Chapter 8

Ultrastructure of Cuticular Exudates and Related Cuticular Changes in Juveniles of the Soybean Cyst Nematode, Heterodera glycines ⁷

Thick cuticular exudates on the surface of adult females of the sugar beet cyst nematode, Heterodera schachtii Schmidt, 1871, were named "subcrystalline layer" by Schmidt (1871, 1872). The layer was thought to be a byproduct that was produced by an outside organism such as a fungus (Brown et al. 1971). This concept has been altered because exudates are produced on the cuticle of the same species feeding on host plants grown under monoxenic culture conditions (Zunke 1985). Recent ultrastructural studies of second- (J2) and third-stage juveniles (J3) of H. schachtii support the concepts that the exudate is produced by the nematode alone and that the cuticle is a relatively porous structure providing continuity between secretory granules in the hypodermis and fibrillar exudates on the cuticle surface (Endo and Wyss 1992).

This study was initiated to elucidate formation of cuticle exudations in the soybean cyst nematode, Heterodera glycines, and to compare the results to those obtained for H. schachtii.

Fibrillar exudates were formed on the cuticle surface of parasitic H. glycines second-stage juveniles during feeding on soybean roots. The accumulation of cuticular exudates was correlated with the fibrillar and porous nature of the epicuticle, exocuticle, and endocuticle. The apparent source of the exudates was the hypodermis, where coalesced secretory vesicles were assembled by Golgi bodies and transferred to the inner surface of the apical membrane of the hypodermis. Products of the secretory vesicles were apparently released into a secretion accumulation zone at the base of the endocuticle by some mechanism and then extruded through and onto the cuticle surface.

Golgi bodies occurred in large expanded regions of the hypodermis, especially in the hypodermal cords where prominent nuclei and other cellular components were located. During ecdysis of the J2 cuticle and early stages of third-stage-juvenile cuticle formation, fine reticulate material accumulated at the secretory-excretory pore. Concurrently, moderately electron-opaque material occurred in the invaginated cephalic region and in the space extending between the molted J2 cuticle and the entire J3 body.

The transition from J2 to J3 of the soybean cyst nematode is accompanied by fundamental changes in the accumulation of electron-opaque material in the invaginated anterior region of the developing J3 (Endo 1985). During molt, the boundary of the electron-opaque zone merges with the widening zone between the J2 cuticle and the J3 body, which is bounded initially by the membranes of the hypodermal cells. Investigations are needed to determine the relationship between the reticulate material formed near the secretory-excretory terminus and the accumulated electron-opaque material within the invaginated anterior of a developing J3.

Ultrastructure of the cuticular exudates and related cuticular changes in juveniles of the soybean cyst nematode are shown in figures 156–157, figures 158-160, figures 161-163, figures 164-165, figures 166-167, figures 168-171, figures 172-175, and figures 176-178.

⁷ Reprinted in modified form with permission of the Helminthological Society of Washington from Journal of Helminthological Society of Washington 60:76–88, 1993b.

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