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Microplitis croceipes teratocytes cause developmental arrest of Heliothis virescens larvae.

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Archives of Insect Biochemistry and Physiology 12:51-61 (1 989)
Microplitis croceipes Teratocytes Cause
Developmental Arrest of Hekothis
virescens Larvae
Deqing Zhang and D.L. Dahlman
Department of Entomology, University of Kentucky, Lexington, Kentucky
Microplitis croceipes teratocytes placed into nonparasitized Heliothis virescens
larvae survived in the absence of a parasitoid larva and caused developmental changes in the host. Expressions of these changes included delayed larval
mortality, incomplete larval-pupal ecdysis, or delayed pupation. Two day old
4th stadium H. virescens larvae were more sensitive t o injected teratocytes
than were 5th stadium larvae. Three day old teratocytes were more effective
than were 6 day old teratocytes. The degree of response was related to the
number of injected teratocytes. For example, 750 three day old teratocytes
(the approximate number from a single parasitoid egg) caused delayed larval
mortality in 96% of the treated larvae whereas 175 three day old teratocytes
caused delayed larval mortality in only 33% of the treated larvae. Even a dose
of 80 teratocytes resulted i n 15% incomplete larval-pupal ecdysis compared
to 0% for controls. Treatment with hernocyte- and teratocyte-free hernolymph
from parasitized larvae, hemocytes from nonparasitized H. virescens, unfertilized M. croceipes eggs, Cotesia congregata teratocytes, or Micrococcus
lysodeikticus cells all had very little effect either on larval growth or development time.
Key words: tobacco budworm, Braconidae, trophoserosa cells, juvenile hormone esterase,
polydnavirus
INTRODUCTION
Many previous studies have described changes in host development, behavior, physiology and morphology caused by its parasitoid [l-41. These changes
were considered to result from either direct feeding and secretions by the young
Acknowledgments: The authors express appreciation for the technical assistance of T.J. Neary
and Becky Wilson and Deidre Jacobson for help with insect rearing. The authors gratefully
acknowledge support of the U.S. Department of Agriculture (85-CRCR-1-1764)and a grant from
R.J. Reynolds. This is paper 89-7-56 of the Kentucky Agriculture Experiment Station, Lexington,
KY 40546-0091.
Received March 20,1989; accepted August 10,1989.
Address reprint request to Dr. D.L. Dahlman, Dept. Entomology, University of Kentucky, Lexington, KY 40546-0091.
0 1989 Alan R. Liss, Inc.
52
Zhang and Dahlman
parasitoid larvae within the host andior some factors injected by the adult female
during oviposition. Numerous changes in the host have been observed during
the development of the parasitoid. The simultaneous expression of interdependent factors associated with host regulation makes it difficult to determine
the specific role of individual components. These parasitoid-derived components [5] include polydnavirus [6], secretions from the venom and other accessory glands [7], and teratocytes [8-111. The teratocytes are cells derived from
the serosal membrane of the egg of parasitic braconids [8], and although these
cells undoubtedly play an important role in the parasitoid-host interaction,
they have not been as extensively studied as the other factors. Vinson and
Iwantsch [ 121 thoroughly reviewed the teratocyte literature through 1979.
Several studies have examined the transfer of teratocytes to nonparasitized
hosts. For example, Cardiochiles nigriceps Viereck teratocytes injected into
nonparasitized Heliothis virescens (Fab.) larvae lived and grew in the absence
of the parasitoid, and the injected larvae all eventually pupated [lo]. Several
years later it was shown that teratocytes from this same species exhibited juvenile hormone activity [13]. Buhler et al. [14] suggested that teratocytes from a
Chelonus species may be responsible for altered development of Trichopluszu ni
(Hubner), but they could show no conclusive evidence to support their suggestion. Subsequently, Jones [15] demonstrated that teratocytes were not
involved in this system. Teratocytes have been shown to improve in vitro growth
of some parasitoids [16,17] but not others [MI. None of the investigations to
date has examined the host response to different numbers of teratocytes,
teratocytes of different ages, and hosts of various ages. To define the potential
role of teratocytes in the parasitoid-host relationship, we transplanted Microplitis
croceipes (Cresson) teratocytes into nonparasitized H. virescens hosts of different ages and demonstrated that teratocytes cause retardation of host H. virescens
larval development.
