Studies were conducted on the physiological mode of action and mode of perception of a queen pheromone that inhibits dealation (wing-shedding) and ovary development in virgin queens of the fire ant Solenopsis inuicta. Winged virgin queens were removed from the pheromonal signal (queen) to compare the response time for dealation, a behavioral response, and ovary development, a physiologi- cal response. Dealation was always accompanied by some degree of ovary development, whereas some individuals exhibited slightly developed ovaries before dealation occurred, suggesting that ovary development precedes dealation by several hours to a day. The response time of virgin queens following extinction of the pheromonal signal was highly variable and was related to colony source and body weight. Individuals from monogyne (single functional queen) colonies were more responsive than those from polygyne (multiple functional queens) colonies, a result consistent with the high cumulative levels of queen pheromone in the latter colonies. Virgin queens treated topically with the juvenile hormone (JH) analogue, methoprene, dealated and developed their ovaries in the presence of the pheromone (queen), suggesting that the pheromonal mode of action involves the suppression of JH titers. To obtain more precise information on the physiological processes underlying the inhibition of ovary develop- ment, vitellogenin titers were determined for virgin queens and functional (egg-laying) queens. Despite having undeveloped ovaries, virgin queens had vitellogenin titers that were as elevated as those of functional queens. This suggests that the effect of the low JH titers resulting from the primer pheromone is on the uptake of vitellogenin by the oocytes rather than on vitellogenin synthesis. The possible mode of perception of the inhibitory pheromone was also investigated. Virgin queens whose antennae had been removed dealated in the presence of the queen, suggesting that the pheromone acts by stimulating sensory cells in the antennae. The results are incorporated into a general model for the mode of action of the queen primer pheromone.
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Pergamon 0022-1910(94)E0013-6
J. fns~ct Phwiol. Vol. 40, No. 7, pp. 601-610. 1994
Copyright :C 1994 Elsevier Science Ltd
Printed in Great Britain. All rights reserved
0022-1910/94 $7.00 + 0.00
Studies on the Mode of Action of a Queen
Primer Pheromone of the Fire Ant
Solenopsis invicta
EDWARD L. VARGO,* MICHELE LAUREL*
Received 5 October 1993: reaised 16 December 1993
Studies were conducted on the physiological mode of action and mode of perception of a queen
pheromone that inhibits dealation (wing-shedding) and ovary development in virgin queens of the fire
ant Solenopsis inuicta. Winged virgin queens were removed from the pheromonal signal (queen) to
compare the response time for dealation, a behavioral response, and ovary development, a physiologi-
cal response. Dealation was always accompanied by some degree of ovary development, whereas some
individuals exhibited slightly developed ovaries before dealation occurred, suggesting that ovary
development precedes dealation by several hours to a day. The response time of virgin queens following
extinction of the pheromonal signal was highly variable and was related to colony source and body
weight. Individuals from monogyne (single functional queen) colonies were more responsive than those
from polygyne (multiple functional queens) colonies, a result consistent with the high cumulative levels
of queen pheromone in the latter colonies. Virgin queens treated topically with the juvenile hormone
(JH) analogue, methoprene, dealated and developed their ovaries in the presence of the pheromone
(queen), suggesting that the pheromonal mode of action involves the suppression of JH titers. To obtain
more precise information on the physiological processes underlying the inhibition of ovary develop-
ment, vitellogenin titers were determined for virgin queens and functional (egg-laying) queens. Despite
having undeveloped ovaries, virgin queens had vitellogenin titers that were as elevated as those of
functional queens. This suggests that the effect of the low JH titers resulting from the primer
pheromone is on the uptake of vitellogenin by the oocytes rather than on vitellogenin synthesis. The
possible mode of perception of the inhibitory pheromone was also investigated. Virgin queens whose
antennae had been removed dealated in the presence of the queen, suggesting that the pheromone acts
by stimulating sensory cells in the antennae. The results are incorporated into a general model for
the mode of action of the queen primer pheromone.
Formicidae Reproduction Primer pheromone Queen Juvenile hormone
INTRODUCTION
The study of insect chemical communication is a
rapidly expanding field. The last 30 years have seen
tremendous progress in our understanding of the chem-
istry of insect pheromones, their mode of perception,
and the regulation of their biosynthesis and release
(Mayer and Mankin, 1985; Tamaki, 1985; Kaissling,
1986; Prestwich and Blomquist, 1987; Mayer and
McLaughlin, 199 1; Schneider, 1992; Raina and
Menn, 1993). However, nearly all of this progress has
concerned releaser pheromones, i.e. intraspecific chemi-
cal signals that elicit behavioral responses, such as
sex attractants. In contrast, progress has been much
slower in our understanding of primer pheromones,
*Department of Zoology, University
U.S.A. of Texas, Austin, TX 78712,
which have less rapid but more profound effects on the
physiology of the target individuals (Wilson and Bossert,
1963).
Primer pheromones are thought to be widespread
among the social insects. They are produced by the
primary reproductives (queens in the social Hymen-
optera) and serve to regulate ovary development in
adult female members of the colony and influence the
developmental fate of larvae (Wilson, 1971). We cur-
rently have little information on the chemical nature of
social insect queen primer pheromones, their mode of
perception, or their physiological mode of action. The
only social insect primer pheromone that has been
identified is that of the queen honey bee, which orig-
inates primarily in the mandibular gland [reviewed in
Winston and Slessor (1992)]. Originally, this queen
mandibular pheromone, especially its major constituent,
(E)-9-keto-2-decenoic acid, was reported to inhibit both
601
602 EDWARD L. VARGO and MICHELE LAUREL
the rearing of new queens (Butler, 1959a, 1960, 1961;
Butler and Paton, 1962; Butler and Callow, 1968) and
ovary development in workers (de Groot and Voogd,
1954; Butler 1959b; Butler et al., 1962; Butler and Fairey,
1963). However, recent studies using synthetic com-
ponents in natural proportions suggest that, although
this pheromone complex inhibits the rearing of new
queens (Winston et al., 1989, 1990), it does not inhibit
worker ovary development (Willis et al., 1990).
There are few data concerning the mode of perception
of the honey bee queen pheromone. Kaissling and
Renner (1968) found that (E)-9-keto-2-decenoic acid
elicits firing by specialized sensory cells in the antennae
of worker honey bees. However, the response of worker
antennae to the other components of the queen
mandibular gland secretion has not been investigated.
Information is also lacking concerning the physiological
mode of action of the honey bee queen pheromones.
Kaatz et al. (1992) recently found that (E)-Pketo-2-
decenoic acid inhibits juvenile hormone (JH) secretion
by the corpora allata in young workers. The conse-
quences of lower JH synthesis in response to (E)-9-keto-
2-decenoic acid in young bees are unclear, however,
because this pheromone component may not inhibit
ovary development (Willis et al., 1990). Furthermore,
developed ovaries in queen and worker honey bees is not
related to elevated JH titers (Robinson et al., 1991,
1992), suggesting that in honey bees JH does not play
the role it typically assumes in regulating insect ovary
development (Koeppe et al., 1985).
In the bumble bee, Bombus terrestris, ovary develop-
ment appears to follow the more general insect scheme
involving regulation by JH (Riiseler, 1977; Riiseler and
Rdseler, 1978). In this species queens produce a phero-
mone of known composition in the mandibular glands
that inhibits ovary development in workers by suppress-
ing JH synthesis (Riiseler et al., 1981). No information
is avaiIable on the possible mode of transmission or
perception of this bumble bee queen pheromone.
Several effects of queen pheromones have been docu-
mented in the fire ant Solenopsis invicta. These include
the development of male and female sexuals (Vargo and
Fletcher, 1986), dealation (wing shedding) and ovary
development in winged virgin queens (Fletcher and
Blum, 1981a), and ovary development among cohabiting
egg-laying queens in polygyne (multiple-queen) colonies
(Vargo, 1992). Although there is little specific infor-
mation available on the mode of action of these phero-
mones, evidence to date suggests that pheromonal
regulation of ovary development involves JH.
Results of studies in which virgin queens in isolation
were treated topically with synthetic JH, either by itself
(Kearney et al., 1977) or in combination with allatec-
tomy (Barker, 1978, 1979), indicate that both dealation
and ovary development are under the control of JH.
