-68.5% of pharynx length. Position of deirids in lateral field at isthmus level, at 68.3-76.6% of total neck length.

Female: Reproductive system monodelphic-prodelphic. Vagina 0.3 body-diameters long. Post-uterine sac 0.7-0.9 times the body width. Uterus 1.5-1.8 body-diameters long. Spermatheca inconspicuous, 0.8-1.7 times the body width. Oviduct short. Ovary directed posteriorly, with or without double flexure, with a single row of oocytes. Rectum length 1.2-1.3 times the anal body diameter. Tail elongated conoid, with small acute mucro. Phasmids at 33.8-36.6% of tail length.

Male: Genital system monorchic. Testis reflexed ventrad anteriorly. Spicules curved ventrad, 22.8-23.6 μm long; manubrium rounded; calamus as wide as manubrium; without hump or velum; with one longitudinal incisure. Gubernaculum curved ventrad. Three pairs of precloacal subventral papillae (one pair close to anus level) present. Six pairs of post-cloacal genital papillae: one subventral and one lateral located anterior to phasmid; one lateral located posterior to phasmid; two ventral and one subdorsal located at tail terminus region. Tail conoid, blunt terminus, with spike-shaped mucro. Phasmid opening at 39.2-46.4% of tail length.

Distribution. South Korea, Spain.

Habitat. Soil sample from a potato farm.

Remarks. The Korea population is similar to P. (B.) pseudolatus in having visibly lateral lips, a short post-uterine sac, an elongated conoid tail in the female, an acute mucro on the tails of both sexes, and in the positions of the nerve ring and excretory pore (both on posterior part of corpus). However, our Korean specimens differ from the previous descriptions of Hernández (1990) and Abolafia and Peña-Santiago (2013), in terms of body length (574-692 vs. 400-470 μm in female and 576-608 vs. 343-400 μm in male), body thickness (a=19.3-21.9 vs. 15-19 in female and 21.2-24.3 vs. 17-21 in male), tail length (c=11.2-11.8 vs. 8.4-9.9 in female and c’=3 vs. 3.3-4.5 in female and 2.6-3.4 vs. 2.0-2.5), ratio between corpus and isthmus in males (3.0-3.1 vs. 2.2), spicule length in males (22.8-23.6 vs. 15-20 μm), phasmid position on tail in female (33.8-36.6 vs. 40-42%) and incisures in the lateral field in the male (extending over phasmids vs. fading out at phasmid region). Although some morphometric values between our specimens and previously described populations are different, the degree of difference is not meaningful and the morphologically important characters such as the shape of the head, stoma and tail, and the acute mucro on the tail of both sexes are very similar. Pseudacrobeles (Bunobus) pseudolatus is reported for the first time from South Korea.