METHODS
H.virescens larvae were obtained from the laboratory colony reared on
Vanderzant’s artificial diet [19]. The developmental terms for specific physiological phases of the larvae used in this study were described by Webb and
Dahlman [4].
M. croceipes larvae were maintained in the laboratory using H . virescens larvae as hosts. Parasitoid adult wasps were held in plastic boxes (27 x 9 x 19cm)
with tap water and 1:l honey:water available on cotton plugs. An equal number male and female wasps were placed in the box and were allowed to mate
for at least 24 h before females were used for parasitization.
Premolt third-instar H. virescens larvae were parasitized by exposing them
to female wasps in a 15 cm glass Petri dish under a high-intensity, IR-filtered
lamp for approximately 50 min. The ratio of H. virescens larvae to M . croceipes
females was about 8:l. Each parasitized larva was held in a 18.5 ml cup holding 6-9 ml of artificial diet; the temperature was 25 2°C under long-day conditions (16L:8D). Under these conditions, the M . croceipes embryo hatched in
approximately 2 days and the host was in the second day of the 4th stadium
*
Teratocytes Cause Larval Development Arrest
53
(D2-4+).The host larva was a newly molted 5th instar (N-5) 2 days later, and
after an additional 4 days, it had reached the burrowing-digging stage (BD-5).
In nonparasitized larvae, pupation occurred four days after BD-5. The age of
the teratocytes was measured from the estimated time of egg hatch. Therefore, for example, 2 day old teratocytes were obtained from larvae that had
been parasitized 4 days earlier. At least 30 larvae of each stage were injected
either with 10 p1 of Pringle's saline [20] or with a suspension of teratocytes in
saline obtained from larvae of different ages.
Approximately 750 teratocytes are derived from each M . croceipes egg [21],
and their size increases with age [ll]. Teratocytes from 4, 5, 6, 7, or 8 day
post-parasitized larvae were collected from the hemolymph by cutting an anterior proleg of the parasitized larva and collecting the teratocyte-containing
hemolymph in a 1.5 ml microcentrifuge tube containing 1ml of Pringle's saline.
Teratocytes were concentrated by a 30 s spin on a microcentrifuge. Following
centrifugation, approximately the upper half of the supernatant was discarded
before the soft pellet containing teratocytes and hemocytes was resuspended
in the remaining supernatant. This suspension was mixed with Pringle's saline
to a final volume of 2.5 ml in a 13 x 100 mm tube held in ice. The teratocytes
were allowed to settle for approximately 15 min, and approximately 1.25 ml of
the upper portion was removed and replaced with an equal volume of fresh
Pringle's saline. The teratocytes were resuspended by gentle swirling of the
tube. This washing procedure was repeated at least three times and was estimated to remove at least 90% of the hemocytes. After the final wash, the
teratocytes were allowed to settle for approximately 30 min and all but 250 pl
of the overlying saline was removed. The teratocyte concentration was then
determined and Pringle's saline was gradually added until the desired dilution was obtained. Dilutions were verified by actual counts of the number of
teratocytes in a 2 pl suspension placed in a depression slide.
Parasitized larval hemolymph was obtained as above from 5 day postparasitized larvae by collecting 1ml of hemolymph into a 1.5ml microcentrifuge
tube containing 0.5 ml Pringle's saline. The hemolymph was centrifuged for 2
min to ensure complete removal of cellular material, and the supernatant was
used for injection. Control hemolymph was collected from S-5 nonparasitized
larvae that were equivalent in age to 5 day post-parasitized larvae. The hemolymph was treated as described above, and the supernatant was used for injection. Hemocytes from nonparasitized S-5 larvae were concentrated from 300
pl of hemolymph diluted with 600 pl of Pringle's saline by centrifugation for 2
min followed by resuspension of the hemocyte pellet in 200 pl of Pringle's saline.