Using topical application of the JH analogue, metho-
prene, Vargo (1992) obtained evidence that the rate of
oviposition among queens in polygyne colonies is regu-
lated by JH. Fletcher and Blum (1983) and Vargo (1992)
hypothesized that fire ant queen primer pheromones
prevent ovary development in winged queens and repro-
ductively active queens, respectively, by inhibiting JH
synthesis. However, the possible JH-suppressing effect of
the pheromone has not been tested.
In this paper, we investigate several aspects of the
mode of action and mode of perception of the phero-
mone inhibiting dealation and ovary development in
S. invicta virgin queens. Specifically, we address four
questions. First, how closely linked are the responses of
ovary development and dealation? These two responses
are very different; dealation is a behavioral response in
which an individual breaks off her wings with the hind
legs, whereas ovary development is a physiological re-
sponse. Although these two responses occur in individ-
uals removed from the influence of the queen, it is not
known whether they occur simultaneously or whether
there is a delay separating them. Second, can the inhibi-
tory effects of the pheromone be overcome with topical
application of a JH analogue? Third, is ovary develop-
ment in virgin queens blocked at the vitellogenin-
synthesis or vitellogenin-uptake stage? Finally, are
antenna1 receptors in the virgin queens involved in the
perception of the pheromone? Answers to these ques-
tions give a more complete picture of the physiological
mode of action of this queen primer pheromone.
MATERIALS AND METHODS
Source and maintenance of ants
All ants used in these experiments originated from
Travis and Hays Counties, TX. The colonies were
collected by excavation of the mound, and the ants were
removed from the soil by flooding (Jouvenaz et al.,
1977). Dates of collection are given below. Colonies were
housed in plastic trays (40 x 52 x 8 cm) equipped with
four nests (14 cm dia Petri dishes half-filled with damp
plaster) and maintained in the laboratory at 29 + 2°C
and natural photoperiod. The ants were fed crickets
(Acheta domesticus) daily and given sugar water and tap
water ad libitum.
Timing of dealatdon and ovary development
Monogyne colonies (n = 9) used in this experiment
were collected on 6 May 1991 and polygyne colonies
(n = 9) were collected on 13 May 1991. All colonies had
numerous winged virgin queens present at the time of
collection.
Dealation and ovary development were studied using
two winged virgin queens placed in small colony frag-
ments (2.5 ml workers and brood) as described by
Fletcher and Blum (1981 b). Virgin queens were moni-
tored every 12 h for dealation, defined as having lost at
least three of the four wings. The time elapsed for
dealation to occur in at least one of the two virgin queens
present in each fragment was recorded. Ovary develop-
ment was assessed by dissecting virgin queens in 70%
ethanol under a dissecting microscope and counting the
FIRE ANT PRIMER PHEROMONE 603
total number of chorionated eggs (fully-formed eggs, i.e.
those 20.4 mm wide x 0.28 mm long) present in the
ovarioles and common oviduct. At the beginning of each
experiment, a sample of virgin queens from each colony
was weighed and dissected to assess ovary development.
In this experiment, the timing of dealation and ovary
development was investigated in virgin queens originat-
ing from monogyne and polygyne colonies as well as the
effect of the weights of individual queens on response
time. Ten virgin queens from each colony were weighed
and then dissected to assess ovary development. Ten
others were weighed and placed in pairs into small
colony fragments (n = 5 fragments per colony); each
individual was marked with one of two colors of paint
(Tex Pen"', Mark-Tex Corp., Engelwood, NJ) for later
identification. The fragments were examined every 12 h
for the presence of dealates. As soon as at least one
dealate was observed in a fragment, both virgin queens
present were removed, dissected and their ovary develop-
ment assessed. The experiment ran for three weeks, after
which time all remaining virgin queens were dissected to
determine the degree of ovary development.
Methoprene treatment
The ants used for this experiment were of the mono-
gyne form and were collected 26 and 30 May 1992 from
Hays and Travis Counties, TX. All colonies contained
numerous alate queens. Approximately one month after
collection, the colonies were sieved to remove all sexual
forms [adults, pupae and large fourth instar larvae;
Vargo (1988)] and reduced to a size of 10 g adult workers
and brood (approx. 5500 adult workers, 4000 worker
pupae and 8500 larvae of all stages). The colonies were
housed in rearing trays (40 x 52 x 8 cm), each contain-
ing a single nest as described above. The mother queen
of the colony was placed in the fragment. Twenty virgin
queens, all field-reared, were returned to each colony.
Half of these were topically treated with 0.5 pg metho-
prene in 0.5 ~1 acetone by placing the solution on the
abdominal dorsum. The other half of the virgin queens
was treated with only the acetone carrier. Each of the
queens was marked with a spot of Texpen ink identifying
it to group. The colony fragments were monitored daily
for three days for the presence of any dealated virgin
queens. All dealates present on a given day were re-
moved and dissected to assess ovary development. At the
end of three days, all remaining winged virgin queens
were removed and their ovaries examined.
Vitellogenin titers
Vitellogenin titers were determined by SDS-PAGE
based on the methods of Martinez and Wheeler
(1991a, b). Hemolymph from individual queens was
collected in 0.5 ~1 capillary tubes through a small in-
cision on the thoracic dorsum and immediately trans-
ferred into a tube containing 10 ~1 SDS sample buffer
(62 mM Tris-HCl, pH 6.8, 10% glycerol, 5% 2-mer-
captoethanol, 2% SDS, 0.005% Bromophenol Blue).
Freshly-laid eggs were homogenized directly into 10 ~1
SDS sample buffer. Samples were centrifuged for 3 s at
8000g and stored at -20°C. Just prior to loading, the
samples were thawed, boiled for 5 min, centrifuged for
3 s at SOOOg, and transferred to ice. Aliquots from each
sample were used for SDS-PAGE analyses.
Samples were subjected to SDS-PAGE using a 7%
polyacrylamide gel with a 4% stacking gel. Electrophor-
esis was performed at constant voltage with a starting
current of 20 mA. Gels were stained with 0.24%
Coomassie Brilliant Blue R 250 in 45.6% methanol,
9.2% acetic acid and destained in 7.5% acetic acid, 5%
methanol.
The density of the vitellogenin bands was quantified
for each sample using a video densitometer designed by
L. L. Poulsen (U.S. Patent No. 5194949). The band
corresponding to the most abundant protein in freshly-
laid eggs (approx. 180 kDa) was determined for each
sample. The quantity of vitellogenin present was deter-
mined by comparison with the density of a known
quantity of myosin standard (200 kDa).
All queens sampled were dissected to assess ovary
development. The ovaries were removed and the maxi-
mum number of opaque vitellogenic oocytes per ovariole
was determined (Tschinkel, 1988) by examining several
ovarioles.
Antennectomy
Five monogyne colonies, containing numerous winged
queens, were collected 26 and 30 May 1992. Approxi-
mately four weeks after collection, the colonies were
sieved to remove sexuals, and the workers and brood
were divided into two equal halves based on weight
(mean + SD weight of colony halves = 11.5 + 2.8 g). The
mother queen was returned to one of the halves (queen-
right halves). Each colony half was given a total of 20
nestmate winged virgin queens, all of which were field
reared and at least one month post emergence age. The
antennae were removed from 10 of the virgin queens in
each colony half by cutting the scape near the insertion
point with fine dissecting scissors. The 10 control virgin
queens in each colony half were amputated at a middle
leg by cutting the proximal region of the femur. The
colony halves were monitored daily for the presence of
dealates. Any dealates observed were removed and dis-
sected to assess ovary development as above. Ovary
development was also assessed for 10 winged virgin
queens from each colony at the time of colony division.
The experiment was terminated after 15 days.
RESULTS
Timing of dealation and ovary development
There was a large difference between monogyne and
polygyne colonies in the frequency of virgin queen
dealation; by 72 h dealation had occurred in all
25 fragments containing virgin queens from mono-
gyne colonies, whereas dealation had occurred in only
three (12%) of the 25 fragments from polygyne colonies
(X: = 25.1, P < 0.0001). The number of polygyne
604 EDWARD L. VARGO and MICHELE LAUREL
fragments containing dealates had risen to only 7 (28%)
by the end of the experiment. Virgin queens from
polygyne colonies that dealated took nearly four times
longer to do so than did those from monogyne colonies
(mean = 120.0 + 156.0 and 31.7 + 15.8 h for polygyne
and monogyne, respectively; t,, = 2.90, P < 0.01, two-
tailed test). Among the monogyne colonies, there was a
significant effect of colony on the time to dealation
(F4.20 = 4.49, P < 0.01).
