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First published online August 9, 2007
Journal of Experimental Biology 210, 2939-2947 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.005033
Nature beats nurture: a case study of the physiological fitness of free-living and laboratory-reared male Anopheles gambiae s.l.
1 Public Health Entomology Unit, Ifakara Health Research and Development
Centre, PO Box 53, Off Mlabani Passage Ifakara, Tanzania
2 Department of Zoology and Marine Biology, University of Dar es Salaam, PO
Box 35064, Dar es Salaam, Tanzania
3 School of Biological and Biomedical Sciences, Durham University, Durham
DH1 3LE, UK
4 Laboratory of Entomology, Wageningen University and Research Centre, PO
Box 8031 6700 EH Wageningen, The Netherlands
5 Division of Infection and Immunity, and Division of Environmental and
Evolutionary Biology, University of Glasgow, Glasgow G12 8TA, UK
* Author for correspondence (e-mail: bjohn{at}ihrdc.or.tz)
Accepted 12 May 2007
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Summary |
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Key words: male Anopheles, insect fitness, laboratory colonization, genetically modified mosquito, energetic reserves
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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Much of our knowledge of insect ecology and evolution comes from laboratory
experimentation. However, the accuracy with which these laboratory-derived
estimates of insect life history and behaviour can predict the fitness and
population dynamics of insects in the wild is uncertain. Unlike homeotherms,
the development and demography of insects is heavily regulated by climate and
other environmental variables (Carey,
2001
), and can also vary substantially in response to subtle
differences in diet (Chang,
2004; Gary and Foster,
2001; Held and Potter,
2004; Jorgensen and Toft,
1997; Straif and Beier,
1996). Given their dependence on environmental variation,
behavioural and life-history traits documented under standardized laboratory
conditions could grossly misrepresent the complexity and norms of insect
behaviours. Critically, laboratory studies using insects reared in captivity
may not represent the resilience of their populations to natural disturbances
and/or human interventions.
In the case of insect vectors of disease, inappropriate extrapolation of
laboratory results could have substantial economic and public health
implications. The recent development of genetically modified (GM)
Anopheles mosquitoes that block the development of malaria parasites
(Christophides, 2005;
de Lara Capurro et al., 2000;
Ito et al., 2002;
Tabachnick, 2003), and the use
of sterile insects to suppress pest population growth
(Benedict and Robinson, 2003;
Dyck et al., 2005), serve as
excellent examples of this issue. Both these approaches require the release of
laboratory-reared individuals in the wild, with the GM approach seeking to
reduce malaria by introducing a parasite refractory gene into natural
populations, and the Sterile Insect approach to suppress population growth by
inducing wild females to mate with infertile males. Ethically, only male
mosquitoes could be released in such programmes as the release of more
blood-feeding females would at best increase the biting nuisance, or at worst,
the transmission of other vector-borne pathogens and possibly even malaria
itself if transgenic females are not 100% refractory.
The mating ability and survival of laboratory-reared, GM or sterile males
when released into the wild is thus critical to the success of these
enterprises. However, comparisons of the fitness of genetically modified and
wild-type mosquitoes have thus far been made only under laboratory conditions
(Catteruccia et al., 2003;
Irvin et al., 2004;
Moreira et al., 2000;
Moreira et al., 2004).
Colonization can alter the mating behaviour of laboratory-reared mosquitoes
and generate selection for assortative mating traits. The evolution of
assortative mating preferences reduces ability to mate with wild-type female
conspecifics, and can occur in as few as three generations of laboratory
maintenance (Reisen, 2003).
Direct field tests of the competitiveness of laboratory-reared genetically
modified mosquitoes when pitted against wild males must necessarily wait until
concerns regarding the ethics, biosecurity and efficacy of this approach are
resolved (Knols et al., 2007;
Mshinda et al., 2004). In the
meantime, substantial progress towards assessing the effectiveness of the GM
and Sterile Insect approach could be made by contrasting the fitness of male
mosquitoes when mass-reared in the laboratory, and allowed to forage freely in
nature; this to our knowledge has never been conducted on male African
Anopheles.
