Juvenile hormone and division of labor in honey bee colonies: effects of allatectomy on flight behavior and metabolism
,1
1 Department of Entomology
2 Neuroscience Program, University of Illinois at Urbana-Champaign, Urbana,
IL 61801, USA,
3 Department of Biological Sciences, Arizona State University, Tempe, AZ
85287-0002, USA
Author for correspondence at address 1 (e-mail:
generobi{at}uiuc.edu)
Accepted 4 April 2003
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Summary |
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Key words: Apis mellifera, behavioral development, corpora allata, division of labor, flight, honey bee, juvenile hormone, metabolism
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Introduction |
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The regulation of division of labor by honey bees involves several
physiological changes, including changes in the level of circulating juvenile
hormone (JH) (Robinson, 2002).
JH levels rise before the onset of foraging
(Elekonich et al., 2001
;
Jassim et al., 2000
) and
typically are much higher in foragers compared with nurses (reviewed by
Bloch et al., 2002
). In
addition, treatment with the JH analog methoprene results in precocious
development of foraging (reviewed by Bloch
et al., 2002
). Recently it was discovered that JH does not
activate foraging, but rather is involved in controlling the pace at which
bees develop into foragers (Sullivan et
al., 2000
). Bees that have had their corpora allata (the sole
source of JH) surgically removed still became foragers, but at an older age
than intact bees. Treatment with methoprene after allatectomy eliminated this
delay (Sullivan et al.,
2000
).
Sullivan et al. (2000) also
reported that allatectomy had another effect on honey bees: it caused them to
disappear around the time they began taking their first orientation flight.
This effect does not appear to reflect direct allatectomy-induced mortality,
because there were no differences in mortality within the hive, but it was
correlated with the onset of flight activity. Additional observations
(Sullivan, 2001
), conducted in
different years on bees of different genotypes, in varied landscapes,
confirmed that allatectomy can decrease the probability of returning to the
hive upon initiating flight.
Sullivan et al. (2000)
suggested that this allatectomy effect was due to a deficit in either the
physical aspects of flight behavior or the ability to learn/remember the
location of the hive. Both suggestions are plausible. JH affects flight
muscles (Rankin, 1989
;
Wyatt and Davey, 1996
) and
respiratory metabolism (Novak,
1966
) in other insects. Experiments with a Drosophila
cell line have shown that exposure to JH causes an increase in cytochrome
oxidase activity and protein synthesis in the mitochondria
(Stepien et al., 1988
). In
addition, there is a rich literature of hormone effects on cognition in
vertebrates (reviewed by Bottjer and
Johnson, 1997
; McEwen,
2000
; Pfaff et al.,
2000
; Welberg and Seckl,
2001
), and a recent report of effects of JH on performance of
honey bees in a learning assay (Maleszka
and Helliwell, 2001
).
Three experiments were performed to determine the nature of the allatectomy
effect that causes honey bees to fail to return to their hive upon initiating
flight. In Experiment 1, the naturally occurring flights of allatectomized
bees were tracked using radar (Capaldi et
al., 2000) to obtain indications about the nature of the deficit.
We reasoned that flight tracks might reveal deficits in navigation expressed
as errors in flight path, while a physiological deficit would be suspected if
allatectomy caused effects on flight distance, duration or speed. In
Experiment 2, intensive observations were made at the hive entrance to find
evidence of allatectomy-induced flight impairment, especially during
take-off.
Experiment 3 was performed in the laboratory, after the results from
Experiments 1 and 2 indicated that allatectomized bees had impaired flight. We
tested the hypothesis that allatectomy causes a decrease in metabolic rate;
such a decrease might explain the flight impairment because of the intense
energetic demands of flight. Just as there is an age-related increase in
circulating JH, there are also age-related increases in both honey bee
metabolic rate (Harrison,
1986) and the abundance of key respiratory metabolic enzymes in
flight muscle (Maruyama and Moriwaki,
1958
; Herold and Borei,
1963
; Herold,
1965
), with foragers showing the highest levels. In addition,
forced flight in the laboratory complements the field analyses in Experiments
1 and 2 by providing a context that is independent of any motivation to fly or
forage.
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Materials and methods |
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Allatectomy
Allatectomy was performed as described in Sullivan et al.