Dealation was related to both higher weights and
increased ovary development. For the colony fragments
in which only one of the two virgin queens dealated, the
dealated individual had an initial weight that was on
average 4% more than the alate (mean differ-
ence = 0.58 4 0.85 mg; t,, = 3.22, P < 0.005, two-tailed
paired test). When the virgin queens from monogyne and
polygyne colonies were analyzed separately, the dealates
in the polygyne form had an initial weight that was 8.6%
greater than alates (mean difference = 1 .l + 0.9 mg,
t5 = 3.0, P < O.OS), whereas in the monogyne form
dealates weighed only 2.8% more (mean differ-
ence = 0.4 + 0.8 mg, t,, = 2.0, P > 0.05). The difference
in ovary development between the dealate and alate in
the same fragment was even more pronounced. When
virgin queens from monogyne and polygyne colonies
were taken together, the dealated females had an average
of 7.6 f 2.0 chorionated oocytes compared to 3.2 + 2.8
for the alates (tz, = 6.85, P < 0.0001, two-tailed paired
test). Again, when analyzed separately, this difference
was greater among females of the polygyne form, where
dealates had 6.2 f 2.0 more chorionated oocytes than
did alates (ts = 7.4, P < 0.001); in fragments from
monogyne colonies dealates had 3.8 f 3.2 more
(tls = 4.8, P < 0.0002).
A comparison of the degree of ovary development in
relation to dealation status was performed by means of
a two-way ANOVA, with social form and dealation
status as the independent variables. In order to detect
changes in ovary development under queenless con-
ditions, winged virgin queens from the queenright
colonies at the start of the experiment were also included
in this analysis. Both social form (F,,,9, = 75.4,
P < 0.0001) and dealation status (F2.,9, = 105.1,
P < 0.0001) had highly significant effects, and the inter-
action term was also significant (F2,19, = 3.1, P < 0.05).
Therefore, the effects of dealation status were analyzed
separately for each form. In both social forms, there was
a highly significant difference in the degree of ovary
development among all three groups, i.e. dealates and
alates in the queenless colony fragments and alate virgin
queens in queenright colonies at set-up (Table 1;
F2;.97 = 80.3, P < 0.0001 and F2,94 = 72.8, P < 0.0001, for
the monogyne and polygyne forms, respectively). In
both forms, dealates showed the most ovary develop-
ment, whereas alate virgin queens in the queenless
colony fragments had significantly more developed
ovaries than did those in the presence of the queen at the
start of the experiment (P < 0.05, Tukey test). These
results suggest that the responses of dealation and ovary
TABLE 1. Ovary development (mean k SD) of virgin
queens in response to queenlessness and dealation
Form/condition n
No.
chorionated
oocytes
Results of the
Tukey test
Monogyne
Q+ 50 2.1 + 2.1 A
Alate 16 4.3 +_ 2.4 B
Dealate 34 8.7 f 2.7 C
Pol.vgyne
Q+ 50 0.2 f 0.5 A
Alate 39 1.0* 1.2 B
Dealate 8 5.4 _+ 2.8 C
Queenright condition (Q +) refers to condition of the virgin
queens in their natal colonies at the time the experiment
was started. Virgin queens were set up in pairs in small
queenless colony fragments. As soon as at least one
individual in each fragment dealated, both virgin queens
present were dissected to assess ovary development.
Included in the polygyne form are those fragments in
which no dealation occurred during the 3-week exper-
iment. Treatments with different letters within each form
differed significantly (P < 0.05, Tukey test).
development are tightly linked, but that ovary develop-
ment begins slightly before dealation.
Methoprene treatment
Of the 50 methoprene treated virgin queens, 44 (88%)
individuals dealated by the end of the 3-day period
(Fig. 1), with a mean of 8.8 + 1.3 individuals per colony
100 y
0
Methoprene Control
FIGURE 1. Effect of topical treatment of methoprene (0.5 ng in 0.5 ~1
acetone; n = 50) on dealation and ovary development (mean & SE) in
the presence of queen inhibitory pheromone. Controls (n = 45) were
treated with 0.5 ~1 of the acetone carrier.
FIRE ANT PRIMER PHEROMONE 605
(range = 7-10). In contrast, none of the control virgin
queens shed their wings during this time. The difference
between the two groups was highly significant when
analyzed either by pooling individuals from all colonies
(P < 0.0001, Fisher exact test) or by examining each
colony separately (all P < 0.0025, Fisher exact test). A
total of five control individuals from three different
colonies were found dead of unknown causes.
Individuals in the methoprene treatment had far more
developed ovaries (Fig. 1). A two-way ANOVA showed
a significant effect of both treatment (F,,, = 74.6,
P < 0.0001) and colony (F4.85 = 3.2, P < 0.02) but not of
the interaction term (F4.85 = 1.3, P > 0.25).
Vitellogenin titer determinations
As shown in Fig. 2, there was no significant difference
among the three categories of queens in their titers of
vitellogenin (F2,2, = 0.6, P > 0.5). This is in sharp con-
trast to the large differences in ovary development
(Fig. 2; F2.2, = 221.5, P < O.OOOl), in which all categories
differed significantly from the others (P < 0.05, Tukey
test). Thus, inhibited virgin queens with undeveloped
ovaries had vitellogenin titers comparable to those of
40
s 30
a
s
.; 20
B
B
5 10
0
n .5! 20
b
P 15
s
ii
.g 10
g
al
.? 5
$
>i
r la
r" 0 WQ PQ MQ
FIGURE 2. Relationship between vitellogenin titers (pg/pl hemo-
lymph, expressed as myosin equivalents) and ovary development in S.
inuicta queens. Shown are means f SE. WQ, winged virgin queens in
the presence of a pheromone producing queen (n = IO); PQ, egg-laying
dealated queens from polygyne colonies (n = 9); MQ, egg-laying
dealated queens from monogyne colonies (n = 5). Groups with differ-
ent lower case letters differed significantly (P < 0.05, Tukey test). absence (P = 0.15, Fisher exact test).
FIGURE 3. Effect of removing antennae on dealation in the presence
and absence of queen inhibitory pheromone. The antennectomized
(-antennae) group (n = 9 and 25 in the presence and absence of the
queen, respectively) had their antennae removed by cutting them off
at the proximal end of the scape. The control group (n = 41 and 49 in
the presence and absence of the queen, respectively) had one middle
leg removed by cutting at the proximal end of the femur. A signifi-
cantly higher frequency of antennectomized individuals dealated in the
presence of the queen (P <O.OOl. Fisher exact test) but not in her
actively laying queens with highly developed ovaries.
These results suggest that the effect of the inhibitory
pheromone is to block ovary development primarily
by suppressing the uptake of vitellogenin rather than
affecting its rate of synthesis.
Antennectomy
There was considerable mortality of the virgin queens
whose antennae had been removed. Of the 50 treated
individuals, 25 (50%) died in the queenless colony halves
during the experiment (range = 3-9 per colony half). In
contrast only a single control individual died in the
queenless halves. Mortality was higher in the queenright
'halves, in which only nine of the antennaeless individuals
survived (range = 4-o survivors per colony half), as
opposed to 38 survivors in the control group. In some
cases, workers were seen attacking individuals, all of
which were wingless. It is not known whether dealation
occurred before attack by workers or as a result of
attack. However, since no alates were seen to be at-
tacked, it is likely that worker attack followed dealation
rather than causing it.
Among the surviving virgin queens, there was a strong
difference in the frequency of dealation in the queenright
halves but not the queenless halves (Fig. 3). In the
queenright halves, all-nine of the surviving ir tdividuals
20
0 - Antennae Control
606 EDWARD L. VARGO and MICHELE LAUREL
whose antennae had been removed dealated, whereas
only one (2.6%) of the controls shed her wings
(P < 0.001, Fisher exact test). In contrast, the frequency
of dealation in the two groups was similar in the
queenless halves (P = 0.145, Fisher exact test).
