Soil samples containing a population of dagger nematodes resembling X. ifacolum were collected at a depth of 5 cm to 30 cm from the rhizosphere of kola nut (Cola sp.) trees in Litaye village, Ondo State, southern Nigeria. Nematodes were extracted from soil by a modified sieving and decanting method (Brown and Boag, 1988). Extracted specimens were processed to glycerol and mounted on permanent slides (Hooper, 1986). The light micrographs and measurements of the nematode population, including the main diagnostic characteristics (i.e., de Man indices, body length, odontostyle length, lip region, tail shape, amphid shape, and oral aperture-guiding ring) were performed using a Leica DM6 compound microscope with a Leica DFC7000 T digital camera (Leica Biosystems, Wezlar, Germany). All other abbreviations used were as defined by Jairajpuri and Ahmad (1992).
For molecular analysis, specimens were preserved in DESS (Yoder et al., 2006), and DNA was extracted from single individuals as described by Archidona-Yuste et al. (2016a). Primers and polymerase chain reaction conditions used in this research were according to De Ley et al. (1999) and Lazarova et al. (2006). Single amplicons of ca 800 bp and 400 bp were obtained and sequenced for D2-D3 and COI regions, respectively. The newly obtained sequences were deposited into the GenBank database under the accession numbers OM777137-OM777138, OM758274-OM758275. Voucher specimens of this species have been deposited in the nematode collection of Institute for Sustainable Agriculture, IAS-CSIC, Córdoba, Spain.
Soil samples from the rhizosphere of kola nut (Cola sp.) trees in southern Nigeria yielded a Xiphinema population, including a moderate density (5–8 dagger nematodes/500 cm3 of soil) resembling X. ifacolum. The identity of the recovered population was further corroborated by molecular markers, including 28S rRNA and COI sequences, which were between 99.2% and 99.4% (differing in 3 indels, 2 gaps) identical in D2-D3 marker with a population of the same species from Sri Lanka (MH012181-MH012182), and between 96.2% and 96.3% (differing in 14 indels) identical in COI marker with the aforementioned population (MH013395-MH013396) (Susulovska et al., 2018). Although only two populations of this species have been sequenced from Sri Lanka and this one from Nigeria, intraspecific diversity in D2-D3 and COI follows the same pattern as other Xiphinema non-americanum-group species (Palomares-Rius et al., 2017). Maximum intraspecific distances for D2-D3 and COI sequences showed a higher molecular variability associated with COI than with D2-D3, establishing optimal barcoding differences of 2.87% and 6.36%, respectively (Palomares-Rius et al., 2017). Since the identity of this species has been confirmed by D2-D3 28S rRNA and the COI genes, no further studies than morphometrics were developed on this Nigerian population.
Light micrographs of Xiphinema ifacolumLuc, 1961 from Nigeria (A–I). (A) Whole female, (B,C) Female lip region showing amphidial fovea, (D–G) Detail of Z-organ in uterus, (H–I) Female tail regions. Indicators: af = amphidial fovea; a = anus; Zorg = Z-organ. (Scale bars: A = 200 μm; B, C, G–I = 10 μm; D–F = 20 μm).
Morphometrics of Xiphinema ifacolumLuc, 1961 from Nigeria. All measurements are in micrometer, with the exception of L, which is expressed in millimeter; and all values take the form: mean ± s.d. (range).
‡d = anterior to guide ring/body width at lip region (Brown et al., 1994)
⚔d′ = body width at guide ring/body width at lip region (Brown et al., 1994)
The Nigerian population of X. ifacolum is characterized by a moderate body length, a broadly rounded lip region that is slightly offset from body contour, a robust odontostyle and odontophore, and a double guiding ring (Fig. 1). The female reproductive system is didelphic-amphidelphic, with a tripartite uterus composed of short pars dilatata uteri followed by a well-developed Z-organ, comprising three to five sclerotized bodies of variable shape (from round to star-shaped) and size, and a tubular region devoid of spiniform structures (Fig. 1). The tail is a short, convex conoid, ventrally curved with a fine canal (“blind canal”), and surrounded proximately by a fine sleeve. According to the polytomous key proposed by Loof and Luc (1990), and as supplemented by the researches of Loof and Luc (1993), Loof et al. (1996) and Peraza-Padilla et al. (2017), the Nigerian population has the following specific a-numeric codes (codes in parentheses are exceptions): A4 - B1 - C3 - D4 - E6 - F34 - G2(3) - H2 - I3 - J3 - K2 – L1.