(2000) on adult bees <6 h
old (the cuticle is sufficiently elastic at this time to avoid cracking and
bleeding following incision). Before treatment, bees were transferred into
small cages placed in an incubator (33°C, 95% relative humidity) with
access to honey in the comb and water ad libitum. Bees were
immobilized prior to surgery by transfer to a small glass vial placed on
crushed ice for approximately 3 min. Previous studies revealed that cold
anesthesia has no effects on bee flight
(Ebadi et al., 1980
;
Robinson and Visscher, 1984
).
After anesthetization, bees were mounted in Plasticine on the stage of an
Olympus SZH10 stereo microscope and an incision was made across the back of
the head. Each corpus allatum was grasped and removed with forceps; the
cuticle then rapidly resumed its original shape, thereby sealing the incision.
Sham allatectomy was identical except the CA were only gently touched with the
tip of the forceps. Untreated bees were anesthetized, as were sham and
allatectomized (CA-) bees, but were otherwise unmanipulated. Sullivan et al.
(2000
) validated this
allatectomy technique with measurements of JH titer and histology and found no
or extremely low levels of JH, and no evidence for regeneration of the CA.
After surgery, each CA- bee was marked for individual identification with a numbered, colored plastic tag (Opalithplâttchen, Chr. Graze, KG, Endersbach, Germany) on the thorax and a spot of paint (Testor's PLA) on the dorsal tip of the abdomen. Bees were returned to the same cage in the incubator prior to introduction to an unrelated host colony. Bees were introduced by opening the cage and placing it over a hole in the top of the hive; treated bees entered the host colony overnight. Descriptions of host colonies are given for each experiment.
Hormone replacement was used in Experiment 3. CA- bees were treated by
applying 200 µg methoprene/5 µl acetone to the dorsal abdomen when they
were 2 days old. This dose is sufficient to eliminate the delay in age at
onset of foraging caused by allatectomy
(Sullivan et al., 2000), and
is used routinely to induce precocious foraging
(Bloch et al., 2002
). The dose
that is used is high, but methoprene, though more resistant to JH-specific
degradative enzymes than native JH
(Weirich and Wren, 1973
), is
also broken down rapidly. More than 69% of a methoprene treatment applied to
ants (Solenopsis invicta) was broken down in 24 h
(Bigley and Vinson, 1979
). In
addition, treatment with the same dose of an inactive JH analog has no
behavioral effects, suggesting that methoprene's effects are due to hormonal
activity (Robinson, 1987
).
Hormone replacement was not employed in Experiments 1 and 2 because this
treatment did not overcome the `missing bee effect' first noted in Sullivan et
al. (2000), suggesting that a
different timing, delivery or treatment may be required in this context. Bees
in Experiment 3 were held in the cage in the incubator for an additional day
prior to introduction to their host colony.
Experiment 1: Effects of allatectomy on radar-tracked flight
behavior
Harmonic radar was used to track orientation flights of bees over flat
farmland (700 mx900 m) at Rothamsted, UK using established methods
(Riley et al., 1996;
Capaldi et al., 2000
). Harmonic
radar detects the position signal of a transponder attached to the thorax of
the bee by sending one wavelength and receiving a unique harmonic frequency
re-radiated from the transponder every 3 s. The transponder consisted of a 16
mm vertical dipole aerial and a Schottky diode weighing 12 mg, less than the
average mass of a load of nectar or pollen
(Winston, 1987
). No
significant effects of the transponder on honey bee flight behavior have been
detected (Capaldi et al.,
2000
).
A total of approximately 250 untreated, sham and CA- bees (derived from a
naturally mated queen) were introduced to the host colony in cohorts of
approximately 25 per treatment group over 11 days. The host colony occupied
one British standard hive box and had an adult population of about 10 000 bees
with a naturally mated queen. The hive entrance was observed from 08:30 h to
17:30 h daily, except during rain. An extended entrance board was attached to
the hive; it had an acrylic cover and several sliding locks that separated
incoming and outgoing bees, to facilitate capturing focal bees (leaving the
hive naturally) and fitting them with a transponder
(Capaldi et al., 2000). The
departure behavior of transponder-affixed bees was observed by eye. Observers
at the hive entrance were informed of flight progress by members of the
research team monitoring the radar and recaptured the bee to remove the
transponder upon return to the hive. Each bee was tracked once.