Cotesiu congregutu (Say)teratocytes obtained fromManducu sextu (L.) hemolymph
were prepared similar to the Microplitis teratocytes. Unfertilized M . croceipes
eggs were dissected from the ovaries. Two to four eggs were injectedlHe2iothis
larva. Lyophilized Micrococcus lysodeikticus cells (Sigma Chemical Co., St. Louis,
MO) (200 pg/ml) were suspended in Pringle's saline. Larvae in stage D2-4, N-5,
or BD-5 were injected with 10 pl volumes of the different preparations.
*Abbreviations used: BD-5 = burrowing-digging, 5th stadium; CFI-5 = cell formation, day 1;
CF2-5 = cell formation, day2; D2-4 = day 2,4th stadium; D1-5 = digging, day 1,5th stadium;
D2-5 = digging, day 2, 5th stadium; D3-5 = digging, day 3, 5th stadium; DLM = delayed
larval mortality; DP = delayed pupation; ILPE = incomplete larval pupal ecdysis; N-5 = new
5th stadium; P = normal pupae at expected time; S-5 = slender 5th stadium.
54
Zhang and Dahlman
All the larvae were rinsed in 75% ethanol solution for 2-3 s to retard subsequent fungal and bacterial growth on the larvae and then anesthetized with
C 0 2before injection. Injections were performed using a 50 pl Hamilton syringe.
The needle was directed posteriorly and entered dorsally through either the
left or right side behind the head capsule. Each larva received a total volume
of 10 pl- Larvae that bled profusely following injection were discarded. Care
was taken to prevent puncturing of the gut wall, which would result in early
(1-2 days post-injection) death of the larvae. Data from these larvae were not
included in the study. After injection, larvae were removed and placed in individual fresh diet cups to insure an optimum growth condition. Larvae were
weighed daily, and the behavioral and morphological markers were observed.
RESULTS
Preliminary studies with teratocyte injections of H . virescens larvae suggest
that teratocytes interfered with the larval-pupal molt. Four categories of host
response were established as follows: DLM (usually occurred near the expected
time of CF2 when larvae were injected at D2-4 or N-5 or occurred 1to several
days later than the expected pupation date when injected at BD-5); ILPE (evidence of an attempt to pupate included formation and tanning of pupal cuticle underneath the unshed larval cuticle, anterior projection of the larval head
capsule, and/or partial shedding of the larval cuticle accompanied with partial removal of the tracheal cuticle); DP (normal pupae but occurred 1-4 days
later than expected); and P (normal pupae at expected time). DLM was considered to be the maximum response because of termination of growth at an
earlier point in the developmental process.
Three day old teratocytes were more effectivethan were 6 day old teratocytes
(Table 1)with greater than 70% of the mortality observed as DLM compared
to less than 20% with 6 day old teratocytes. However, 6 day old teratocytes
were active enough to produce approximately 70% ILPEs when injected into
either D2-4 or N-5 larvae. When compared to injections with saline, both 3
and 6 day old teratocytes significantly reduced subsequent weight gain when
injected into D2-4 larvae (Table 1).The only combination that produced a minimum response was when 6 day old teratocytes were injected into BD-5 larvae. However, even then, 36% of the larvae failed to pupate as compared to
100% pupation of the controls. The only apparent effect of the saline injection
was a 1 day delay in pupation compared to noninjected controls. The actual
delay in development occurred just after injection because most saline-treated
larvae spent an extra day in the active feeding period of the ultimate larval
stadium.
