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Citation Information : Journal of Nematology. Volume 50, Issue 1, Pages 69-76, DOI: https://doi.org/10.21307/jofnem-2018-013
License : (PUBLISHER)
Accepted: 07-July-2017 / Published Online: 31-May-2018
Three populations of neotylenchid nematodes were isolated in Ningbo, P. R. China, from white pine lumber (
Thorne (1941) grouped tylenchid nematodes without a valvular median bulb into three subfamilies, namely Neotylenchinae, Paurodontinae, and Nothotylenchinae. In 1949, Thorne combined all these subfamilies under the family Neotylenchidae when the order Tylenchida was proposed (Thorne, 1949). Although for several decades, the status of the family Neotylenchidae had many changes (Jairajpurmi and Siddiqi, 1969; Husain, 1972), the family and its placement under Tylenchida were mostly accepted. Most disagreements have been related with the subfamily level classification, and the genera within the family, which were predicted by Thorne in 1949 when he proposed the family, partly due to the fact that many species have morphologically different entomophagous and non-entomophagous forms. These disagreements have been well summarized by Fortuner and Raski (1987), who also stated that those complexities should not discourage from descriptions of the new taxa.
Siddiqi (2000) placed the family Neotylenchidae along with Allantonematidae and Sphaerulariidae under suborder Hexatylina within the order Tylenchida, and placed Hexatylus Goodey, 1926 Deladenus Thorne, 1941, Gymnotylenchus Siddiqi, 1961, Fergusobia Currie, 1937, and Rubzovinema Slobodyanyuk, 1991 within the family Neotylenchidae based on the fact that all these genera possess two life forms: entomophagous and non-entomophgaous. The non-entomophagous forms had been well studied in this family and shared significant characters such as the absence of isthmus, the esophageal glands arranged as long lobes free in the body cavity, and the pharyngo-intestine junction at or anterior, or immediately posterior to the nerve ring.
Recent molecular phylogenetic studies also supported Siddiqi’s (2000) classification at different levels. For example, Holterman et al. (2009) used ribosomal DNA (rDNA) small subunit (SSU) sequences from members of the order Tylenchida and placed Neotylenchidae, Allantonematidae, and Sphaerulariidae under Hexatylina. Yu et al. (2009) came to similar conclusions. Other studies using sequences of the rDNA large subunit (28 S) also supported the Hexatylina as a suborder under Tylenchida (Subbotin et al., 2006; Kanzaki et al., 2016).
In the present study, a new neotylenchid nematode which was intercepted in Ningbo Port in a quarantine inspection of imported wood from the USA is described. This nematode did not fit any of the presently described genera. The nematode morphologically shares characters of Hexatylus and Deladenus, and also has morphological similarity to Gymnotylenchus, Fergusobia, and Rubzovinema. It has unique morphological features separating it typologically from all other Neotylenchidae. This nematode is described and illustrated herein as Delatylus andersoni n. gen., n. sp. Phylogenetic analysis inferred from the 18 S, 28 S D2-D3 region and rDNA internal transcribed sequence (ITS) gene sequences are presented.
Sawn wood samples were taken from white pine lumber of lots #: 9095, 480-T, 484-T from the USA, intercepted in Ningbo Port, China, in June and August of 2015 and July of 2016, respectively. The samples were cut into small pieces of less than 1 cm long at the port of entry. Nematodes were extracted with the Baermann funnel technique for 24 h. The nematodes were then heat killed, and kept in a refrigerator at 5°C for both morphological and molecular studies.
The nematodes were all fixed in TAF (2% triethanolamine, 7% formaldehyde, and 91% water) and mounted in anhydrous glycerin on slides for morphological studies. The specimens were examined using a Leica DM5500 B compound microscope using differential interference contrast and pictures were taken with a Leica DFC 420 digital camera. The measurements were made using a Leica micro application system on the images. The details of the observational techniques are given by Yu et al. (2014). Scanning electron microscope (SEM) pictures were taken with an FEI Quanta 600 SEM.
DNA samples were prepared according to Gu & Wang (2010). Three sets of primers (synthesised by Invitrogen, Shanghai, China) were used in the PCR analyses to amplify the near-full length 18 S, 28 S D2-D3 and ITS. The 18 S region was amplified as two partially overlapping fragments; the first fragment by 988 F and 1912R and the second fragment by 1813F and 2646 R (Holterman et al., 2006). The 28 S D2-D3 region was amplified with the forward primer D2A and the reverse primer D3B (De Ley et al., 1999). Primers for amplification of ITS were the forward primer F194 (Ferris et al., 1993) and the reverse primer 5368r (Vrain, 1993). PCR conditions were as described by Ye et al. (2007). The PCR products were separated on 1.5% agarose gels and visualized by staining with ethidium bromide. High-quality PCR products were purified for cloning and sequencing by Invitrogen.