Tracking was conducted only when blue sky was visible. Anemometry stations in five locations took wind velocity readings at 9 s intervals to calculate the average wind vector for each track. Wind speed during tracking of CA-, sham and untreated bees did not vary significantly (analysis of variance, ANOVA: F(2,82)=2.61, P=0.08). The colony was located in the center of plots of cereals and blooming oilseed rape and field beans 260 m NNW of the radar. Radar detection ranges in this landscape were 700 m radially and 3 m above the ground. Within a reference frame centered on the hive, bees were tracked 500 m to the west, 500 m to the east, 325 m to the south, and 325 m to the north. The limits of the flight area were detected by tracking long-distance foraging flights (results to be presented elsewhere); no orientation flight extended beyond the range of the radar.
The radar tracks were used to calculate the following details of
orientation flights (Capaldi et al.,
2000): flight duration (s), round trip distance (m), maximum range
(m), area covered (m2) and mean ground speed (m s-1).
Duration was the total time the bee was tracked. Round trip distance was the
sum of the linear distances between one point in space occupied by a bee and
its subsequent position, from the first to the last radar signal. A linear
flight path was assumed between every two positions, regardless of the period
of time elapsed between them. Maximum range was the distance of greatest
radial departure from the hive. Area covered was calculated using the minimum
convex polygon method of home range analysis from the Antelope spatial
analysis software package
(http://www.nbb.cornell.edu/neurobio/jbsv_downloads/programs.html).
Ground speed was calculated by averaging the point-to-point speed of the
moving bee based on 3 s radar sampling intervals. Track segments >9 s in
duration were used for this calculation, to match flight-speed sampling
intervals to wind-speed information. In cases when a radar track had a missing
signal, the calculated flight speed ignored the time the bee spent in these
gaps unless the plotted data indicated that the flight path continued on the
same course and at the same speed. The proportion of time spent in gaps
probably represents short landings on vegetation; it was not correlated with
age or the number of flights taken by each bee (data not shown). Tracks of
foragers were identified based on departure behavior, track shape
(Capaldi et al., 2000
), or
return with pollen loads or a distended abdomen; these were excluded (1 CA-, 3
sham, 1 untreated).
Statistical analyses of radar data were conducted as follows. Normality was
tested and log- or square-root transformation used when appropriate
(Zar, 1996). ANOVA was
performed to determine the effects of allatectomy on flight duration, round
trip distance, maximum range, area covered and ground speed, followed by the
least significant difference multiple-comparison test (SPSS 10.0.5 software,
SPSS, Inc., Chicago, IL, USA). Outliers were excluded (2 CA-, 4 sham, 4
untreated) after testing the coefficient of symmetry (g1) for highly
significant (P<0.001) departure from normality
(Grubbs, 1969
;
Zar, 1996
).
2
analyses of the proportions of bees from each treatment group that failed to
fly with the transponder or did not return from a flight (SAS 6.12 software)
were followed by pairwise comparisons corrected for continuity.
Experiment 2: Effects of allatectomy on flight ability and
survival
The behavior of CA- bees as they departed on orientation flights was
studied by making detailed observations at the hive entrance. Three serial
trials (replicates) of this experiment were performed with the same host
colony, each time with approximately 50 untreated, 50 sham and 50 CA- bees
derived from a (different) queen instrumentally inseminated with semen from a
single drone. The host colony occupied one Langstroth hive box and had an
adult population of about 12 000 bees and a naturally mated queen, with 6
frames of honeycomb containing brood and 4 frames of comb containing honey and
pollen.
Observations were recorded on audio cassettes from 13:00 h to 18:00 h daily
when the bees were 214 days of age. An extended entrance covered with
Plexiglas (15 cmx15 cm) facilitated observation of identification tags
on the bees. To induce bees to walk with their tags facing upward, a thin film
of petroleum jelly was applied on the underside of the Plexiglas at the end
touching the hive (Winston and Katz,
1982). Flight ability was assessed based on criteria described
below. In addition, we determined the age at first entrance appearance (based
on the age at which a bee walked out of the hive into any part of the
entrance) and the age at first flight.