Teratocytes were checked for viability using absorption of 0.1% solutions of
neutral red and exclusion of trypan blue in Pringle's saline as criteria. Teratocytes
harvested from all parasitized larvae 3-10 days post-parasitization turned red
when placed in a solution of neutral red but retained their original pale-yellow
color for at least 20 min when placed in a solution of trypan blue. Both tests
confirmed the teratocytes were living cells. Damage to teratocytes resulting
from collection, centrifugation, and washing procedures ranged between 0.9
and 2.7%. In order to determine viability of transplanted teratocytes, approx-
Teratocytes Cause larval Development Arrest
55
TABLE 1. Heliothis virescens Host Response as a Function of DifferentAge Microplitis croceipes
Teratocytes and Host Age*
Response
Host
age
DLM
(%)
Days to
larval
mortality
( 2SD)
ILPE
(%)
Days to
ILPE
( tSD)
Delayed
pupation”
(days)
( 2 SD)
(%)
0
-
0
0
-
0
+ 161.4B
( f 45.7)
+ 150.7B
0
-
0
( f 39.6)
-
3.8A
( f 0.1)
3.3AB
(20.8)
2.9 A
( 20.3)
0
DP
(%)
P
Weightb
gain
(mg)
3 day old teratocytes
D2-4
N-5
BD-5
72.7
83.3
84.3
9.2A‘
( + 1.7)
7.8B
( 21.0)
7.7B
( 20.6)
27.3
7.1 A
( 2 0.4)
16.7
7.0A
( t0.8)
15.7
6.8A
(r 0.4)
6 day old teratocytes
D24
16.7
8.0B
66.7
( ? 0.4)
N-5
18.1
7.6 B
72.7
( 20.2)
BD-5
18.1
6.7C
( + 0.3)
7.5A
1.1)
7.5A
( 20.2)
7.3 A
( 20.4)
16.7
(i-
18.1
0.9
63.8
0
+ 158.2B
( *40.1)
+ 133.1B
(k
0
49.6)
-
Pringle’s saline
D2-4
0
0
-
100.0
1.2 c
0.4)
1.1 c
( k 0.2)
0.9C
( 20.1)
0
(k
N-5
0
BD-5
0
-
0
-
100.0
0
-
100.0
0
+ 358.3 A
(+ 40.9)
+ 145.78
( 253.6)
0
-
*Each treatment consisted of between 30 and 45 larvae injected either with Pringle‘s saline or
with 750 teratocytes in saline.
”Daysafter expected pupation day.
bDifferencebetween initial and maximum weight.
‘NOS. in the same column followed by the same letter are not significantly different (P < .05)
by Duncan’s multiple range test.
imately 350 2 day old teratocytes were injected into a group of N-5 larvae and
subsampled on successive days after treatment (5-5 to pharate pupa). Nearly
all the teratocytes were viable for the first 5 days, but on the sixth day all had
lost their viability.
The effect of different numbers of teratocytes on the development of host
larvae was also examined. Based on the results previously described, 3 day
old teratocytes were used with the BD-5 stage host because this stage was larger
and responded as well as the smaller sized, younger N-5 stage. We found a
dose-dependent effect and as few as 175 three day old teratocytes were able to
prevent pupation of 80% of the injected larvae (Table 2). A shift in mortality
from DLM to ILPE was observed with decreasing teratocyte numbers. Even a
dose of 80 teratocytes resulted in 15%ILPEs.
An injection of 750 teratocytes of different ages produced an inverse relationship between teratocyte age and Heliothis larval response (Table 3). Two
56
Zhang and Dahlman
TABLE 2. Host Response of BD-5 Heliothis virescms Larvae to Injections of Various Numbers
of 3 day old Microplitis croceipes Teratocytes*
Response
No.