Phylogenetic analysis was performed as described previously (Ye et al., 2007). The sequences were deposited in the Genbank database. DNA sequences were aligned by Kalign (http://www.ebi.ac.uk/Tools/msa/kalign/) using default settings. The DNA sequences of D. andersoni n. sp. n. gen. were compared with those of the other nematode species available in Genbank using the BLAST homology search program. For phylogenetic analyses, the model of base substitution was first evaluated and selected using MODELTEST, and the Akaike-supported model, the base frequencies, the proportion of invariable sites and the gamma distribution shape parameters, and inferred substitution rates were specified in phylogenetic analyses. Bayesian analysis was performed to infer the tree topology for each gene separately using MrBayes 3.1.2 (Huelsenbeck and Ronquist, 2001) running a Markov Chain Monte Carlo chain for 1 × 106 generations, discarding trees from the first 2,500 generations as burnin after confirming convergence of chain. Node support was evaluated by their posterior probabilities of the phylogenetic trees using a 50% majority rule.
Neotylenchidae: Female body ranges from nonobese to semiobese and from straight to dorsally curved when heat relaxed; lateral fields raised; lip region moderately high, continues with body contour; the head with six sectors; four submedian and two smaller ones laterally; oral aperture elongated; amphidal apertures round and not dorsally displaced; stylet tylenchoid type, with three distinct knobs of equal size, slopped posteriorly, cone is slightly shorter than the shaft, with 4 to 5 guide rings (one or two on the cone, three always on the shaft); pharynx typical of other neotylenchoid: without a median bulb, and glands free in the cavity; pharyngo-intestinal junction located immediately posterior to the nerve ring; excretory pore posterior to the nerve ring; intestinal lumen widens immediately after the pharyngo-intestinal junction, with a broad rectum; the genital system consists of ovary, oviduct, quadricolumella, uterus, and the vagina without a discernible spermatheca (one specimen was observed with a degenerate spermatheca); vagina is directed anteriorly; vulva not protruding, a long transverse slit, about one-body width; anus located about the third distance between the vulva and the tail terminus.
Type species by monotype: Delatylus andersoni n. sp.
Delatylus. n. gen. is distinguished from Hexatylus, Deladenus, Gymnotylenchus, Fergusobia, and Rubzovinema, the genera of the family Neotylenchidae by a combination of the body shape, the head framework, the numbers of guide rings on the stylet, the absence of spermatheca, the anterior directed vagina, the nonprotruding vulva, and the relative proximity of vulva to the anus. The new genus differs from Hexatylus (Shepherd et al., 1983) by its semiobese and dorsally curved mature females in a heat-relaxed state vs. nonobese and straight or ventrally curved ones, the six-sectored lip region vs. eight, the elongated oral aperture vs. the round one, the immediately broadened intestine after the pharyngo-intestinal junction vs. a narrow tube, the absence of male, the absence of spermatheca, and the closer proximity between the vulva and the anus; from Deladenus by its semiobese and dorsally curved mature females in a heat-relaxed state vs. nonobese and straight or ventrally curved ones, the 4 to 5 guide rings vs. 2 to 3 on the stylet, vulva not protruding, the vagina directed anteriorly, and the diminished or absent spermatheca; from Gymnotylenchus by its wide vulva opening vs. a narrow one and the diminished or absent spermatheca vs. a normal one; from Fergusobia by its nonobese juveniles, the diminished or the absent spermatheca and the non-offset lip region vs. an offset one, and the six unequal sectored head framework vs. six equal sectored (Taylor and Davies, 2008); from Rubzovinema by its semiobese and dorsally curved mature females upon heat relaxation vs. nonobese vermiform, stylet with distinct knobs vs. indistinct and minute knobs, the excretory pore posterior to the nerve ring vs. anterior and the spermatheca diminished or absent vs. the one present (Siddiqi, 2000).
Key to genera of Neotylenchidae (based on non-entomophagous forms)
Juveniles and adults obese……….……………………………………………..Fergusobia
Juveniles and young adults not obese, mature adults obese………………Delatylus n. gen.
Juveniles and adults not obese…………………………………………………………….2
Males without bursa or gubernaculum…………………………………….Gymnotylenchus
Males if present with bursa and gubernaculum…..……………………………………….3
Spermatheca present in female. ……………………………………………………………4
Female without spermatheca……………………………………………………..Hexatylus
Vulva position over 93% ……………………………………………………….Deladenus
Vulva position between 83.3 and 92.5%……………………………………………………Rubzovinema
Only females of one form were found from these samples. Although some morphometric differences were observed between the three populations, the main morphological characters and the 28S sequences were identical.
Measurements of the holotype and paratype females of D. andersoni n. sp. are given in Table 1.
