The soybean cyst nematode larva, like other plant-parasitic species (Bird 1971), uses its stylet to emerge from the egg, to penetrate the host tissue, and to initiate and feed on a syncytium (Endo 1964). Contraction of the protractor muscles that extend from the base of the cephalic framework to the stylet knobs thrusts the stylet forward and through the stomatal aperture. During stylet extension, the anterior part of the stylet, the cone, and portions of the extended stylet shaft are guided by the stomatal wall and the supporting cephalic framework.
A study of the ultrastructural morphology of the infective juveniles (J2) of the soybean cyst nematode reveals an extensive cephalic framework, stylet musculature, and a cuticular support system that are required for host penetration and feeding. The cephalic framework and portions of the stomatal wall have a fibrillar matrix that merges with and supports the surface cuticle. The flat broad base of the dorsal and ventral cephalic framework blades imparts a bilateral orientation to the framework to which stylet protractor and somatic muscles are attached by hemidesmosomes.
Juveniles of the soybean cyst nematode have robust stylets that are supported by an extensive network of protractor muscles. These muscles, in turn, are attached to the stylet, cephalic framework, and body cuticle through a network of irregular and linearly oriented hemidesmosomes. Protractor muscles consist of 10 muscle elements that extend posteriorly and make lateral hemidesmosomal contacts with the stomatal wall and adjacent somatic muscles. The 10 muscle elements merge to form 3 distinct muscle groups that attach to the stylet knobs. The protractor muscles are attached to the stylet knobs by extensive hemidesmosomes. Interaction between the stylet knobs and sarcoplasm is provided by the extensive hemidesmosomes and interhemidesmosomal membrane evaginations that extend from the stylet knob surface into the protractor muscle cells.
The extensive stylet protractor muscle system observed in Heterodera glycines juveniles supports studies conducted on the mechanics and design of tylenchid stylets (Seymour 1975). Because of the anatomical structure of the stomatal region of its juveniles, the soybean cyst nematode is well adapted to penetrate soybean roots and establish a feeding site that can cause irreparable damage to the host plant.
1 Reprinted in modified form with permission of the Helminthological Society of Washington from Proceedings of Helminthological Society of Washington 50:4361, 1983.
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