The reproductive potential and fitness of male mosquitoes can be indirectly
measured by their energetic reserves as adults
(Briegel, 1990;
Van Handel, 1984). These
reserves, accumulated during larval development and/or from blood or
sugar-feeding as adults (Briegel,
2003; Foster,
1995), are critical determinants of adult survival and mating
ability (Briegel, 1990;
Timmermann and Briegel, 1999;
Van Handel, 1988). Three key
energetic reserves of adult mosquitoes are lipids, glycogen and sugar. Lipids
are required for long-term maintenance (e.g. survival), and are primarily
acquired from feeding during larval development, and sugar feeding as adults
(Briegel et al., 2001;
Van Handel, 1984). Flight is a
requirement for mosquito mating, an activity fuelled by sugars or glycogens,
derived from sucrose or its components fructose and glucose, in nectar,
honeydew and fruit juices (Briegel,
2003; Foster,
1995; Nayar and Sauerman,
1977; Rowley and Graham,
1968; Van Handel,
1984). Body size is another indirect measure of mosquito
reproductive success, with several studies showing that larger individuals
have greater reproductive success (Ng'habi
et al., 2005; Takken et al.,
1998; Yuval et al.,
1993).
Given previous observations of poor survival and reproduction in
laboratory-reared mosquitoes when released into the wild
(Ferguson et al., 2005), it is
often assumed that free-living insects are subject to much harsher
environmental conditions and may generally be smaller in size and have lower
levels of energetic reserves than those reared in standardized controlled
environments. This suggests that mosquitoes reared in standardized laboratory
conditions should be better provisioned to out-compete wild individuals upon
release. If this is not true, any fitness cost conferred by a refractory gene
(Catteruccia et al., 2003;
Irvin et al., 2004;
Moreira et al., 2004), or
irradiation (Helinski et al.,
2006b) in the case of the sterile insect technique, will be
further inflated by the poorer physiological condition of laboratory-reared
mosquitoes. In the present study we investigated how key nutritional resources
and body size vary between laboratory-reared and free-living male mosquitoes
from southern Tanzania. We focused on male An. gambiae s.s. Giles and
its sibling species An. arabiensis Patton, because little is known
about the biology of this sex (Ferguson et
al., 2005), and because these species are the most important
vectors of malaria in Africa (Gillies and
DeMeillon, 1968; White,
1974) and thus a leading target for control measures based on the
release of genetically modified (Ito et
al., 2002; Riehle et al.,
2003) and/or sterile males
(Helinski et al., 2006a).
Consequently there is an urgent need to understand the life history and
performance of free-living male An. gambiae s.l., and evaluate the
extent to which their behaviour, physiology and reproductive potential can be
inferred from laboratory observation.
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). Over a 4-week period in 2005 (mid May-mid June), we
conducted daily resting catches in the morning (06:00-08:00 h) in
approximately 10 houses and outdoor toilets to collect Anopheles
mosquitoes. Daily temperatures during this collection period fluctuated
between 28-30°C, which matched the ambient conditions under which our
laboratory colony was maintained (field site was located at the same altitude,
approximately a 1-h drive from our laboratory). Males visually identified as
belonging to the An. gambiae s.l. species complex
(Gillies and DeMeillon, 1968)
were kept for dissection, which was done within 1 h after collection.
Mosquitoes were killed by shaking them in a holding cup; one leg was then
removed from each male An. gambiae s.l. and stored in an Eppendorf
tube containing silica gel for genotypic identification to sibling species
level using PCR (Scott et al.,
1993).
In addition, one wing was removed and measured under a dissecting
microscope fitted with an ocular micrometer (1 unit=0.35 mm). Mosquito wing
length is often used as a proxy for body size as it is a fixed trait that is
relatively easy to measure, and is positively correlated with body mass in
most species (Koella and Lyimo,
1996; Nasci, 1990;
Siegel et al., 1992). The
relationship between wing length and body mass is variable, and its exact
nature can differ between mosquitoes of different species, strain and rearing
background (Nasci, 1990;
Siegel et al., 1992;
Siegel et al., 1994). Despite
this limitation, Anopheline mosquito wing length has consistently been shown
to be a significant predictor of traits such as fecundity and survival
(Ameneshewa and Service, 1996;
Hogg et al., 1996;
Kittayapong et al., 1992;
Lehmann et al., 2006;
Lyimo and Takken, 1993), and
thus was selected as a useful approximator of mosquito fitness for our
purposes.