In comparing ovary development, individuals were
grouped according to whether queen pheromone was
present or absent (queenright vs queenless colony
halves), and whether they had dealated in the course of
the experiment (alate vs dealate). Out of the eight
possible groups (2 pheromone conditions x 2 treat-
ments x 2 states for wing presence), three groups were
not included because one group (antennaeless alates in
queenright colony halves) had no members and two
groups (antennaeless alates in queenless colony halves
and control dealates in queenright colony halves) had
very few members (n = 2 and 1, respectively). Individ-
uals that were in the queenright colonies on the day the
experiment started also were included in the analysis. An
ANOVA showed a significant difference among the
groups (& = 1.3, P < 0.0001). As seen in Table 2, this
difference was due to the greater degree of ovary devel-
opment in the control dealates in the queenless colony
halves than in those of all the other groups. Thus,
although antennectomy caused virgin queens to dealate,
even when in the presence of a pheromone producing
queen, development of the ovaries that normally accom-
panies wing shedding did not occur.
DISCUSSION
The finding that topical application of a JH analogue
to virgin queen fire ants overcomes the inhibitory effects
of the queen pheromone strongly suggests that the
pheromone inhibiting dealation and ovary development
acts by suppressing JH titers. The precise mechanism of
JH titer suppression was not investigated, but a likely
candidate is inhibition of JH synthesis by the corpora
allata. There is growing evidence from social insects that
queen primer pheromones affect JH titers by suppressing
the activity of the corpora allata. In the bumble bee,
B. terrestris, Roseler et al. (1981) found that extracts of
queen mandibular glands prevented ovary development
in workers by inhibiting JH production by the corpora
allata (Riiseler, 1977; Rijseler and Roseler, 1978). Re-
cently, Kaatz et al. (1992) showed in the honey bee that
synthetic (E)-9-oxo-2-decenoic acid, the principal com-
ponent of the honey bee queen mandibular gland se-
cretion, inhibits JH synthesis by the corpora allata in
young (8 day-old) workers. However, the effects of lower
rates of JH synthesis in young bees is not clear, because
JH does not appear to be the gonadotropic hormone in
this species (Robinson et al., 1991, 1992). In the termite,
Zootermopsis angusticollis, Greenberg and Tobe (1985)
reported that larvae had lower rates of JH synthesis by
the corpora allata in the presence of the king and queen
than in their absence. It is likely that a pheromone
produced by the royal pair is responsible for this effect,
but the existence of an inhibitory pheromone in
Z. angusticollis has not been conclusively demonstrated
(Stuart, 1979).
Although it is well established that social insect queens
inhibit ovary development in nestmates, little is known
about the process of oogenesis and the physiological
mechanisms underlying its control. The present results
demonstrate that virgin queens of S. invicta, whose
ovary development is blocked by the queen inhibitory
pheromone, have vitellogenin titers comparable to those
of egg-laying queens. Thus in fire ants, the presence of
vitellogenin in the hemolymph is not sufficient to induce
yolk deposition, a phenomenon noted in other insects
(Bell and Barth, 1971), including the honey bee (Engels,
1974) and the ant Camponotus festinatus (Martinez and
Wheeler, 1991a). Evidently, in many insects a specific
trigger is necessary for vitellogenin uptake to occur, and
it is such a trigger that is blocked by the fire ant queen
inhibitory pheromone. As discussed below, the trigger in
S. invicta queens may be a relatively high titer of JH.
Although the main effect on ovary development appears
to be inhibition of oogenesis, it is nevertheless possible
that the queen pheromone reduces the rate of vitel-
logenin synthesis in inhibited virgin queens compared to
reproductively active queens. However, because vitel-
logenin is not incorporated in the former group, it
accumulates and eventually reaches titers as elevated as
those in egg laying queens, which may be synthesizing
TABLE 2. Effect of antennectomy and queen pheromone on ovary development in virgin
queens
No. chorionated Results of
oocytes Tukey test
Treatment Dealated n (mean * SD) (P < 0.05)
Queenright colony at start _ 50 1.5 f 2.4 B
Queenright colony half
Antennae removed + 9 1.2k2.9 B
Control _ 37 0.9 * 1.5 B
Queenless colony half
Antennae removed + 23 1.9k2.6 B
Control - 9 2.6 & 3.6 B
Control + 40 1.3 & 4.6 A
Treatments with different letters differed significantly (P < 0.05. Tukey test).
FIRE ANT PRIMER PHEROMONE 607
vitellogenin at higher rates. Studies of the rate of
vitellogenin synthesis and turnover in inhibited and
reproductively active queens are needed to determine
possible effects of the pheromone on vitellogenin
synthesis.
In this study, we found that virgin queens amputated
of their antennae dealated in the presence of a queen
producing the primer pheromone. This result is expected
if the primer pheromone is perceived through sensory
cells in the antennae, and the inhibitory signal is then
transmitted via the nervous system to the target tissues,
presumably the corpora allata. In many insects, the
activity of the corpora allata is regulated by neuro-
secretory cells in the brain [reviewed in Feyereisen
(1985)]. Results from extirpation experiments on
S. inuicta virgin queens (Barker, 1978) are consistent
with this general model and suggest that the median
neurosecretory cells regulate the activity of the corpora
allata. In line with the present results, Sorensen et al.
(1985) found that the pheromone components, which
appear to be relatively non volatile (Fletcher and Blum,
198la), could be quickly and efficiently transported
throughout fire ant colonies via surface contact.
A puzzling result of the antennectomy experiment was
that ovary development in amputated individuals did
not coincide with dealation. This is unexpected if inhi-
bition of both dealation and ovary development is
achieved simply by pheromonal stimulation of sensory
cells in the antennae, which in turn act on the corpora
allata to inhibit JH synthesis. The reasons for the
uncoupling of dealation and ovary development in this
case are not clear, but possible explanations include:
(1) the presence of other sensory cues perceived
by the antennae that are needed to stimulate
oogenesis; and
(2) antennectomy induces dealation by affecting the
nervous system via some alternate route not
involving JH.
We do not yet have sufficient data to exclude either of
these possibilities.
Based on the results of the present study, we propose
a general model, illustrated in Fig. 4, for the mode of
action of the fire ant queen pheromone that inhibits
dealation and ovary development. The pheromone is
perceived by sensory cells in the antennae. Stimulation
of these sensory cells inhibits the brain from releasing
chemical and/or neural signals that stimulate the cor-
pora allata to synthesize JH. Consequently, the JH titer
is maintained at relatively low levels in the presence of
the pheromone. While the JH titer is kept relatively low,
dealation and oogenesis by virgin queens are prevented.
Presumably, JH acts directly on the nervous system to
elicit dealation, whereas it acts indirectly on ovary
development by stimulating the oocytes to absorb pre-
viously synthesized vitellogenin. The uncoupling of
dealation and ovary development following antennec-
tomy, as discussed above, suggests the physiological
control of these processes is more complicated than
indicated here, but the precise nature of these compli-
cations has yet to be determined.
We also hypothesize that JH controls vitellogenin
synthesis by the fat body, as occurs in many insects
(Koeppe et al., 1985), but that vitellogenin synthesis has
a lower JH threshold than does vitellogenin uptake by
the ovaries. Thus the pheromone does not completely
suppress JH synthesis, but reduces it to a low level,
keeping the JH titer below the vitellogenin absorption
threshold but above the vitellogenin synthesis threshold.
Further studies are needed to test and refine this model.
Of critical importance in this regard will be the identifi-
cation of the active pheromone components and JH titer
determinations.
Once removed from the queen primer pheromone,
ovary development in virgin queens appears to begin
slightly ahead of dealation. Although several studies
have shown that dealation by virgin queens of S. invicta
is associated with eventual oogenesis and oviposition
(Kearney et al., 1977; Barker, 1978; Tschinkel and
Howard, 1978; Fletcher and Blum, 198la. 1983; Fletcher
et al., 1983) the precise timing of these events was
previously unknown. Fletcher and Blum (1981a) dis-
sected dealate virgin queens one week after queenlessness
and found some with developed ovaries. Fletcher et al.
(1983) reported that some dealate virgin queens exam-
ined 72 h after queenlessness exhibited ovary develop-
ment. The present study, in which virgin queens were
dissected within 12 h post-dealation, offers a more de-
tailed view of the timing. All of the dealates showed
some signs of ovary development, whereas only some of
the alates, which had been free from the queen phero-
mone for the same period of time as the dealates,
exhibited signs of ovary development. The finding that
ovary development appears to precede dealation
suggests that either dealation has a slightly higher JH
threshold than vitellogenin absorption by the ovaries, or
dealation proceeds more slowly after the JH threshold is
crossed. Histolysis of the flight muscles, another re-
sponse of virgin queens to queenlessness, takes longer to
occur than does dealation; it begins just after wing
shedding and is complete after about 10 days (Barker,
1979).
