The soybean cyst nematode, Heterodera glycines, penetrates soybean roots as a second-stage juvenile (J2) and undergoes three molts before maturing. Regeneration of a functional stylet, lost early in each molt, is critical for survival and is characteristic of the ontogeny of the Tylenchida. Light microscopy studies have elucidated the molting process in tylenchid nematodes (Anderson and Darling 1964, Andrássy 1962, Hechler and Taylor 1966, Roman and Hirschmann 1969). Stomatal wall and stylet components disappeared or remained attached to the cuticle, and tissues from which new stomatal wall and stylet components arose were identified. The parts of the stylet of the adult tylenchid nematode were named, and attempts were made to homologize the parts with those of the cylindrical stomata of rhabditids (Andrássy 1962, Chitwood and Chitwood 1950, Goodey 1963, Steiner 1933).
The relationship of the molting cuticular lining of the stoma to the subjacent cells was established in the rhabditid Caenorhabditis elegans (Wright and Thomson 1981). In contrast to the cell-to-cell reassembling of stomatal parts as described in rhabditid nematodes, tylenchid species such as H. glycines showed a stylet initiating zone at an invaginated central region of the J3.
Observations were made on the morphology and alterations of infective juvenile J2 body components, with emphasis on the body wall, stomatal wall, stylet, and sensilla of H. glycines. During the molt of J2 to J3, the J2 hypodermis separates from the J2 cuticle and forms an extracellular space, continuous with an invagination of the anterior center of the J3. The space between the J2 cuticle and the enlarged J3 hypodermal cells is filled with electron-opaque material resembling the fluid observed in insects during molt.
Regeneration of the J3 during molt was traced in a series of ultrathin sections. The site of stylet regeneration is in the hypodermal and myoepithelial tissues of the invaginated anterior center of the J3. Four layers of arcadelike cells are related to specific components of the stomatal wall, the stylet cone, and the stylet shaft of the J3. The first and second arcadelike cells are primarily related to stomatal wall development, whereas the third and fourth arcadelike cells are related to stylet cone and shaft development. Spherical, electron-translucent vacuoles that occur in myoepithelial cells just posterior to the arcadelike cells appear to be progenitors of the stylet knobs. Early stages of protractor muscle attachment to the vacuolar membrane were observed.
Future investigations should be directed toward elucidation of the intermediate stages of stylet development as well as the source and nature of the molting fluid. A broader understanding of the mechanisms that influence molting may provide targets for disruption and may lead to improved nematode-control strategies.
Ultrastructure of the head region of molting second-stage juveniles of the soybean cyst nematode is shown in figures 146147, figure 148, figures 149-150, figure 151, figures 152-153, and figures 154-155.
6 Reprinted in modified form with permission of the Society of Nematologists from Journal of Nematology 17:112123, 1985.
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