After wing removal, the remainder of the mosquito body was placed in a drop
of phosphate-buffered saline (PBS) on a cavity microscope slide. The
reproductive system of males was removed using dissecting pins under a
dissecting microscope (10x), and examined under a compound microscope
(50x). Three key features of the male reproductive system that have been
associated with male An. gambiae age
(Huho et al., 2006) were
observed and scored: the number of spermatocysts in the testes, proportion of
the testes occupied by the sperm reservoir, and presence or absence of a clear
border surrounding the edge of the accessory gland. Remaining male body parts
and fluids were washed into a test tube using 100 µl 100% ethanol. In the
field, these tubes were heated at approximately 90°C for 10 min over a
heating block in order to temporarily fix and preserve energetic reserves for
subsequent biochemical analysis in the laboratory. Following this protocol,
samples can be stored for up to 2 weeks at room temperature before being
processed (H. Briegel, personal communication).
Mosquito species identification
DNA was extracted from legs of individual wild-caught male An.
gambiae by placing them individually in an Eppendorf tube containing 15
µl of Tris-EDTA (TE) buffer, and then crushing them using a micropestle. 3
µl of this solution was used for DNA extraction. A master mix containing
DNA templates for the An. gambiae species complex was prepared, and
added to each DNA sample to initiate the PCR
(Scott et al., 1993). Only two
An. gambiae s.l. species were represented within our field sample,
namely An. arabiensis and An. gambiae s.s. Giles.
Laboratory reared mosquitoes
Male An. gambiae s.s., from the insectary at Ifakara Health
Research Development Centre, were used for comparison with wild mosquitoes.
These mosquitoes originated from a sample colonized from wild individuals
collected in 1996 at Njage village (8°20'00,05 South,
36°05'30,57 East). Since then, these mosquitoes have been reared in
laboratory conditions perceived as ideal for survival and reproduction. As
larvae, they are maintained on a standard diet of TetraMin® fish food
(Tetra GmbH, Melle, Germany) at densities of 150-200 larvae per 100 ml of
water in a larval tray (32 cmx12 cmx15 cm). Upon emergence, adult
males were pooled in a separate cage and maintained on a 10% glucose solution,
at ambient conditions (approximately 28-30°C, 70-80% relative humidity)
and a photoperiod of 14 h:10 h (L:D). From these cohorts of males, groups of
different age (1-20 days) were randomly sampled and subjected to biochemical
analysis to assess their energetic reserves. Their body size was also
estimated from their wing length as described above. As with the
wild-collected mosquitoes, laboratory-reared males had one leg, one wing and
their reproductive system removed before their remaining parts were fixed in
ethanol and stored for further biochemical analysis. Anopheles gambiae
s.s. was the only captive reference strain available in the laboratory
group.
Laboratory quantification of sugars, glycogen and lipids
We determined the contents of three key energetic reserves in field- and
laboratory-collected mosquitoes using a spectrophotometric method originally
devised by Van Handel (Van Handel,
1985a; Van Handel,
1985b). Standard curves for converting absorbency readings into
quantities of lipids, sugars and glycogen were obtained from two replicate
series of experiments, in which the absorbency of known concentrations of each
reserve were measured.
Age grading of wild male mosquitoes
Previously we have shown that an age-grading method based on male
reproductive morphology originally devised for Asian Anopheles
(Mahmood and Reisen, 1982;
Mahmood and Reisen, 1994) can
be successfully adapted for male An. gambiae s.s.
(Huho et al., 2006).
Information on the number of spermatocysts, relative size of the sperm
reservoir, and presence of a clear area surrounding male accessory glands was
used to classify male An. gambiae s.s. into age categories of `young'
(
4 days post emergence) and old (>4 days) with 89% accuracy
(Huho et al., 2006). We
applied this model here to age-grade wild-collected males, and test for any
association between age and reserve abundance.