The following procedure was used to obtain additional information on flight
ability during times when no observations were taking place. The host colony
was set on a stand inside two swimming pools (1.5 m and 3 m diameter, each
containing at least 5 cm of water). We assumed that the pools would act as
water traps and only bees with impaired flight ability would end up in them.
Observations (Sullivan, 2001)
confirmed this assumption. A circle of closely mown grass about 6 m in
diameter surrounded the pools to further facilitate observation of marked bees
on the ground. Mortality was monitored by installing a dead bee trap prior to
the onset of observations (Gary and
Lorenzen, 1984
). In addition, censuses of the host colony were
performed every other day by visually inspecting each frame of honeycomb in
the hive twice and recording the identity of each bee present on audio tape.
We positioned several small colonies near the host colony to help recover any
marked bees that drifted (only 1 bee in all three replicates). Flight ability
was categorized as: `Flightworthy', `Impaired Flight', `Hive Bee', `Unknown'
and `Dead'.
The `Flightworthy' category was composed of bees that foraged or flew normally. Flightworthy foragers took multiple flights >15 min duration or returned from a flight with pollen or a distended abdomen (forager). Flightworthy pre-foragers showed normal flight behavior but were not observed to forage. The `Impaired Flight' category was composed of missing fliers, weak fliers and flightless bees. Missing fliers took at least one successful (roundtrip) flight, but did not return from a subsequent flight. Some missing fliers probably occurred due to random factors such as predation rather than impairment. We included all missing fliers in the `Impaired Flight' category; if this caused a bias it would be to decrease the magnitude of any observed treatment effects on flight. Weak fliers were those found in the pools. Observations of some of the bees that ended up in the water revealed that they climbed up the front of the hive, lifted the front legs and took off, but then flew down into the pool. These bees then `swam' slowly and were unable to pull themselves from the water onto the hive stand. This was distinct from casual observations of bees foraging for water, which were able to get out of the pools. Flightless bees did not fly, and fell from the entrance after attempting to take off. Flightless bees had weak wing motion; the wings were not moving rapidly enough to even become blurred to the eye of the observer. The `Hive Bee' category was composed of bees present in the hive at the end of the experiment and observed during censuses or at the entrance, but never observed to attempt to fly. The `Unknown' fate category was composed of bees never observed or only observed in a census or at the entrance but not present in the hive at the end of the experiment; their corpses were not recovered in either the dead bee trap or the pools. `Dead' bees were corpses that were retrieved from the dead bee trap or from within the hive during a census.
Statistical analyses of the effects of allatectomy on flight ability were performed as follows. Contingency table analysis was used to compare the distributions of bees in the five categories. ANOVA was performed on age at first appearance at the entrance and age at first flight, after data were square-root-transformed to meet the assumptions of normality. Survival was calculated based on data from the observations, water traps, dead bee traps and censuses, adjusted for censorship due to termination of the experiment. Differences in survival between groups were analyzed by KaplanMeier survival analysis (yielding the Breslow statistic, B), performed with SPSS 10.0.5 software.
Experiment 3: Effects of allatectomy on metabolic rates
Bees for this experiment were taken from three small host colonies. Each
host colony occupied a small hive and had an adult population of about
30004000 bees and a naturally mated queen, with 1 honeycomb frame
containing honey and pollen and 1 containing brood. 23 cohorts of
1-day-old bees (derived from 2 naturally mated queens) were added to each host
colony, about 25 bees per treatment group: CA-, CA- treated with methoprene
(MCA-), sham and untreated. The hive entrance was closed until bees were
sampled at about 8 days of age (see below) to ensure that they had had no
flight experience prior to testing. Sampling was performed by removing a hive
top and randomly collecting focal bees from the tops of the honeycomb frames
and placing them in small glass vials. They were then transferred into a small
cage and incubated (35°C) with access to honey and water ad
libitum until assayed. Bees were collected when they were about 89
days old (untreated: 8.2±0.01, N=102; sham: 8.2±0.01,
N=92; CA-: 8.0±0.01, N=89; and MCA-: 8.8±0.02,
N=58; means ± S.E.M.).