In the present study, polygyne virgin queens were
much less responsive than monogyne virgin queens to
removal of the pheromone-producing queens. Keller and
Ross (1993a) also found polygyne queens to be less
responsive. These authors reported that genotype greatly
affects the response of virgin queens from polygyne but
not monogyne colonies. They showed that in polygyne
colonies from Georgia, mature virgin queens with the
genotype Pgm-3"/-3" weighed 13% more than virgin
queens of the other two genotypes that occur at this
locus (Pgm-3"/-3' and Pgm-3b/-3 b, and, following re-
moval of all reproductively active queens, were 5 times
more likely to dealate and begin laying eggs during the
3-day test period. Although the frequency of the Pgm-3
alleles has not been investigated in our Texas study
population, the small number of responsive individuals
608 EDWARD L. VARGO and MICHELE LAUREL
Y
Neural
and/or
chemical 1 0
signal y
CA
3
JH
Low tiler 0
Vitellogenin
FIGURE 4. Proposed general model for the mode of action of the primer pheromone of queen fire ants that inhibits dealation
and ovary development in virgin queens. In this model, the pheromone triggers antenna1 receptors which send inhibitory signals
to the median neurosecretory cells in the brain. Largely inhibited, the median neurosecretory cells only weakly stimulate the
corpora allata to synthesize JH, maintaining low titers of this hormone. At low levels, JH stimulates vitellogenin synthesis in
the fat body. In the absence of the pheromone, the disinhibited neurosecretory cells send a stronger chemical and/or neural
signal that triggers the corpora allata to produce larger quantities of JH. At higher titers, JH stimulates vitellogenin uptake
by the ovaries and dealation, the latter process possibly involving an effect of JH on the nervous system. Dealation may result
from a JH-independent pathway in the nervous system in lieu of or in addition to the JH-mediated pathway. These two possible
pathways for control of dealation are flagged with question marks.
from polygyne colonies found here (12% after 3 days) is
similar to the low frequency of the more responsive
Pgm -3 "l-3" individuals occurring in a Georgia polygyne
population [l 1%; Keller and Ross (1993a)], suggesting
that the frequency of the Pgm-3"/-3" genotype may be
similar in the two populations. Interestingly, this geno-
typic effect on reproductive maturation does not occur
in monogyne colonies, in which all three genotypic
classes are equally likely to undergo reproductive devel-
opment under queenless conditions (Keller and Ross,
1993a).
As hypothesized by Keller and Ross (1993a), the
low responsiveness of virgin queens in polygyne colonies
probably results from the pheromonal milieu of the
colony. Because of the relatively high levels of queen
pheromone that likely accumulate in polygyne colonies
(Vargo and Fletcher, 1987), polygyne virgin queens
are subjected to more intense inhibition than virgin
queens in monogyne colonies. This increased phero-
monal influence likely affects the rate of virgin queen
reproductive maturity in a genotype-specific manner.
Evidence for such a social environment-genotype inter-
action has been recently documented by Keller and Ross
(1993b), in which queen pupae from monogyne and
polygyne colonies were cross-fostered in colonies of the
alternate type. In addition to an effect of social form on
FIRE ANT PRIMER PHEROMONE 609
reproductive development of virgin queens, the results of
the present study indicate there is a strong effect of
source colony in the monogyne form. The causes of this
variability are not known but could involve one or
more of the following: age differences, genotype, social
environment and ecological factors.
REFERENCES
Barker J. F. (1978) Neuroendocrine regulation of oocyte maturation
in the imported fire ant Solenopsis invicta. Gen. camp. Endocrinol. 35,
234-237.
Barker J. F. (1979) Endocrine basis of wing casting and flight muscle
histolysis in the fire ant Solenopsis invicta. Experientia 35, 552-554.
Bell W. J. and Barth R. H. (1971) Initiation of yolk deposition by
juvenile hormone. Nature, New Biol. 230, 220-222.
Butler C. G. (1959a) Queen substance. Bee World 40, 269-275.
Butler C. G. (1959b) The source of the substance produced by a queen
honey bee (Apis mell~fira L.) which inhibits development of ovaries
of the workers of her colony. Proc. Ro_v. ent. Sot. London (A) 34,
137-138.
Butler C. G. (1960) The significance of queen substance in swarming
and supersedure in honeybee (Apis mell[fera L.) colonies. Proc. Roy.
ent. Sot. London (A) 35, 129-132.
Butler C. G. (1961) The scent of queen honeybees (A. mellifera L.) that
causes partial inhibition of queen rearing. J. Insect Physiol. 7,
258-264.
Butler C. G. and Callow R. K. (1968) Pheromones of the honeybee
(Apis mell@ra L.): the "inhibitory scent" of the queen. Proc. Roy.
em. Sot. London B 43, 62-65.
Butler C. G. and Fairey E. M. (1963) The role of the queen in
preventing oogenesis in worker honey bees. J. apic. Res. 2, 14-18.
Butler C. G. and Paton P. N. (1962) Inhibition of queen rearing by
queen honey-bees (Apis mellifera L.) of different ages. Proc. Roy. ent.
Sot. London (A) 37, 114-l 16.
Butler C. G., Callow R. K. and Johnston N. C. (1962) The isolation
and synthesis of queen substance, 9-oxodec-trans-2-enoic acid, a
honey bee pheromone. Proc. Ro)). ent. Sot. London (B) 155,417432.
Callow R. K., Chapman J. R. and Paton P. N. (1964) Pheromones of
the honey bee: chemical studies of the mandibular gland secretion
of the queen. J. upic. Res. 3, 77-89.
Engels W. (1974) Occurrence and significance of vitellogenins in female
castes of social Hymenoptera. Amer. Zool. 14, 1229-1237.
Feyereisen R. (1985) Regulation of juvenile hormone titer: synthesis.
In Comprehensive Insect Physiology, Biochemistry and Pharma-
cology. Vol. 7: Endocrinology I (Eds Kerkut G. A. and Gilbert L. I.),
pp. 391429. Pergamon Press, Oxford.
Fletcher D. J. C. and Blum M. S. (1981a) Pheromonal control of
dealation and oogenesis in virgin queen fire ants. Science 212,
73-75.
Fletcher D. J. C. and Blum M. S. (1981 b) A bioassay technique for an
inhibitory primer pheromone of the fire ant, Solenopsis invicta
Buren. J. Ga. ent. Sot. 16, 352-356.
Fletcher D. J. C. and Blum M. S. (1983) The inhibitory pheromone of
queen fire ants: effects of disinhibition on dealation and oviposition
by virgin queens. J. camp Physiol. A 153, 467475.
Fletcher D. J. C., Cherix D. and Blum M. S. (1983) Some factors
influencing dealation by virgin queen fire ants. Insectes sot. 30,
443-454.
Greenberg S. and Tobe S. S. (1985) Adaptation of a radiochemical
assay for juvenile hormone biosynthesis to study caste differentiation
in a primitive termite. J. Insecr Physiol. 31, 347-352.
de Groot A. P. and Voogd S. (1954) On the ovary development
in queenless worker bees (Apis mell$ca L.). Experientia IO,
384-385.
Jouvenaz D. P., Allen G. E., Banks W. A. and Wojcik D. P. (1977)
A survey for pathogens of fire ants, Solenopsis spp.. in the south-
eastern United States. Fla. Em. 60, 275-279.
Kaatz H., Hildebrandt H. and Engels W. (1992) Primer effect of queen
pheromone on juvenile hormone biosynthesis in adult worker honey
bees. J. camp Physiol B 162, 588-592.
Kaissling K.-E. (1986) Chemo-electrical transduction in insect olfac-
tory receptors. A. Rev. Neurosci. 9, 121-145.
Kaissling K.-E. and Renner M. (1968) Antennale Rezeptoren fiir
Queen Substance und Sterzelduft bei der Honigbiene. Zeit verg.
Physiol. 59, 357-361.
Kearney G. P.. Toom P. M. and Blomquist G. L. (1977) Induction of
de-alation in virgin female Solenopsis invicta with juvenile hormones.