Female: Body of the young female is nonobese and straight when heat relaxed, semiobese and dorsally curved for mature ones; cuticle with indistinct annulations; lateral fields raised with 10-12 straight lines in mid-body, fewer lines toward the ends; deirids papilla shaped; lip region about 2 µm high, the oral area slightly raised, when viewed with SEM: the en face view shows both inner labial and outer labial plates; the inner labial plate is rounded and has a raised, dumbbell-shaped oral disk with six inner labial papillae, three on each side of a dorsal ventrally elongated oral aperture; the out labial plate with six sectors: four submedian ridges with four outer labial papillae and two smaller lateral ridges; amphidal apertures on and at the end of the lateral ridges, round, and not dorsally displaced; stylet with three distinct knobs of equal size that slope backwards; cone slightly shorter than the shaft, with 4 to 5 rings (1 or 2 on the cone); pharynx spindle shaped, nonmuscular, and without apparent median bulb and isthmus; dorsal gland opening about 2 µm behind the base of the stylet knobs, and subventral gland openings about two stylet lengths behind the knobs, both dorsal gland and subventral glands free in the body cavity, with larger dorsal gland; pharyngo-intestinal junction forming a distinct valve or cardium of different shapes, and located immediately posterior to the nerve ring; excretory pore posterior to the nerve ring; hemizonid immediately after the excretory pore; intestinal lumen widens immediately following the pharyngo-intestinal junction, and with a broad rectum; genital system with one outstretched gonad outstretched that overlaps the dorsal gland in mature females, without a discernible spermatheca (one specimen showed a diminished spermatheca), and consists of ovary, oviduct, quadricolumella, uterus, and vagina; oocytes at anterior half with two or three rows of cells; oviduct of three-celled quadricolumella distinct with 20 to 30 cells; uterus cylindrical; and vagina directed anteriorly; vulva not protruding, the opening as wide as body width; post-uterine sac absent; anus located about the 1/3 the distance between the vulva and the tail terminus; tail narrows quickly with a conical terminus.
Male: not found.
Juvenile: Similar to female, but smaller; the body straight when relaxed; stylet knobs, pharynx, and the pharyngo-intestinal junction valve similar to those in females; the intestinal lumen relative to the body width wider than in females; tail similar to females.
Type habitat and locality: Lumber of white pine from the USA, intercepted in Ningbo Entry–Exit Inspection and Quarantine Bureau, China, in 2016.
Type specimens: Holotype female and 10 paratypes of D. andersoni n. sp. were deposited in the Canadian National Collection of Nematodes; Ottawa, ON, Canada, under the accession number T547. Five paratypes were deposited in each of the following collections: USDA Nematode Collection, Beltsville, MD under the accession numbers XXX and the Ningbo Entry–Exit Inspection and Quarantine Bureau Nematode Collection, Ningbo, China. The new genus has been registered in the ZooBank database (zoobank.org) under the identifier XXXXX, and the new species binomial under the identifier XXXXX.
Etymology: The specific epithet is patronymic, honoring Dr. Roger Anderson, a distinguished nematode taxonomist with Agriculture and Agri-Food Canada.
Molecular characterization and phylogenetic analysis: The 18S, ITS1, 5.8S rDNA, ITS2, 28S (3256 bp) and 28S D2/D3 (805 bp) of D. andersoni n. gen., n. sp. were sequenced with GenBank accession numbers KY907662 and KY907663, respectively. The blast search on 1749 bp 18S yielded a significant match as Hexatylina species with 95% to 97% identity, including Deladenus, Howardula, Fergusobia, Tylenchomorpha, Hexatylus, and Rubzovinema. The blast search on 240 bp 5.8S is 91% identical with Deladenus, 86% identical with Tylenchomorpha, and 85% identical with Rubzovinema. The blast search on 805 bp 28S is 93% identical with Fergusobia, 88% identical with Deladenus, Psyllotylenchus, and Tylenchomorpha. Sequences of ITS1 and ITS2 are very variable and did not yield any significant match with the other sequenced Hexatylina genera.
The inferred phylogeny based on 18S (Fig. 5) placed D. andersoni n. gen.; n. sp. in a basal position for sequenced genera in Hexatylina, no close grouping was identified between the new genus and the other sequenced Hexatylin genera. The phylogenetic tree inferred by 28S D2/D3 (Fig. 6) placed it in the same clade with a Fergusobia species with 100% support. This clade is sister to Deladenus, Rubzovinema, Tylenchomorpha, and Psyllotylenchus.
Remarks: Delatylus andersoni n. gen., n. sp. shares the characters of Deladenus (pharyngo-intestinal junction at the nerve ring), Hexatylus (anterior directed vagina), and Fergusobia (obese and dorsally curved mature female). Although nematodes recovered from the lumber of the pine trees were of the mycetophagous form, and no other forms were found; this does not exclude the possibility that it may have an insect parasitic phase. Most species of the family Neotylenchidae are insect associates (Siddiqi, 2000) with the exception of a few Deladenus, but these Deladenus species were not from pine wood. In addition, all the nematodes from this family that were extracted from pine lumber intercepted at the same port using the same method were insect associates from our previous studies (Yu et al., 2013, 2014, 2017).
Micrographs of females of
Micrographs of females of