injected
teratocytes
DLM
1,400
86.7
1%)
Days to
larval
mortality
(? SD)
8.0 A“
ILPE
(%)
13.3
Days to
ILPE
(-+ SD)
175
33.3
7.4 A
( * 1.2)
46.7
80
0
-
15.4
Saline
0
-
0
8.0 A
( + 0.7)
7.7 AB
( * 1.2)
6.8 B
( * 1.0)
5.3 c
( * 0.4)
5.0 C
(-+ 0.4)
-
Control
0
-
0
-
( ? 1.3)
750
76.2
8.1 A
23.8
( ? 1.2)
350
46.7
7.5 A
40.0
(t0.5)
DP
(days)
( * SD)
(%)
0
-
0
0
-
0
2.8A
0
DP
(%)
13.3
P
( * 0.2)
20.0
84.6
97.6
0
1.9B
( * 1.0)
1.6B
( ? 1.3)
1.OB
( ? 0.1)
-
0
0
2.4
100.0
“Each treatment consisted 30-45 BD-5 larvae.
a N ~in~the
. same column followed by the same letter are not significantly different (P > .05) by
Duncan’s multiple test.
and 3 day old teratocytes prevented pupation in all treated larvae, whereas 4
and 5 day old teratocytes successfully prevented pupation in approximately
one-half of the treated larvae (Table 3). Although 6 day old teratocytes were
even less effective, they still prevented pupation in 36%of those treated. Delayed
pupation associated with injected teratocytes was significantly longer than control individuals (Table 3, compare teratocyte treatments to hemolymph plasma
and saline treatments).
The effect of an injection of the equivalent of lop1 of either hemocyte- and
teratocyte-free hemolymph from parasitized larvae or hemocyte-free hemolymph from control larvae were evaluated and the results are included in Table
3. The 13.3% ILPE obtained from the parasitized hemolymph treatment may
have been in response to unidentified cell-free materials associated with the
parasitized condition of the host from which the hemolymph plasma was
derived. Hemolymph from nonparasitized HeZiothis larvae and Pringle’s saline
did not produce either DLM or ILPEs even though approximately one-half of
the larvae experienced a delay in pupation, presumably as the result of trauma
related to the injection (see also saline controls in Tables 1 and 2). Injections
of H. virescens hemocytes, C. congregutu teratocytes, unfertilized M. croceipes
eggs or a suspension of lyophylized M. lysodeikticus cells all failed to produce
anything other than a 1day delay in pupation time.
The behavioral and morphological changes in 5th-instar Heliothis larvae
injected at the N-5 stage with 750 teratocytes included changes in pigmentation, heart beat, ocellar pigmentation, ocellar bar, feeding behavior, and tactile response (Table 4). These characters were first described nonparasitized
and parasitized larvae by Webb and Dahlman [4].
Teratocytes Cause Larval Development Arrest
57
TABLE 3. Host Response of BD-5 Heliofhis virescens Larvae to 750 Microplitis croceipes
Teratocytes of Different Ages*
Response
Teratocyte
age
(days)
DLM
2
85.0
(%)
Days to
larval
mortality
( 2SD)
ILPE
(%)
15.0
4
-
0
0
-
0
3.9A
1.8)
3.5AB
( k 1.0)
3.0B
( k 1.5)
1.5C
( t1.2)
1.4C
( t0.2)
1.OC
( * 0.1)
0
0
-
0
0
-
0
-
63.3
0
-
0
-
0
40.0
6
18.1
Control
0
0
43.3
5
Parasitized
hemolymph'
Control
hemolymph'
Saline
(%)
5.5 AB
( + 0.7)
5.2 AB
(+ 0.3)
5.0 B
( t1.1)
-
33.3
6.4 A
(a)
DP
(days)
( tSD)
DP
9.2 A"
0.7)
7.8 B
( t0.6)
7.7 B
( k 1.1)
6.5 C
( t1.0)
6.2 C
( k 1.5)
-
P
( & 1.3)
(k
3
Days to
ILPE
( + SD)
66.7
6.1 AB
( & 0.7)
16.7
5.7 AB
40.0
( ? 1.4)
6.7
18.1
13.3
(k
53.3
63.8
46.7
50.0
-
0
0
40.0
50.0
36.7
100.0
*Each treatment consisted of between 30-45 BD-5 larvae.
a N ~in~the
. same column followed by the same letter are not significantly different ( P > .05) by
Duncan's multiple range test.