Ann. ent. Sot. Am. 70, 699-701.
Keller L. and Ross K. G. (1993a) Phenotypic basis of reproductive
success in a social insect: genetic and social determinants. Science
260, 1107-1110.
Keller L. and Ross K. G. (1993b) Phenotypic plasticity and "cultural
transmission" of alternative social organizations in the fire ant
Solenopsis invicta. Behav. Eeol. Sociohiol. 33, 121-129.
Koeppe J. K., Fuchs M., Chen T. T.. Hunt L. M., Kovalick G. E. and
Briers T. (1985) The role of juvenile hormone in reproduction. In
Comprehensive Insect Physiology. Biochemistry and Pharmacology.
Vol. 8: Endocrinology II (Eds Kerkut G. A. and Gilbert L. I.).
pp. 165-203. Pergamon Press, Oxford.
Martinez T. and Wheeler D. (1991a) Effect of the queen, brood and
worker caste on haemolymph vitellogenin titre in Camponotus
Jtistinatus workers. J. Insect Physiol. 37, 347-352.
Martinez T. and Wheeler D. (1991b) Identification of vitellogenin in
the ant, Camponotus&stinatus: changes in hemolymph proteins and
fat body development in workers. Arch. Insect. Biochem. Physiol. 17.
143- 155.
Mayer M. S. and McLaughlin J. R. (1991) Handbook qf Insect
Pheromones and Sex Attractants. CRC Press, Boca Raton. FL.
Mayer M. S. and Mankin R. W. (1985) Neurobiology of pheromone
perception. In Comprehensive Insect Physiology, Biochemistry and
Pharmacology, Vol. 9: Behaoiour (Eds Kerkut G. A. and Gilbert
L. I.), pp. 95-144. Pergamon Press, Oxford.
Prestwich G. D. and Blomquist G. J. (1987) Pheromone Biochemistry.
Academic Press. New York.
Raina A. K. and Menn J. J. (1993) Pheromone biosynthesis activating
neuropeptide: from discovery to current status. Arch. Insect. Bio-
them. Physiol. 22, 141k151.
Robinson G. E.. Strambi C.. Strambi A. and Feldlaufer M. F. (1991)
Comparison of juvenile hormone and ecdysteroid haemolymph
titres in adult worker and queen honey bees (Apis mellifera). J. Insect
Physiol. 37, 929-935.
Robinson G. E., Strambi C.. Strambi A. and Huang Z. (1992)
Reproduction in worker honey bees is associated with low juvenile
hormone titers and rates of biosynthesis. Gen. camp. Endocrinol. 87,
47 I-480.
RGseler P. (1977) Juvenile hormone control of oiigenesis in bumble bee
workers, Bombus terrestris. J. Insect. Physiol. 23, 985-992.
Rijseler P. and Riiseler I. (1978) Studies on the regulation of the
juvenile hormone titre in bumblebee workers. Bombus terrestris.
J. Insect. Physiol. 24, 707-713.
Riiseler P., Raseler I. and van Honk C. G. J. (1981) Evidence for
inhibition of corpora allata activity in workers of Bombus terrestris
by a pheromone from the queen's mandibular glands. E.xperientiu
37, 348-35 I.
Schneider D. (1992) 100 years of pheromone research. Naturwissen-
schaften 79, 241-250.
Sorensen A. A.. Fletcher D. J. C. and Vinson S. B. (1985) Distribution
of inhibitory queen pheromone among virgin queens of an ant.
Solenopsis invicta. Psyche 92, 5749.
Stuart A. M. (1979) The determination and regulation of the neotenic
reproductive caste in the lower termites (Isoptera): with special
reference to the genus Zootermopsis (Hagen). Sociobiology 4,
223-237.
Tamaki Y. (1985) Sex pheromones. In Comprehensive Insect Physi-
ology, Biochemistry and Pharmacology. Vol. 9: Behaviour (Eds
Kerkut G. A. and Gilbert L. I.), pp. 145-191. Pergamon Press.
Oxford.
Tschinkel W. R. (1988) Social control of egg-laying rate in queens of
the fire ant. Solenopsis inricta. Physiol. Ent. 13, 327T350.
610 EDWARD L. VARGO and MICHELE LAUREL
Tschinkel W. R. and Howard D. F. (1978) Queen replacement in
orphaned colonies of the fire ant, Solenopsis invicta. Behav. Ecol.
Sociobiol. 3, 297-310.
Vargo E. L. (1988) A bioassay for a primer pheromone of queen fire
ants (Solenopsis inuicta) which inhibits the production of sexuals.
Insectes sot. 35, 382-392.
Vargo E. L. (1992) Mutual pheromonal inhibition among queens in
polygyne colonies of the fire ant Solenopsis invicta. Behav. Ecol.
Sociobiol. 31, 205-210.
Vargo E. L. and Fletcher D. J. C. (1986) Evidence of pheromonal
queen control over the production of sexuals in the fire ant,
Solenopsis invicta. J. camp. Physiol. A 159, 741-749.
Vargo E. L. and Fletcher D. J. C. (1987) Effect of queen number on
the production of sexuals in natural populations of the fire ant,
Solenopsis invicta. Physiol. Ent. 12, 109-l 16.
Willis L. G., Winston M. L. and Slessor K. N. (1990) Queen honey bee
mandibular pheromone does not affect worker ovary development.
Can. Ent. 122, 1093-1099.
Wilson E. 0. (1971) The Insect Societies. Belknap Press of Harvard
University Press, Cambridge, MA.
Wilson E. 0. and Bossert W. H. (1963) Chemical communication
among animals. Rec. Prog. Hormone Res. 19, 673-716.
Winston M. L. and Slessor K. N. (1992) The essence of royalty: honey
bee queen pheromone. Am. Sci. 80, 374-385.
Winston M. L., Slessor K. N., Willis L. G., Naumann K., Higo H. A.,
Wyborn M. H. and Kaminski L. A. (1989) The influence of queen
mandibular pheromones on worker attraction to swarm clusters and
inhibition of queen rearing in the honey bee (Apis mel/zj&a L.).
Znsectes sot. 36, 15-27.
Winston M. L., Higo H. A. and Slessor K. N. (1990) Effect of various
dosages of queen mandibular gland pheromone on the inhibition of
queen rearing in the honey bee (Hymenoptera: Apidae). Ann. ent.
Sot. Am. 83, 234238.
Acknowledgements-We thank W. Donaldson for valuable technical
assistance and L. L. Poulsen for the generous use of his video
densitometer. The methoprene was kindly provided by M. A. Rankin.
This material is based in part upon work supported by the Texas
Advanced Technology Program under Grant No. 003658134.
... For the expression analysis, the untreated control was used to calibrate the Vg relative expression. As a positive control to validate the application of the JH analog to S. invicta, virgin alate queens were tested following the protocol by Vargo and Laurel (Vargo & Laurel, 1994) since JH application to alate virgin queens causes them to dealate. Twelve hours following the topical application virgin queens were checked for dealation. ...
... The JH analog, S-hydroprene, had no effect on the expression of the four Vgs 12 h after topical application on the abdomen of workers. Previously, it was reported that topical application of a JH analog to virgin queens resulted in queen dealation coupled with some degree of ovary development 8 and 12 h after JH analog application to queens (Vargo & Laurel, 1994). It was suggested that in S. invicta queens, Vg is constitutive, while yolk formation is regulated through the level of Vg uptake into the oocyte, rather than at the level of Vg synthesis by the fat body (Chen et al., 2004). ...
- Chloe Hawkings
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![Cecilia Tamborindeguy]()
Vitellogenin has been proposed to regulate division of labor and social organization in social insects. The red imported fire ant ( Solenopsis invicta ) harbors four distinct, adjacent vitellogenin genes (Vg1, Vg2, Vg3, and Vg4). Contrary to honey bees that have a single Vg ortholog as well as potentially fertile nurses, and to other ant species that lay trophic eggs, S. invicta workers completely lack ovaries or the ability to lay eggs. This provides a unique model to investigate whether Vg duplication in S. invicta was followed by subfunctionalization to acquire non-reproductive functions and whether Vg was co-opted to regulate behavior within the worker caste. To investigate these questions, we compared the expression patterns of S. invicta Vg genes among workers from different morphological subcastes or performing different tasks. RT-qPCRs revealed higher relative expression of Vg1 in major workers compared to both medium and minor workers, and of Vg2 in major workers when compared to minor workers. Relative expression of Vg1 was also higher in carbohydrate foragers when compared to nurses and protein foragers. By contrast, the level of expression of Vg2, Vg3, and Vg4 were not significantly different among the workers performing the specific tasks. Additionally, we analyzed the relationship between the expression of the Vg genes and S-hydroprene, a juvenile hormone analog. No changes in Vg expression were recorded in workers 12 h after application of the analog. Our results suggest that in S. invicta the Vg gene underwent subfunctionalization after duplication to new functions based on the expression bias observed in these data. This may suggest an alternative and still unknown function for Vg in the workers that needs to be investigated further.
