Question

Identify the nematode genera based on the following characters- Small to medium-sized (0.4-1.2 mm) ectoparasitic nematodes, females spirally coiled or rarely arcuate, cephalic region low or elevated, continuous or rarely offset, with or without annulation. Stylet robust, about three to four times maximum width of cephalic region. Both branches of female reproductive organs well developed and functional.

• A. Dorylaimus
• B. Longidorus
• C. Helicotylenchus
• D. Pratylenchus

Hint:

The spiral shape in plant-parasitic nematodes is the primary diagnostic feature of "spiral nematodes" belonging to the genus Helicotylenchus.
This characteristic posture, combined with a robust stylet and two functional ovaries, is key to identifying this genus.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Identifying plant-parasitic nematodes relies on key morphological characteristics of the adult female, including body size, posture upon fixation, characteristics of the feeding apparatus (stylet), and the structure of the reproductive system.

Step 2: Detailed Explanation:
Let us analyze the morphological characters described in the question:
- Female body shape: "spirally coiled or rarely arcuate." This is a classic characteristic of the "spiral nematodes," which belong to the subfamily Hoplolaiminae. Upon heat fixation, these nematodes relax into a characteristic spiral shape.
- Body size: "Small to medium-sized (0.4-1.2 mm)." This is the typical size range of hoplolaimid nematodes.
- Stylet: "robust, about three to four times maximum width of cephalic region." The stylet is strong and adapted for ectoparasitic or semi-endoparasitic feeding on root cells.
- Female reproductive system: "Both branches of female reproductive organs well developed and functional." This means the female is didelphic and amphiadelphic (having two functional ovaries extending in opposite directions).
These combined features point to the genus Helicotylenchus.
Let us review why the other options are incorrect:
- Dorylaimus belongs to the order Dorylaimida. These are typically large nematodes with a dorylaimid odontostyle, which does not match the tylenchid stylet characters described.
- Longidorus (needle nematodes) are much larger (usually 2 to 8 mm in length) with an extremely long, slender odontostyle.
- Pratylenchus (root-lesion nematodes) are migratory endoparasites. They are smaller (typically under 0.7 mm), have a flat cephalic region, a short stylet, and a single, forward-directed ovary (monodelphic, prodelphic), which contradicts the didelphic character described.
Therefore, the correct genus is Helicotylenchus.

Step 3: Final Answer:
The morphological description, particularly the spiral posture and didelphic reproductive system, identifies the genus as Helicotylenchus.