'Hemocyte- and teratocyte-free.
'Hemocyte-free.
The nonparasitized larva takes 9 days to complete 5th-instar development
and pupate. As the insect aged, the integument pigmentation changed from
very dark to light, the dorsal heart became very apparent on BD-5, the ocellar
pigments were retracted to the maximum on PP, and the ocellar bar was completely gone from CF2-5 [4].
Injections of N-5 larvae with 10 p1 of Pringle's saline modified this scheme
to some degree. The primary effect was to add an additional day, digging 3
(D3-5) to the normal development time. This resulted in slight modification
of the schedule of morphological and behavioral changes that occurred in
noninjected controls [4], including slight differences in pigmentation, an extension of the time the dorsal vessel was apparent, and a delay in the complete
disappearance of the ocellar bar until early CF2-5 (Table4). Pupation of Pringle's
saline-injected larvae was normal, but usually 1day later than untreated larvae.
In contrast, teratocyte-treated larvae appeared to stop development at CF1-5.
These individuals did not pupate and only gradually, over a period of days,
attained some of characteristicsassociated with CF2-5 (Table 4). Pigmentation
did not change as much as saline-injected larvae, heart beat was observed on
D3-5 and BD-5, but was not as pronounced, ocelli began to retract 3 days following cell formation (a delay of 2 days), and the ocellar bar was still recognizable at the time of larval death. After treatment, larval feeding was restricted
to the diet surface for 4-5 days in contrast to only 3 days for saline-injected
S
N
7
-
++
+
CF
C
++
CF
C
W
CF
-
+ ++++
-
-
+
PP
8
++ ++
CF1 CF2
6
D1
S
N
-
-
-
-
-
+
+
-
+
-
-
-
-
-
R
SU
SU
E
SU
R
E
SU
E
SU
CF
CF
CF
-
CF
++
+
CF
+
++
-
E S L S L S L S L S L
D
++++ ++++ ++++ ++++ ++++ ++++ +++ +++ ++
-
-
+ + + + + + + + + + + + + + + + +++ +++ +++ +++ +++ +++ ++
2
1
0
Days from treatment
3
4
5
6
7
8
9
1
0...
Phases
D2
D3
BD CF1 CF CF CF . . .b
+lDa +2D +3D
750 Microplitis croceipes teratocytes
”Daysafter CFl .
bAfterCF + 3D, ILPE, DLM or Larvae were observed.
“Intensityof response indicated by the number of + ’s.
dFeedingbehavior: SU = surface; D = digging; CF = cell formation; W = wandering.
‘Tactile response: R = rearing, mandibles extended; E = active escape; C = curling; W = Wriggling; SL = sluggish.
Pigment++++‘++++++++ +++ ++
++
ation
Heartbeat
+ ++++
visible
Ocellar
pigment
retracted
Ocellarbar + + + + + + + + + + + + + + + + + + + + + + +
W
SU
SU
SU
D
D
Feeding
behaviord
E
R
R
R
E
E
Tactile
responsee
1
0
Pringle‘s saline
Days from treatment
2
3
4
5
Phases
D2
D3
BD
D1
TABLE 4. Behavioral and Morphological Changes in 5th-Instar Heliothis virescens Larvae Subjected to Injection
Teratocytes Cause Larval Development Arrest
59
controls. On the 6th day, treated larvae formed a cell and remained there until
they died. Tactile response was very sluggish during the latter portion of development. Death ultimately occurred, either as DLM or ILPE.