Data analysis
Preliminary analysis of the total glycogen, sugar and lipid content of male
mosquitoes indicated that these reserves did not follow a normal distribution
(Kolmogorov-Smirnov normality test, P<0.001). Consequently we used
the non-parametric Mann-Whitney test for two independent samples to test for
differences in reserve levels between the following treatment groups: (1)
laboratory-reared and field-collected males, (2) field-collected An.
gambiae s.s. and An. arabiensis males, and (3) males of
different age categories (two age groups: 4 days post-emergence, or
older). Laboratory-reared males were excluded from the analysis of
between-species variation in reserve levels to avoid confounding species
differences with those generated by rearing condition (as only one species,
An. gambiae s.s. was represented in the laboratory group).
Relationships between male body size and reserve levels were investigated
using Spearman's correlation coefficient (non-parametric), and analysis of
variance (ANOVA) was used to test if there were differences in body size
between treatment groups that could account for observed differences in
reserve levels. All data were statistically analyzed using SPSS (version
11.5). Unless otherwise stated, numbers in parentheses following means
represent one standard error (s.e.m.).
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2=158.15, P<0.01, odds
ratio=12.93, 95% CI: (8.31-20.13)], and four times more likely to test
positive for glycogen [2=34.29, P<0.01, odds
ratio=4.32, 95% CI: (2.57-7.24)]. In contrast, lipids were detected at a much
higher frequency in wild than in laboratory-reared males [97.3% vs
78% prevalent in wild and laboratory males, respectively:
2=54.78, P<0.01, odds ratio=9.98, 95% CI:
(5.0-19.91)]. Not only the prevalence, but also the abundance, of sugars was higher in laboratory-reared male An. gambiae s.s. than in their wild conspecifics (Mann-Whitney U=13397.0, P<0.01, MedianLAB=8.01 µg, MedianFIELD=0 µg, Fig. 1A,B). Similarly, glycogen content was higher in laboratory-reared males, being on average three times greater than the amount found in wild males (Mann-Whitney U=19783.5, P<0.01, MedianLAB=15.26 µg, MedianFIELD=4.21 µg, Fig. 1C,D). In contrast, lipid content in wild An. gambiae s.s. males was more than twice that of laboratory-reared individuals (Mann-Whitney U=23035, P<0.01, MedianLAB=4. 6 µg, MedianFIELD=9.6 µg, Fig. 1E,F).
Adult body size also varied significantly between laboratory-reared and field-collected An. gambiae s.s. (F1,554=436.77, P<0.001). Wild males were approximately 17% larger than laboratory-reared individuals [MeanLAB=2.17 mm (0.011), MeanFIELD=2.54 mm (0.010)]. Body size was substantially more variable in field-collected males (range 1.86-3.14 mm) than in the laboratory-reared males (range 1.89-2.57 mm). Male body size was positively correlated with lipid stores in both laboratory-reared and wild male An. gambiae s.s. (Table 1; Fig. 2C). In contrast, the amount of glycogen and sugars in males was not associated with the body size of either laboratory or field-collected males (Table 1; Fig. 2A,B).
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Between-species differences in energetic reserves of wild collected mosquitoes
Restricting analysis to field-collected mosquitoes, the quantity of stored
reserves did not vary between An. arabiensis and An. gambiae
s.s. (Table 2). Despite
the lack of variation in reserve abundance between mosquito species, An.
arabiensis males were significantly larger than An. gambiae s.s.
[F1,457=11.38, P<0.01,
MeanARABIENSIS=2.63 mm (0.024), MeanGAMBIAE=2.54 mm
(0.010)]. Thus for a given unit of body length, An. gambiae s.s.
contained a higher abundance of energetic reserves than An.
arabiensis. Body size was positively correlated with sugar abundance in
An. arabiensis but not in An. gambiae, as detailed above
(Table 1). Lipids were
positively correlated with body size in field-collected An. gambiae
but not An. arabiensis (Fig.
3C). Neither species showed any association between body size and
glycogen (Table 1;
Fig. 3B).
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Variation in energetic reserves with age
The age of laboratory-reared males was known with certainty because this
was tracked from emergence. To facilitate comparison with age grades available
for the field sample, we pooled our laboratory sample into two age groups of
`young' (4 days post-emergence) and `old' (>4 days post-emergence).