Flight ability was assessed in the laboratory (room temperature: 26°C) immediately before measuring metabolic rate. To assay flight ability, a bee was removed from the cage in the incubator and transferred to an open glass vial; bees uniformly attempted to initiate flight within 1 min. `Flightless' bees fell immediately to the floor; `Poor Fliers' descended gradually to the floor; and `Flightworthy' bees flew up to a fluorescent light or window. Poor Fliers were retrieved and given a second opportunity to fly; in all cases the result was the same. All bees measured for metabolic rate were also tested for flight ability except for 13 untreated, 11 sham, 13 CA- and 13 MCA- individuals (due to logistical limitations).
We used whole body in vivo measurement of metabolic rate under
forced flight conditions. This technique provides a good estimate of the
metabolic capacity of honey bee flight muscles, given that flight muscles
consume more than 90% of the O2 taken up by the body when active
(Rothe and Nachtigall, 1989;
Suarez, 1992
). Metabolic rate
was assayed using a standard respirometry technique validated for the honey
bee (Harrison, 1986
;
Harrison and Hall, 1993
;
Suarez et al., 1996
) that
measures CO2 production. O2 consumption and
CO2 production are regarded as interchangeable estimates of
metabolic rate under steady-state activity for honey bees; their metabolism of
hexose sugars during flight is completely aerobic, yielding a respiratory
exchange ratio of 1.0 for O2 and CO2 (e.g.
Beenakkers, 1969
;
Micheu et al., 2000
). A bee
was placed in a 20°C respirometry chamber (a 300 ml Plexiglas cylinder)
and the chamber was flushed for 2 min at 2 l min-1 with dry
(Drierite, W. A. Hammond Drierite Co. Ltd., Xenia, OK), CO2-free
(Ascarite, Mallinckrodt Baker, Inc., Phillipsburg, NJ, USA) air. Flight
metabolic rate was then measured over 1 min of flight using flow-through
respirometry (Harrison et al.,
1996
). The chamber was shaken and beaten with a padded stick to
agitate the bee and cause it to fly continuously during the time of
measurement. A magnesium perchlorate column was used to remove water vapor
from the air leaving the respirometry chamber, and the excurrent carbon
dioxide was measured with a Licor 5152 carbon dioxide analyzer (±1
p.p.m. CO2, Lincoln, NE, USA). The flow rate of air through the
chamber was approximately 0.8 l min-1, measured to ±0.001 l
min-1 with an Omega mass flow meter (Stamford, CT, USA). The rate
of CO2 production was measured once per second, digitized, and
recorded with a Sable Systems AD-1 data acquisition system (Las Vegas, NV,
USA).
Any bee that did not fly continuously throughout the trial was excluded.
After respirometry, bees were removed from the chamber, cooled on ice, and
weighed (to ±0.1 mg; Mettler Toledo, Switzerland). CO2
production rates were converted to metabolic rates assuming that only
carbohydrates are catabolized during flight
(Rothe and Nachtigall, 1989).
Metabolic rate was represented as W g-1 body mass.
Statistical analyses of the effects of allatectomy on flight ability and
metabolic rate were performed as follows. Differences between the treatment
groups in the proportion of individuals in each flight ability category were
determined by contingency table analysis. Because the bees were different ages
and derived from different queens, the CochranMantelHaenszel
test was used to control for variation in age and genotype. Differences
between treatment groups in metabolic rate were analyzed by ANOVA (on
square-root-transformed data), again controlling for variation in age and
genotype, which are important variables (e.g.,
Allen, 1958;
Harrison et al., 1996
). SAS
(SAS Institute Inc., Cary, NC, USA) and SPSS (SPSS, Inc., Chicago, IL, USA)
software were used for statistical analyses. A correlation analysis was also
performed, based on the a priori hypothesis that hormone replacement
(via methoprene) would partially rescue an allatectomy deficit.
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Results |
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Experiment 2: Effects of allatectomy on flight ability and
survival
Survival varied significantly among the groups (Replicate 1:
B=35.32, P<0.00001; Replicate 2: B=23.36,
P<0.00001; Replicate 3: B=28.15, P<0.00001).
The CA- and sham groups had significantly lower survival rates than the
untreated group in all replicates (Fig.
1). The CA- group had significantly lower survival than the sham
group in the first replicate but did not differ in the other replicates.