DISCUSSION
It should be recognized that, although not free of host hemocytes, the method
used to prepare the teratocytes reduced the number of hemocytes by at least
90%. The fact that injections of concentrated preparations of hemocytes from
nonparasitized larvae did not alter the growth parameters of the larvae further supports our contention that the hemocytes are not a critical component
in causing the observed phenomena. An alternative approach for collection
of teratocytes would be to use in vitro rearing techniques similar to those
described by Greany [16]. Although such a procedure would provide teratocytes
free from host hemocytes, at this time we have not developed expertise sufficient to provide the quantity of teratocytes needed to conduct the work.
A confounding issue that must be considered in future studies is the fact
that polydnavirus from Cumpoletis sonorensis (Cameron) is found in the nuclei
of hemocytes, tracheal epithelium, muscle, and fat body cells of its H . virescens
host even though replication of the virus was not observed [22]. In addition,
both male and female wasps contain DNA sequences homologous to the
polydnavirus DNA and there may be at least some integration of the viral
sequences into the wasp genome [23]. This suggests that the virus is transmitted vertically through the germline. Although C. sonorensis is an Ichneumonidae and does not have teratocytes, both Ichneumonidae and Braconidae
have polydnavirus. It is likely that all tissue associated with the egg, including the teratocytes, contain the polydnavirus genome. Therefore, our conclusions that teratocytes cause the described retardation of host development may
actually be the result of the expression of viral genome via the teratocytes.
Nevertheless, our results clearly demonstrate an important role of the teratocyte
in the parasitoid-host interaction.
The growth and development of teratocyte-injected larvae were very similar to those of larvae following parasitism except for those that developed into
ILPE. Our findings were similar to those of Vinson [lo] who demonstrated
that H. virescens larvae injected with approximately 100 C. nigriceps teratocytes
(50% of the normal number) formed abnormal pupae that became sclerotized
and appeared normal in the head and abdominal regions, whereas the ventral thoracic region did not become sclerotized because of the failure of the
wing pads to develop. Our results suggest that the injected teratocytes inhibit
the process of larval-pupal transformation. According to our investigations,
the inhibitor(s) are more active in younger teratocytes and younger hosts are
more sensitive than are older hosts.
Even though the older teratocytes are less effective in producing altered
growth in H . virescens larvae, it should be emphasized that it is only a case of
degree and even teratocytes that were collected from naturally parasitized hosts
6 days after the estimated eclosion time of the parasitoid egg still had major
impact (Tables 1 and 3). There does seem to be some trauma to the teratocytes
caused by the collection process because the transplanted cells were only via-
60
Zhang and Dahlman
ble for 5 days, whereas teratocytes from natural parasitization were still viable
10 days post-parasitization. It is also possible that a synergisticreaction occurs
between the teratocytes and other factors associated with a truly parasitized
host that enhances teratocyte survival. In at least one case, transplanted
teratocytes attained even larger sizes when introduced into nonparasitized larvae 1101.
The action of the teratocytes appears to be directed toward the latter part of
the last stadium because the response of both N-5 and BD-5 larvae to 3 day
old teratocytes were similar (Table 1). These data support the conclusion that
the action occurs quickly, but they also support the possibility that the larvae
may be conditioned several days prior to the time of response.
The specific nature of the inhibitors remain undetermined. Joiner et al. [13]
reported that extracts of teratocytes of C. nigriceps exert juvenile hormone activity. If this also is the case with M . croceipes, it is likely that the juvenile hormone titer is higher in younger teratocytes. Alternatively, juvenile hormone
titers in the host may be higher as the result of a failure of juvenile hormone
esterase to clear the hemolymph. Dahlman et al. [24] have reported reduced
levels of juvenile hormone esterase in H. virescens parasitized by M. croceipes
and Zhang and Dahlman (unpublished data) have shown that transplants of
M . croceipes teratocytes inhibit juvenile hormone esterase production in H.
virescens larvae. Further studies will undoubtedly enable us to develop an essential understanding of the importance that teratocytes play in this parasitoidhost interaction. This is the first report that has demonstrated quantitative
differences in hosts treated with various numbers and ages of teratocytes in
the absence of a parasitoid larva.
LITERATURE CITED
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