The morphologically based method we applied to age-grade our sample of wild
males into similar categories is approximately 89% accurate
(Huho et al., 2006). Our aim
was to test whether energetic provisions change with age in both wild and
laboratory-reared males. One potentially confounding factor when testing for
age-related changes, or lack thereof, is size-selective mortality. If small
males die earlier than large males, the older age group, both in the
laboratory and the field, may be disproportionately represented by large males
who inherently have greater reserve levels; this phenomenon could obscure any
decline in reserve abundance with age. To rule this out, we first tested
whether the body size of young and old males varied. We found that the average
body size of `old' males was indeed greater than that of `young' males in both
field (F1,389=12.11, P<0.01) and laboratory
samples (F1,157=4.78, P=0.03), indicating
size-selective mortality is operating in both populations. We then
sub-selected from within our field-collected An. gambiae s.s.,
An. arabiensis, and lab-collected An. gambiae s.s., to
obtain samples of `young' and `old' males of approximately equal body size.
This was done by calculating the mean body size for males in each of the three
groups, and eliminating individuals whose body size fell outside one standard
deviation of this mean. Subsequent statistical analysis revealed no
statistical difference in body size between `young' and `old' males within
these sub-samples (P=0.43 for field An. arabiensis;
P=0.79 for field An. gambiae s.s.; P=0.77 for
laboratory An. gambiae s.s.). Within these size-restricted groups,
there was no difference in sugar or lipid mass between `young' and `old' males
(Table 3). However, there was a
substantial increase in glycogen content in older males within field-collected
the An. gambiae s.s. sample (Table
3); this observation was not evident within the laboratory group
or wild An. arabiensis males.
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;
Klowden, 1988). This
discrepancy between studies of males and females suggests the existence of
sex-specific variation in mosquito energetic budget, and that males may
require a broader range of nutritional resources to maximize their energetic
reserves than females. While the higher sugar and glycogen content of laboratory-reared mosquitoes was expected, the greater lipid reserves of wild male Anopheles was not. Unlike wild male mosquitoes, An. gambiae s.s. maintained in the laboratory had a guaranteed supply of sugar at all times of day, and were rendered largely inactive due the limited confines of their cages. The higher accumulation of sugars and glycogen under these conditions in contrast to free-living mosquitoes is thus not surprising, and suggests that wild males do not sugar-feed to repletion, probably due to limitations in the availability of sugar sources. Alternatively, free-living mosquitoes may sugar-feed less during the night than laboratory males, resulting in a lower detectable of carbohydrate reserves when they were sampled in the early morning hours.
We hypothesize that the larger lipid stores of field-collected males is a
by-product of their larger body size. Female anophelines are known to
accumulate lipids in a size-dependent manner
(Briegel, 1990); this
observation is supported here for males. As mosquito body size is determined
almost entirely by larval nutrition and microclimate
(Briegel, 1990;
Timmermann and Briegel, 1999),
the artificial larval habitats we created in the laboratory may have been of
lower quality to those of males sampled in the wild. However, it is not yet
possible to conclude whether our results indicate that natural conditions are
generally more or less `harsh' than the laboratory. The greater body size and
lipid stores of field mosquitoes could imply that natural larval habitats are
generally of higher quality than those in the laboratory. Alternatively it
could be that rates of larval mortality in the field are substantially higher
than in the laboratory, such that the small percentage of individuals that do
survive can rapidly accrue resources without interference from competitors
(Agudelo-Silva and Spielman,
1992). `Common garden' experiments in which mosquitoes from the
field are reared under laboratory conditions, or vice versa, will
help this resolve this issue. Alternatively, these results may have little to
do with the relative benignity of either setting, but reflect that long-term
evolutionary adaptation of Anopheles to field rather than laboratory
conditions. Regardless of the particular ecological or evolutionary mechanism,
our results suggest that mosquitoes reared under standard laboratory
conditions are not of equal quality to free-living male mosquitoes with
respect to at least two key determinants of lifetime reproductive fitness.
