Abstract
Third-stage larvae (L3) of Steinernema feltiae exist as free-living infective juveniles (IJ), with suspended development activities. In contrast, parasitic stages (L1, L2, L4, adult) have mutualistic relations with Xenorhabdus species bacteria, along with unique morphological changes and development inside the cadaver of host insects and/or plant-parasitic nematodes. Commercial IJ strains are tolerant to cucurbitacin-containing phytonematicides, but we have scant information on how morphological adjustments in IJ are achieved. In this study, we investigated the nature of morphological adjustments in commercial S. feltiae IJ strains to Nemafric-BL phytonematicide, which contains cucurbitacin B as active ingredient. Post-72 h exposure to phytonematicide concentration, IJ specimens were fixed on mounting slides. Length (body, excretory pore to anterior end, pharynx, rectum, stoma, tail), diameter (head width, neck base, mid-body, anal body), cuticle thickness and De Man ratios were measured with a computer software programme attached to Omax light microscope. Morphometric data against increasing phytonematicide concentration exhibited either density-dependent quadratic, linear or neutral relations. Increase in body length at low phytonematicide concentration was accompanied by decrease in tail length and pharynx length during muscle contraction when IJ were still alive. After death at high phytonematicide concentration, the opposite morphometric effects ensued due to muscle relaxation. The observed changes in morphometric structures were explained on the basis of morphological adjustments that modulated volumes of pseudocoelom cavity in IJ. The modulation is intended to maintain hydrostatic pressure within permissible upper limits in order to avoid structural damage to internal organs embedded in the pseudocoelom fluids.
Highlights
The body length of S. feltiae infective juveniles (IJ) against increasing concentration of Nemafric-BL phytonematicide exhibited positive quadratic relations, with the model being explained by 91% coefficients of determination (CsOD) (Fig 2)
De Man ratios a, b, c and c’ against increasing phytonematicide concentration each exhibited positive quadratic relations, with CsOD explaining the models by 83% (Fig 3A), 93% (Fig 3B), 96% (Fig 3C) and 94% (Fig 3D), respectively
In the current study we argue that the morphometric adjustment in body length of S. feltiae IJ at low phytonematicide concentration was intended to regulate the volume of pseudocoelomic cavity in order to maintain the hydrostatic pressure in equilibrium [27]
Summary
Materials and experimental designMature fruit of Cucumis africanus were harvested from a cultivated field at 92 days after transplanting seedlings. Fruit were washed using chlorine-free tapwater, cut into pieces and dried at 52 ̊C for 72 h [21]. Dried fruit were ground in a Wiley Mill Model 4 to pass through a 1-mmpore sieve. 40 g ground material was fermented in hermetically-sealed 20-Lplastic containers (4 × containers) using effective microorganisms (EM) at 30 ̊C as explained previously [13]. The EM culture comprised yeast, photosynthetic bacteria, lactic acid bacteria, actinomycetes and fermenting fungi [22]. Post-fermentation at 14 days, 1000 ml sample at pH 3.7 was passed through a Whatmann 1442-125 Ashless Grade 42 Quantitative Filter Paper. Dilutions at 0, 2, 4, 8, 16, 32 and 64% phytonematicide were made in hermetically sealed 150 ml glass measuring cylinders prior to temporary storage in 150 ml opaque plastic containers. The 0% phytonematicide comprised distilled water that served as a negative control
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