Mesocriconema spp. belongs to the family Criconematidae Taylor, 1936 (1914) (Thorne, 1949). Species in this group are known as the ring nematodes, and they are one of important ectoparasitic nematodes that can be a potential threat at high soil population density. Some species of the ring nematodes caused yield loss up to 50% such as Macroposthonia ornatum (Raski, 1958) Loof and De Grisse, 1989 in pod field (Siddiqi, 2000).
Our study recorded Mesocriconema sphaerocephalum (Taylor, 1936) Loof, 1989 on carrot in Hanoi city and Hai Duong province in Vietnam. Mesocriconema sphaerocephalum was found on 10% of 130 soil samples. Density of this nematode was up to 45 individuals/250 g soil. To our knowledge, this is the first report of M. sphaerocephalum on carrot.
Nematodes were extracted from soil samples using modified Baermann tray method (Whitehead and Hemming, 1965). For morphological characterizations, permanent slides of nematodes were observed through the Carl Zeiss Axio Lab.A1 light microscope. Measurements and pictures were taken using a ZEN lite software on ZEISS Axiocam ERc5s digital camera (Nguyen et al., 2017).
For molecular studies, Primers D2A (5′-ACAAGTACCGTGGGGAAA GTTG-3′) and D3B (5′-TCGGAAGGAACCAGCTAC TA-3′) were used to amplify D2D3 of 28S rDNA region (Nguyen et al., 2017). Obtained sequence was used for a Blast search in GenBank (Altschul et al., 1997). The data set was analyzed using maximum likelihood (ML) method in MEGA 6 program with 1,000 bootstrap replications. The best fit model of DNA evolution was obtained using the Model test in MEGA 6 (Nguyen et al., 2017).
Measurements of M. sphaerocephalum in this study are in agreement with measurement of M. sphaerocephalum in Geraert (2010) (Table 1). Females of M. sphaerocephalum on carrots are characterized by the following traits: body curved ventrally (Fig. 1A); lip region bearing two annuli with flattened labial disc (Fig. 1B); first body annulus much smaller than second one with smooth edge, sloping posteriorly (Fig. 1B); cuticle annuli at mid-body 4 to 5 µm wide; anastomoses numerous at lateral field, forming zigzag lines (Fig. 1D,G); stylet robust, knobs 9 to 10 µm wide (Fig. 1C); vulva located near posterior end; tail rounded (Fig. 1F).
Measurements of females of M. sphaerocephalum on carrot in Vietnam. All measurements are in µm and in form: mean ± SD (range), except for ratio.
| M. sphaerocephalum | ||
|---|---|---|
| Measurement | Population in Hanoi city | Population in Hai Duong province |
| n | 10 | 10 |
| L | 298 ± 13.8 (274–316) | 330.7 ± 14.4 (309.4–360) |
| a | 8.6 ± 0.5 (7.5–9.2) | 9.7 ± 0.6 (8.8–10.6) |
| b | 2.8 ± 0.1 (2.5–3) | 3.2 ± 0.2 (3-3.5) |
| c | 44 ± 9.8 (31–65) | 52 ± 9.2 (39–69) |
| c′ | 0.4 ± 0.1 (0.3–0.5) | 0.4 ± 0.1 (0.3–0.5) |
| VL/VB | 0.8 ± 0.1 (0.7–1) | 0.7 ± 0.1 (0.6–1) |
| V | 94 ± 1 (93–96) | 95 ± 0.4 (94–95) |
| Stylet length | 55 ± 1.6 (53–58) | 51 ± 1.5 (48-54) |
| Anterior end to nerve ring | 82 ± 2.4 (78–85) | 76 ± 2.6 (72–82) |
| Anterior end to end of pharynx | 107 ± 2.7 (104–110) | 103.3 ± 3 (98–109) |
| Anterior end to secretory-excretory pore | 110 ± 4 (105–117) | 108 ± 2.3 (104-113) |
| Max. body diam. | 34.8 ± 1 (33–37) | 34 ± 1.4 (31–37) |
| Body diam. at vulva | 23.2 ± 1.4 (21–25) | 24.3 ± 1.7 (22.2–28) |
| Body diam. at anus | 17.7 ± 1.8 (14.3–19.4) | 18 ± 2 (14.2–22) |
| Tail length | 7 ± 1.3 (4.4–8.8) | 6.6 ± 1.2 (4.8–9) |
| Rst | 12.4 ± 0.5 (12–13) | 12 ± 0.3 (12–13) |
| Roes | 20.7 ± 0.8 (20–22) | 20.4 ± 0.6 (20–22) |
| Rex | 21.5 ± 0.7 (20–22) | 21.6 ± 0.5 (21–22) |
| Rv | 4.8 ± 0.4 (4–5) | 4.3 ± 0.4 (4–5) |
| Ran | 2.2 ± 0.4 (2–3) | 2 ± 0.3 (2–3) |
| Rvan | 3 ± 0.3 (2–3) | 2.2 ± 0.4 (2–3) |
| R | 65 ± 1.6 (63–68) | 64 ± 1.4 (62–66) |
Mesocriconema sphaerocephalum on carrot in Vietnam. (A) entire body; (B) head region; (C) stylet; (D) anastomoses at mid-body; (E) pharyngeal region; (F) tail region; (G) anastomoses at tail region (Scale bar: 10 µm).
D2D3 of 28S rDNA sequence of M. sphaerocephalum in this study was 730 bp, submitted to GenBank under accession number: MK026628. It is 99% similar to M. sphaerocephalum (AB933464) in GenBank. This sequence varied 0 to 2% compared to other sequences of M. sphaerocephalum in GenBank. The maximum likelihood phylogenetic tree placed sequence of M. sphaerocephalum on carrots together with other sequences of M. sphaerocephalum from GenBank (Fig. 2).
Phylogenetic tree generated from D2D3 of 28S rDNA sequences based on ML method (TN93 + G model) with 1,000 replications. Sequence of Mesocriconema sphaerocephalum on carrot in Vietnam is in red.