for one or more body waves, hesitation and resumed forward movement
in a new direction. A kink, considered to be a refractory state by Croll and
Sukhdeo (1981), can be formed when backward and forward waves meet.Special problems in using agar
Most plant-parasitic nematodes occupy habitats that are optically opaque.
Most experiments to study directed movementin vitrohave been on the
surface of water agar where nematodes and their tracks can be seen.
These studies suffer the deficiency noted by Croll (1970): bilaterally
symmetrical nematodes typically undulate dorsoventrally on their sides
on a water agar surface, which precludes lateral steering. If the bilaterally
positioned amphids are in fact separated sufficiently in plant-parasitic
nematodes to resolve edaphically realistic chemical and thermal
gradients, as argued by Dusenbery (1988a), and if they normally serve to
guide lateral steering when nematodes move through soil, the primary
orientation mechanism may be seriously handicapped, if not entirely
incapacitated, on agar. In the case where nematodes can burrow through
the agar and rotate on the body axis, gradients to which they respond must
occur within the agar, not just along its surface. Interestingly, the only
nematode that appears to have been critically examined for lateral steer-
ing, the phototaxing adult female of the orthopteran parasiteMermis
nigrescens, shows the response very clearly. When phototaxing, the
anterior end of this large nematode sweeps the air laterally, while dorso-
ventral body waves lift and lower the ventral surface from a horizontal
moist felt-cloth substrate (Gans and Burr, 1994).
Nematode species vary in the way they move on agar (Robinson,
2000). The robust 650 and 1100μm long infective juveniles ofDity-
lenchus dipsaciandSteinernema glaseriburrow easily through 0.75%
agar and become trapped in water films less frequently than do the
350–400μm long infectives of Meloidogyne incognita, Rotylenchus
reniformis and Tylenchulus semipenetrans (A.F. Robinson, personal
observation). Some species (S. glaseriandDitylenchus phyllobius, for
example) readily and others never ascend the walls of plastic Petri dishes.
Some nematodes produce curved tracks on agar, exhibiting slew caused
by dorsoventral asymmetry of the laterally positioned body (Croll and
Sukhdeo, 1981; Green, 1977). Slew is particularly strong in males of
plant-parasitic cyst nematodes with ventrally curved tails. Simulation
models have shown that changes in slew curvature in response to changes
in stimulus intensity within a stimulus field can help males to find
females (Green, 1977). Slew does not seem to account for the circular
tracks made by thermotaxing wild-typeC. eleganson radial temperature
gradients on agar (Hedgecock and Russell, 1975; Mori and Ohshima, 1995;
Bargmann and Mori, 1997).
Some studies have examined horizontal movement in shallow
containers of Sephadex beads, sand or agar coated with a thin or partialHost Finding by Plant-parasitic Nematodes 91