Chapter 7
Ultrastructure of Head Region of Molting Second-Stage Juveniles
of Heterodera glycines, With Emphasis on Stylet Formation
6
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
146–147, 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 17:112–123, 1985.
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