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Decreased survival of CA- bees was reflected in lower proportions of Flightworthy CA- bees (Table 2). The distribution of bees classified as Flightworthy, Impaired Flight, Hive Bee, Unknown or Dead differed significantly between the treatment groups [QMH(2)=57.86, P<0.001]. The CA- group had the smallest percentage of Flightworthy bees, followed by the sham group and the untreated group. The CA- group had the largest percentages of Impaired Flight and Unknown bees.
|
In addition to flight impairment, bees from the CA- and sham groups appeared at the entrance and initiated flight at significantly younger ages than untreated bees (Table 3). There were also significant differences in the age at onset of flight for Impaired flight and Flightworthy bees in the untreated and sham group, but not in the CA- group (Table 3). Untreated bees with impaired flight attempted to fly at a significantly younger age than untreated bees that flew successfully. The same pattern was observed in the sham group. In contrast, there was no significant difference in the CA- group between Impaired Flight and Flightworthy bees.
|
Experiment 3: Effects of allatectomy on metabolic rates
Flight ability in the laboratory also differed significantly among the
groups [QMH(10)=26.39, P<0.003] (results not
shown). The CA- group had the smallest percentage of Flightworthy bees (64%,
N=76), significantly less than for untreated and sham groups (88%,
N=89 and 83%, N=81, respectively). CA- bees treated with
methoprene (MCA-) were intermediate (69%, N=45) and did not differ
significantly from the either the CA- group or the sham and untreated groups.
Although relatively small, the percentage of CA- bees that were `Flightworthy'
in this experiment was higher than in Experiment 2. This might reflect
differences between the laboratory and field; some bees in the CA- group that
flew well in a warm, windless room may not have been able to do so under less
favorable conditions in the field.
There were significant differences between the treatment groups in metabolic rate (Fig. 2). Bees in the CA- group had significantly lower metabolic rates than the untreated and sham groups. The MCA- group was `intermediate' and did not differ significantly from the either the CA- or the sham and untreated groups. To analyze the effects of hormone replacement further, a non-parametric correlation analysis was performed in which the four treatment groups were rank-ordered based on endocrine status and the prediction that methoprene treatment provides a partial rescue of the allatectomy effect on metabolic rate: CA- (1), MCA- (2), sham (3), untreated (4). There was a significant correlation between metabolic rate and endocrine status (MantelHaenszel Statistic=6.813, P<0.01).
|
Allatectomy-induced flight impairment appears to be a consequence of allatectomy-induced effects on metabolic rate. The metabolic rate for Flightworthy bees was about twice as large as for Flightless bees with Poor Fliers intermediate; all flight categories were significantly different from each other (Table 4). This result was obtained regardless of treatment. There were significant differences in metabolic rates for bees in different flight categories for all bees pooled and for bees within each treatment group (Table 4).
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Discussion |
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Sullivan et al. (2000)
reported that allatectomy caused bees to disappear around the time they began
taking their first orientation flight. Our results confirm this effect and
attribute it to allatectomy-induced flight impairment. Radar tracking and
detailed observations at the hive entrance indicate that the impairment does
not involve cognitive deficits in navigation that result in bees becoming
lost. Rather, allatectomized bees are physically impaired: many cannot achieve
proper lift-off and those that can fly show reduced flight speeds. Hormone
replacement was not employed in Experiments 1 and 2, so it is not possible to
ascribe these effects of allatectomy conclusively to an absence of JH. In
Experiment 3, hormone replacement partially eliminated the effects of
allatectomy, though the effect was not overwhelming and was detected with one
statistical analysis, but not another. These results provide some support for
the interpretation that a JH deficiency due to allatectomy caused the effects
on flight and metabolic rate reported here. The same type of methoprene
treatment did not overcome the missing bee effect first noted by Sullivan et
al. (2000
), suggesting that a
different timing, delivery or treatment may be required for a more robust
rescue. Alternatively, allatectomy may have inadvertently affected other
unknown processes that caused these effects; if so, they are unlikely to
involve a blood-borne factor, as the honey bee corpora allata in
vitro produce only one primary product, JH III
(Huang et al., 1991
).
