Comparative Parasitology 68(2) 2001 - Peru State College

G1BLIN-DAVIS ET AL.—CUTICULAR CHANGES IN FERGUSOBIID NEMATODES 243
structure of the cuticle of adults at different
phases of the life cycle of Fergusobia.
Materials and Methods
Multilocular flower bud galls of undescribed species
of Fergusobia and Fergusonina were collected on 9
August 1999 from C. ptychocarpa at the Sherwood
Arboretum in Sherwood, Queensland, Australia
(27°32.06'S; 152°58.39'E). Galls were dissected. Adult
parthenogenetic female and amphimictic male and preparasitic
infective female nematodes present in the
plant tissue were placed separately into Trump's fixative
for transmission electron microscopy or in formalin-aceto-alcohol
fixative (Southey, 1970). Mature
fly larvae (third-instar) and adults were dissected from
the galls in phosphate-buffered saline (pH 7.2). Parasitic
female nematodes were removed from the hemocoel
and placed into Trump's fixative. Specimens
were postfixed in 2% formaldehyde (prepared from
paraformaldehyde), 2% glutaraldehyde in 0.1 M cacodylate
buffer at pH 7.2 for 18 hr at 4°C. After repeated
rinsing in buffer, specimens were postfixed in
2% OsO4 in 0.1 M cacodylate buffer at pH 7.2 for 3
days at 4°C. Nematodes were rinsed in water, fixed
with 1 % aqueous uranyl acetate, dehydrated through
100% ethanol into 100% acetone, and infiltrated with
Spurr's epoxy resin. Blocks were sectioned on an
RMC® ultramicrotome. Sections were poststained with
5% aqueous uranyl acetate and lead citrate before
viewing on a Zeiss EM 10® transmission electron microscope
at 80 kV.
Results
Examination of the cuticle of an adult parthenogenetic
female and a male nematode revealed
a relatively simple cuticle with a striated basal
zone, an amorphous cortical/median zone, and a
distinct epicuticle (Figs. 1, 2). It is underlain by
relatively thin epidermis that covers the striated
somatic muscles.
Comparisons of the preparasitic female nematode
from the plant gall and the parasitic female
nematode from the adult fly show dramatic differences
(Figs. 3-7). The preparasitic female has
cuticle, epidermis, and muscles similar to those
described for the male and parthenogenetic female
from the plant host (Figs. 3, 4). However,
the cuticle appears thinner (200-250 nm vs.
450-550 nm for the parthenogenetic female and
630—680 nm for the male). The parasitic form
of the nematode from the adult fly has no cuticle.
The epidermis is greatly enlarged, and the
outer edge of the epidermis appears to be modified
into microvilli (Figs. 5—7). The somatic
muscles appear degenerated (Fig. 6).
During the transition from the preparasitic to
the parasitic female nematode in the larval fly,
the stylet is lost and the esophagus and intestine
appear to degenerate. In a parasitic female from
a fly larva, the remnant of the adult epicuticle
was present (Fig. 5), but it was not present in
the parasitic female from an adult fly (Figs. 6,
7). The apparent hypertrophy and development
of epidermal microvilli greatly expand the surface
area of the parasitic female and presumably
increase the nematode's ability to absorb nutrients
directly through its epidermis from the
host's hemolymph without cuticular interference.
Interestingly, the cuticle represents a form
of protection against insect host defense mechanisms.
However, these mechanisms may be
modified or lacking in the female larva, pupa,
and adult fly in this mutualistic association.
Whether there is a strong defense system in male
flies to prevent parasitism by Fergusobia or the
nematodes fail to penetrate the male fly larvae
is not known.
Discussion
Riding (1970) reported that microvilli were
present on the outside of the parasitic female
stage of Howardula husseyi Richardson, Hesling,
and Riding, 1977 (=Bradynema sp.) (Allantonematidae),
a tylenchid parasite of the
phorid fly, Megaselia halterata Wood, 1910. A
cuticle was not observed in this stage of the
nematode, suggesting that the microvilli were of
epidermal origin and that there could have been
an additional apolysis and ecdysis without cuticular
replacement, as appears to occur in Fergusobia.
The epidermis in this nematode was hypertrophied.
In addition, the stylet and esophagus
are not present in this form of H. husseyi
(Poinar, 1979). Subbotin et al. (1994) reported
that entomoparasitic females of Wachekitylenchus
bovieni (Wachek, 1955) Slobodyanyuk,
1986 (Parasitylenchidae), and Bradynema rigidum
(von Siebold, 1836) zur Strassen, 1892 (Allantonematidae),
had similar body wall morphology
to H. husseyi. Entomoparasitic females
of the tylenchid Skarbilovinema laumondi Chizhov
and Zakharenkova, 1991 (lotonchiidae),
exhibited a body wall composed of a "spongy"
layer of the epidermis formed by interwoven
and fused microvilli without a cuticle (Subbotin
et al., 1993).
In contrast, the epicuticle is apparently retained
by the entomoparasitic amphimictic female
of Paraiotonchium nicholasi Slobodyanyuk,
1975 (=Heterotylenchus sp.) (lotonchiidae)
(Nicholas, 1972). Ultrastructural differences
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