Chapter 4

Ultrastructure of Esophagus of Juveniles of the Soybean Cyst Nematode,
Heterodera glycines ³

The soybean cyst nematode, Heterodera glycines, is a major pest of soybeans. Host-parasite studies with emphasis on host responses (Endo 1964, 1965; Gipson et al. 1971) were augmented by ultrastructural studies on the morphology of males and juveniles of the root-knot and soybean cyst nematodes (Baldwin and Hirschmann 1973, 1975a,b, 1976; Endo and Wergin 1973, 1977; Wergin and Endo 1976; Baldwin et al. 1977; Endo 1978, 1980). Observations and reviews of other tylenchid species (Bird 1967, 1968a,b, 1971; Wisse and Daems 1968; Yuen 1968b; Byers and Anderson 1972; De Grisse et al. 1974; De Grisse 1977; Coomans 1979b; Natasasmita 1979; Shepherd et al. 1980; Baldwin 1982) provided a substantial base for further investigations of these and other major plant parasitic nematodes. Furthermore, in the evaluation of the ultrastructure of plant parasitic nematodes, it is essential that information gained on fundamental biology of the rhabditid nematode, Caenorhabditis elegans, and a wide range of animal parasitic species be applied to data on plant parasitic species (Yuen 1968a; Wright 1974, 1980; Ward et al. 1975; Ware et al. 1975; Albertson and Thomson 1976; McLaren 1976; Sulston 1976). Terminology proposed by Coomans (1979a) has been used to describe certain regions of the nematode that show well-defined triradiate symmetry.

Previous ultrastructural observations of second-stage juveniles of the soybean cyst nematode concentrated on the anterior neurosensory organs and the stomatal region of H. glycines. This chapter describes the ultrastructure of the esophagus, with emphasis on procorpus, metacorpus, and glandular organs. General morphology of the nerve ring is also included.

The cell bodies of the stylet protractor muscles and of the tissue immediately surrounding the stylet shaft are located in the anterior procorpus alongside the cells of the procorpus proper. The protractor muscle cell bodies are in the dorsal and ventrosublateral sectors, and those related to the shaft are in the dorsosublateral and ventral sectors. The duct from the dorsal gland ampulla joins the esophageal lumen just behind the stylet knobs. Secondary muscle cells lie centripetal to the protractor muscle cells. Sphincter muscles are at the anterior and posterior termini of the metacorpus. The metacorpus consists of a pump chamber operated by a complex of muscle units with their perikaryons and innervations. The subventral gland extensions terminate in ampullae. Quadradiate membrane valves of the ampullae join sclerotized ducts that enter the triangular posteriad vestibule of the pump chamber. The isthmus supports attenuated gland extensions and is encircled by the nerve ring. The dorsal gland occupies most of the anterior of the gland lobe; the two subventral gland cells occupy the posterior region. The esophago-intestinal valve lies adjacent to the dorsal gland nucleus.

Considerable effort is necessary to establish the relationships of nematode structure and function. With the use of existing and future technology, the functions of organs—especially the esophageal glands—that relate to nematode survival may be better defined. As shown in research on inhibitors of molting hormones in insects and other arthropods, inhibitors may be developed to interrupt or deter the feeding process. Very little is known about the formation of the stylet in the tylenchid plant parasitic nematodes. Stylet formation is an integral part of the molting process that involves the tissues of the anterior esophagus. Detailed study of the molting stages of the nematode is required to clarify this site of cellular activity, and this may provide clues for future control technologies.

Ultrastructure of the esophagus of juveniles of the soybean cyst nematode is shown in figures 68–71, figures 72-75, figures 76-80, figures 81-84, figure 85, figures 86-89, figures 90-91, figures 92-93, figures 94-96, figures 97-102, figures 103-104, figures 105-107, figures 108-110, and figures 111-112.

³ Reprinted in modified form with permission of the Helminthological Society of Washington from Proceedings of Helminthological Society of Washington 51:1–24, 1984.

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