Our results suggest that flight impairment in allatectomized bees is a
consequence of decreased metabolic rates, reflecting inadequate development
and/or function of the flight muscles. Previous studies of the Colorado potato
beetle Leptinotarsa decemlineata and the migratory locust Locusta
migratoria implicated JH in the development and maintenance of flight
muscles (Rankin, 1989;
Wyatt and Davey, 1996
), and JH
treatment has been shown to affect flight ability in several insect species
(reviewed by Rankin, 1989
;
Nijhout, 1994
). But if
allatectomy causes an impairment of flight in honey bees because of the
absence of JH, why are all allatectomized bees not more equally affected
(Sullivan et al., 2000
; this
study)? One possible explanation unexplored in this study is genetic
variation, which has been shown to affect metabolic rate, muscle development,
JH levels and rate of behavioral maturation
(Coelho and Mitton, 1988
;
Nachtigall et al., 1995
;
Harrison et al., 1996
;
Giray et al., 2000
). Also, if
flight in allatectomized bees is impaired, why are the radar-tracked flights
of allatectomized bees normal except for decreased ground speed? We speculate
that the radar tracks of allatectomized bees show such a specific, limited
deficit because only the least impaired bees of this group were capable of
flying with the transponder. It should be noted that an additional possible
influence on flight speed is experience. A previous radar study
(Capaldi et al., 2000
) showed
that bees with more flight experience fly faster than less experienced bees.
However, because the allatectomized and control bees used for this study were
selected randomly with respect to flight experience and covered a range of
ages (313 days), the most likely explanation is that allatectomy
exerted a direct influence on flight speed.
Allatectomized bees were observed at the hive entrance and initiated flight
at significantly younger ages than untreated bees. This appears to be a
non-specific effect of surgery, because sham-operated bees showed the same
response. There appears to be an intriguing interaction between flight
impairment and the age at onset of flight. All bees even those in the
untreated group that showed impaired flight also initiated flight at
younger ages than did the Flightworthy bees. Failed flight may occur because
some bees attempt to fly before the flight machinery has matured sufficiently
to support this. Perhaps the stress of surgery causes neurochemical and
endocrine changes that cause an earlier onset of flight, effects that appear
to be opposite to the delay in the onset of foraging caused by allatectomy
(Sullivan et al., 2000).
Starvation causes premature honey bee foraging
(Schulz et al., 1998
), but
effects on pre-foraging orientation flights have not been studied. Starvation
also causes an increase in octopamine immunoreactivity in the bee brain
(Kaatz et al., 1994
) and
octopamine has been implicated in the stress response of several insect
species (Nijhout, 1994
).
Octopamine is involved in the regulation of the onset of honey bee foraging
(Schulz and Robinson, 2001
),
but not orientation (Schulz et al.,
2002
). It also is not clear why about 15% of the untreated bees
showed impaired flight. These considerations suggest that the neural
mechanisms that control the initiation of flight may operate somewhat
independently of the physiological mechanisms that control flight ability in
honey bees.
Bees in the sham group in Experiment 2 showed a deficit in flight ability but in Experiment 3 they did not. This might be because Experiment 2 involved younger bees than those in Experiment 3. If so, these results would be consistent with the sham effect involving a premature initiation of flight, as discussed above.
The metabolic rates reported here are comparable to those reported
previously (reviewed by Harrison and
Fewell, 2002) especially to those in Harrison
(1986
), which were also
derived from relatively young bees, as in this study. Foragers have higher
metabolic rates (Harrison and Fewell,
2002
). Only allatectomized bees had values that were appreciably
lower than those reported by Harrison
(1986
). Allatectomized bees
treated with methoprene showed metabolic rates comparable to untreated bees in
this study and that of Harrison
(1986
).
JH influences several aspects of adult honey bee maturation that are
involved in division of labor, including exocrine gland secretions,
responsiveness to olfactory task-related stimuli, age at onset of foraging
(reviewed by Bloch et al.,
2002), and levels of octopamine in the antennal lobes
(Schulz et al., 2002
). The
results presented here suggest an additional role for JH in honey bees:
maturation of flight ability via effects on metabolic rate. Given
that removal of the corpora allata delays but does not completely repress the
onset of foraging (Sullivan et al.,
2000
), perhaps the effects of allatectomy reported here for young
bees also reflect developmental delays. JH appears to coordinate the timing of
various physiological and behavioral processes associated with honey bee
maturation, thus improving the effectiveness of division of labor.
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Acknowledgments |
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Footnotes |
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Present address: Department of Biology, Bucknell University, Lewisburg, PA
17837-2029, USA
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References |
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