... Similar studies have been conducted in social insects. The best example is perhaps in the fire ant Solenopsis invicta, where several studies showed that a putative queen pheromone inhibits wing-shedding and ovary development in virgin queens and is likely produced in multiple exocrine glands including the poison sac, the mandibular glands and possibly other glands (Robert, 1983;VanderMeer and Alonso, 2002;VanderMeer et al., 1980;Vargo, 1999;Vargo and Hulsey, 2000;Vargo and Laurel, 1994). However, the actual pheromone was never identified. ...
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![Etya Amsalem]()
The reproductive division of labour in social insects is a fascinating phenomenon regulated by diverse chemical signals that vary substantially in structure. Is this diversity an example of one problem (reproductive regulation) and many potential solutions (diverse chemicals)? Or are there hidden shared elements in the pheromonal regulation of reproduction across insects? To address this question, I will first discuss the phenomenon of reproductive division of labour in social insects, particularly, the reproductive conflicts among females and the means by which these conflicts are resolved. I will then focus on the use of pheromones, a mode of communication that has broadly diversified among social insects that live in large complex societies. I will summarize the different approaches to define semiochemicals in the context of the reproductive division of labour and review the state of knowledge of compounds regulating insect reproduction, both solitary and social, demonstrating the structural diversity as well as the potential conservation of the mechanisms regulating signal production and perception. Lastly, I will discuss the different hypotheses underlying the evolution of pheromones regulating reproduction in insects. Our current understanding of reproductive signalling, while extensive within single species, is still limited by the paucity of comparative studies across the Insecta as a whole, and further investigations are sorely needed.
... In insects, reproductive dominance is established by behavioural aggression and/or chemical cues (Le Conte and Hefetz, 2008;Padilla et al., 2016). Chemical cues are often in the form of queen pheromones (Matsuura et al., 2010;Van Oystaeyen et al., 2014;Princen et al., 2019;Vargo and Laurel, 1994;Winston and Slessor, 1992). Perhaps the most well-studied queen pheromone is Queen Mandibular Pheromone (QMP), produced by queen honeybees (Apis mellifera) (Keeling et al., 2003;Pankiw et al., 1996;Slessor et al., 1988). ...
Eusociality is characterised by the reproductive division of labour; a dominant female (queen) or females are responsible for the majority of reproduction, and subordinate females are reproductively constrained. Reproductive constraint can be due to behavioural aggression and/or chemical cues, so-called queen pheromones, produced by the dominant females. In the honeybee, Apis mellifera, this repressive queen pheromone is queen mandibular pheromone (QMP). The mechanism by which honeybee workers are susceptible to QMP is not yet completely understood, however it is thought to be through olfaction via the antennae and/or gustation via trophallaxis. We have investigated whether olfaction is key to sensing of QMP, using both Drosophila melanogaster- a tractable non-eusocial insect which is also reproductively repressed by QMP- and the target species, A. mellifera worker honeybees. D. melanogaster are still capable of sensing and responding to QMP without their antenna and maxillary palps, and therefore without olfactory receptors. When worker honeybees were exposed to QMP but unable to physically interact with it, therefore required to use olfaction, they were similarly not reproductively repressed. Combined, these findings support either a non-olfactory based mechanism for the repression of reproduction via QMP, or redundancy via non-olfactory mechanisms in both D. melanogaster and A. mellifera. This study furthers our understanding of how species are susceptible to QMP, and provides insight into the mechanisms governing QMP responsiveness in these diverse species.
... It influences physiology in queens and guarding behavior in workers of primitive eusocial wasp Polistes canadensis (Giray et al., 2005). JH also affects queen maturation and reproduction in the invasive ant S. invicta where, high levels of JH induces alates to begin oogenesis (Vargo and Laurel, 1994;Brent and Vargo, 2003;Lu et al., 2009). Vg is a yolk precursor protein; its production is typically used to produce egg yolk by oviparous animals (Amdam et al., 2003), but it may also affect behavior (Nelson et al., 2007). ...
Many species of social Hymenoptera demonstrate behavioral flexibility, where older workers that typically forage can revert to younger worker tasks, such as nursing, when these are absent. This flexibility is typical of the sterile worker class, yet rare in queens. In the little fire ant (Wasmannia auropunctata), queens have been reported to perform only egg laying. We examined behavior of queens of W. auropunctata after demographic manipulation. When half of the workers were removed from the colony, queens were observed caring for eggs, larvae and pupae as well as eating outside of the nest, like forager workers. We examined the relationship between these atypical queen behaviors and their juvenile hormone binding protein (JHbp) and vitellogenin (Vg) expression via QRT-PCR method. JHbp and Vg expression decreased when queens were performing worker tasks, resembling the expected expression pattern of typical sterile workers. Flexibility in queen behaviors in the little fire ant may be an important adaptation to changing environments. As a significant invasive species, such adaptation may increase the probability of colony survival during propagation. Our results not only present new insights in behavioral flexibility in social insects, but also increases our understanding of the success of this significant invasive species.
... To 48 achieve this in the Hymenoptera, a clade containing many eusocial species, a mixture of 49 behavioural aggression and chemical inhibition of reproduction is used (Le Conte and Hefetz, 50 2008; Padilla et al., 2016). Chemical inhibition occurs via queen pheromones (Matsuura et 51 al., 2010;Vargo and Laurel, 1994; Winston and Slessor, 1992). These queen pheromones are 52 produced by the reproductively dominant female and are thought to signal her fecundity to 53 ...
Queen pheromones effect the reproductive division of labour, a defining feature of eusociality. Reproductive division of labour ensures that one, or a small number of, females are responsible for the majority of reproduction within a colony. Much work on the evolution and function of these pheromones has focussed on Queen Mandibular Pheromone (QMP) which is produced by the Western or European honeybee (Apis mellifera). QMP has phylogenetically broad effects, repressing reproduction in a variety of arthropods, including those distantly related to the honeybee such as the fruit fly Drosophila melanogaster. QMP is highly derived and has little chemical similarity to the majority of hymenopteran queen pheromones which are derived from cuticular hydrocarbons. This raises the question of whether the phylogenetically widespread repression of reproduction by QMP also occurs with more basal saturated hydrocarbon-based queen-pheromones. Using D. melanogaster we show that saturated hydrocarbons, are incapable of repressing reproduction, unlike QMP. We also show no interaction between the four saturated hydrocarbons tested or between the saturated hydrocarbons and QMP, implying that there is no conservation in the mechanism of detection or action between these compounds. We propose that the phylogenetically broad reproductive repression seen in response to QMP is not a feature of all queen pheromones, but unique to QMP itself, which has implications for our understanding of how queen pheromones act and evolve.
... Queen signals are a subset of caste-recognition signals that distinguish the queen from the workers [5 ]. Within Hymenoptera, such pheromones were only found in A. mellifera where the composition of the pheromone is known [18 ], and in Solenopsis invicta where the composition remains elusive [19,20]. The A. mellifera queen mandibular pheromone blend is responsible for maintenance of the queen reproductive monopoly by inducing worker sterility [21,22]. ...
Pheromones mediating social behavior are critical components in the cohesion and function of the colony and are instrumental in the evolution of eusocial insect species. However, different aspects of colony function, such as reproductive division of labor and colony maintenance (e.g. foraging, brood care, and defense), pose different challenges for the optimal function of pheromones. While reproductive communication is shaped by forces of conflict and competition, colony maintenance calls for enhanced cooperation and self-organization. Mechanisms that ensure efficacy, adaptivity and evolutionary stability of signals such as structure-to-function suitability, honesty and context are important to all chemical signals but vary to different degrees between pheromones regulating reproductive division of labor and colony maintenance. In this review, we will discuss these differences along with the mechanisms that have evolved to ensure pheromone adaptivity in reproductive and non-reproductive context.
... However, several lines of evidence suggest that JH and biogenic amines play a central role in QP-mediated effects. First, the effects of QPs on worker ovarian development and larval caste can be mimicked by topical application of methoprene, a JH analogue (Edwards 1987;Penick and Liebig 2012;Vargo 1992;Vargo and Laurel 1994). In a social wasp, a recent study has shown that this treatment also affects CHC synthesis (Oliveira et al. 2017). ...
Ant queen pheromones (QPs) have long been known to affect colony functioning. In many species, QPs affect important reproductive functions such as diploid larvae sexualization and egg-laying by workers, unmated queens (gynes), or other queens. Until the 1990s, these effects were generally viewed to be the result of queen manipulation through the use of coercive or dishonest signals. However, in their seminal 1993 paper, Keller and Nonacs challenged this idea, suggesting that QPs had evolved as honest signals that informed workers and other colony members of the queen's presence and reproductive state. This paper has greatly influenced the study of ant QPs and inspired numerous attempts to identify fertility-related compounds and test their physiological and behavioral effects. In the present article, we review the literature on ant QPs in various contexts and pay special attention to the role of cuticular hydrocarbons (CHCs). Although the controversy generated by Keller and Nonacs' (Anim Behav 45:787–794, 1993) paper is currently less intensively debated, there is still no clear evidence which allows the rejection of the queen control hypothesis in favor of the queen signal hypothesis. We argue that important questions remain regarding the mode of action of QPs, and their targets which may help understanding their evolution.
... Yet, queen pheromones still exist, for instance in fire ants; here they have other social functions such as inhibiting the development of virgin queens within the natal nest (e.g. Vargo and Laurel 1994). ...
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![Judith Korb]()
Termites evolved eusociality independently from social Hymenoptera. As a common trait, reproductive monopoly is maintained through chemical communication. The queen (and in termites also a king) prevents workers from reproduction by conveying their reproductive status. In termites all soldiers are sterile, but workers' potential to reproduce differs between species. It ranges from totipotency in wood-dwelling lower termites where workers are a transient stage from which all other castes develop, to sterile workers in some higher termites. Intermediate are species in which workers can develop into replacement sexuals within the nest but not into winged sexuals. I summarize the patchy picture about fertility signaling that we currently have for termites, pointing also to potential conflicts over reproduction that differ from those in social Hymenoptera. Recent findings imply that, similar to many social Hymenoptera, wood-dwelling termites that live in confined nests use long-chain cuticular hydrocarbons (CHCs) as fertility signals. Yet other compounds are important as well, comprising proteinaceous secretions and especially volatiles. For a subterranean termite, two volatiles have been identified as primer pheromones that prevent reproductive differentiation of workers. It requires more data to test whether wood-dwelling termites use CHCs, while species with larger colonies and less confined nests use volatiles, or whether all species rely on multicomponent signals. Ultimately, we need more effort to model and test potential conflicts over reproduction between queens, kings and workers. Here results from social Hymenoptera cannot be transferred to termites as the latter are diploid and commonly inbred. This review illustrates promising future research avenues.
- Haolin Zeng
- Jocelyn G. Millar
- Li Chen
- Kenneth G. Ross
Ants use chemical signals to communicate for various purposes related to colony function. Social organization in the red imported fire ant, Solenopsis invicta, is determined by the Sb supergene, with colonies of the monogyne (single-queen) form lacking the element and colonies of the polygyne (multiple-queen) form possessing it. Polygyne workers accept new reproductive queens in their nest, but only those carrying Sb; young winged queens lacking this genetic element are executed as they mature sexually in their natal nest or as they attempt to enter a foreign nest to initiate reproduction after mating and shedding their wings. It has been suggested that queen supergene genotype status is signaled to workers by unsaturated cuticular hydrocarbons, while queen reproductive status is signaled by piperidines (venom alkaloids). We used high-throughput behavioral assays to study worker acceptance of paper dummies dosed with fractions of extracts of polygyne queens, or blends of synthetic counterparts of queen cuticular compounds. We show that the queen supergene pheromone comprises a blend of monoene and diene unsaturated hydrocarbons. Our assays also reveal that unsaturated hydrocarbons elicit discrimination by polygyne workers only when associated with additional compounds that signal queen fertility. This synergistic effect was obtained with a polar fraction of queen extracts, but not by the piperidine alkaloids, suggesting that the chemical(s) indicating queen reproductive status are compounds more polar than cuticular hydrocarbons but are not the piperidine alkaloids. Our results advance understanding of the role of chemical signaling that is central to the regulation of social organization in an important invasive pest and model ant species.
Reproductive division of labor in insect societies is regulated through multiple concurrent mechanisms, primarily chemical and behavioral. Here, we examined if the Dufour's gland secretion in the primitively eusocial bumble bee Bombus impatiens signals information about caste, social condition, and reproductive status. We chemically analyzed Dufour's gland contents across castes, age groups, social and reproductive conditions, and examined worker behavioral and antennal responses to gland extracts. We found that workers and queens each possess caste-specific compounds in their Dufour's glands. Queens and gynes differed from workers based on the presence of diterpene compounds which were absent in workers, whereas four esters were exclusive to workers. These esters, as well as the total amounts of hydrocarbons in the gland, provided a separation between castes and also between fertile and sterile workers. Olfactometer bioassays demonstrated attraction of workers to Dufour's gland extracts that did not represent a reproductive conflict, while electroantennogram recordings showed higher overall antennal sensitivity in queenless workers. Our results demonstrate that compounds in the Dufour's gland act as caste- and physiology-specific signals and are used by workers to discriminate between workers of different social and reproductive status.
- C.G. Butler
- E.M. Fairey
1.Ovary development in worker honeybees is inhibited by 9-oxodecenoic acid or its vapour. A scent from the queen, in addition to her 9-oxodecenoic acid, also inhibits oogenesis in workers, but it is less effective than the acid. The scent and acid do not work together synergically. Neither 9-oxodecenoic acid nor queen scent, nor the two together, are as effective in inhibiting oogenesis in workers as access to a live queen. Older, mated queens are more effective than younger, virgin ones. Injection of 9-oxodecenoic acid partially inhibits ovary development in workers, but does not inhibit queen rearing by them. The mode of action of this acid is still uncertain.
- D. P. Jouvenaz
- G. E. Allen
- W. A. Banks
- Daniel P. Wojcik
In a survey conducted in the Southeastern United States, one colony in a sample of 1,007 colonies of the red imported fire ant, Solenopsis invicta Buren, was infected with a microsporidium (Protozoa: Microsporida). The normal host of this parasite appears to be the tropical fire ant, Solenopsis geminata (F.). A benign or very mildly pathogenic yeast was associated with 93 (9.24%) of the S. invicta colonies, and was most common in areas which have been infested with this ant for the longest periods. No pathogens were associated with 83 colonies of the black imported fire ant, Solenopsis richteri Forel. The apparent rarity of bona fide pathogens in imported fire ants in the United States is in marked contrast to the abundance of pathogens in these and other Solenopsis spp. in South America. Four species of microsporidia (possibly new genera) were detected in 22 (7.2%), 12 (3.9%), 6 (2.0%), and 4 (1.3%) of 307 colonies of the tropical fire ant, S. geminata. One colony of this species was infected by a neogregarine (Sporozoa: Neogregarinida). No pathogens were found in a small sample (53 colonies) of the Southern fire ant, Solenopsis xyloni McCook.
- Mark L. Winston
- Keith N. Slessor
Describes new findings on the mechanisms of coordination of individuals in an Apis mellifera colony. Discusses the composition and role of pheromones on behaviour and development of the colony rearing new queens and the development of ovaries in worker bees; how the fractions of the queen's pheromones that control the retinue response were identified; the functions of the queen's pheromones, including the presence/absence effects on the number of new queens raised and the control of swarming. The quantities of pheromones produced by the queen and how these were transmitted through the colony by retinue workers were studied, showing that most of the pheromones are internalised. -L.J.Chellis
Source: https://www.researchgate.net/publication/242182606_Studies_on_the_mode_of_action_of_a_queen_primer_pheromone_of_the_fire_ant_Solenopsis_invicta
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