In light of these findings, what can we conclude about the likely success
of laboratory-reared versus wild male mosquitoes when competing
against each other in nature? Ultimately the relative success of male
mosquitoes is determined by their lifetime mating success; this is a composite
measure, depending on both their ability to obtain mates on a particular
swarming event, and the number of swarming events in which they can
participate (correlated with survival). With respect to the first component of
male reproductive success, sugars and glycogen are known to determine male
mating success in a swarm, with the ability to initiate and sustain swarming
being positively associated with carbohydrates reserves
(Briegel et al., 2001;
Nayar and Sauerman, 1977;
Rowley and Graham, 1968;
Yuval et al., 1994). Thus the
higher abundance of sugars and glycogen in laboratory-reared males may
predispose them towards greater competitive success in a swarm. However, males
from the field have substantially greater body size than those from the
laboratory, and this trait that has also been associated with greater
competitive success in a swarm in some
(Ng'habi et al., 2005;
Yuval et al., 1994) but not
all studies (Charlwood et al.,
2002).
In terms of the second component of male mosquito lifetime reproductive
fitness, adult survival, free-living males should have an advantage because
they have greater lipid stores than laboratory-derived males. Several studies
have shown that long-term survival is positively associated with lipid
abundance in mosquitoes (Briegel,
1990; Service,
1987; Van Handel,
1984) and in other insects such as Drosophila
melanogaster Meigen (Service,
1987). Adult body size is also positively associated with
survival, with the present study and others showing that larger mosquitoes
live longer (Ameneshewa and Service,
1996; Hawley,
1985; Reisen et al.,
1984). Thus both the body size and lipid provisioning of wild
males incline them towards substantially greater survival than
laboratory-reared individuals. If this advantage outweighs the possibly
shorter-term benefit of relatively higher sugar content, it is likely that
free-living males will have higher physiologically determined reproductive
potential than their laboratory-reared counterparts.
It is generally assumed that in nature, male mosquitoes depend upon sugars
from plant juices for longevity and other reproductive functions
(Foster, 1995;
Van Handel, 1984;
Van Handel and Day, 1990;
Yuval et al., 1994). In this
study, however, we found only small amounts of free sugars in field-collected
males, with the vast majority having no detectable levels of sugar. A finding
similar to these observations was obtained for five species of mosquitoes from
Florida analyzed by gas chromatography
(Burkett et al., 1999;
Burkett et al., 1998). This
contrasts with studies of Anopheles freeborni Aitken, which found
substantial levels of sugars in males sampled in resting catches
(Yuval et al., 1994).
Anopheles gambiae males may have a lower dependence on sugar feeding
or may replenish their reserves at different times of day than An.
freeborni. Consequently, the importance of sugar-feeding for male
Anopheles remains an open question and likely varies substantially
between species, populations, and habitats
(Foster, 1995). Further
comparative analyses of the physiology of Anopheles species in
different environments with different floral sugar resources will help resolve
this issue.
We caution that we may have underestimated the proportion of wild males
feeding on sugar in this study, as the anthrone technique used here may not
reliably detect very low levels of sugar. An alternative method is gas
chromatography, which has also been successfully used to measure sugar
composition and quantity in mosquito crops
(Burkett et al., 1999;
Burkett et al., 1998), and may
be able to detect sugars at lower quantities. However, unlike the anthrone
technique, gas chromatography does not easily facilitate simultaneous
measurement of additional nutritional reserves (e.g. lipids, glycogen and
protein), and requires analysis equipment that is substantially more expensive
and not yet typically available within field settings in Africa. While use of
gas chromatography may have increased the proportion of wild males that we
considered to be positive for sugars, it would not have qualitatively changed
our main conclusion: sugars were much more abundant in laboratory than
wild-caught males. Future studies could make use of this more specific gas
chromatographic method in order to identify the source of sugars consumed by
males (e.g. nectar, plant juices, honeydew), and their relative abundance in
our field site.
There were no measurable between-species differences in the abundance of
energetic reserves in wild An. gambiae s.s. and An.
arabiensis. Interestingly, reserve levels were constant across these two
species, despite the fact that An. arabiensis was significantly
larger than An. gambiae s.s. As lipid levels, both in this study and
others, are known to increase with body size
(Yuval et al., 1994), it is
unclear why An. arabiensis did not gain an energetic advantage from
its increased body size. One possibility stems from the observation that
An. arabiensis generally store more water than An. gambiae
(M. Kirby, personal communication); this feature may explain why they are
capable of tolerating drier conditions than An. gambiae s.s.
(Coluzzi et al., 1979). Thus
An. arabiensis may devote a smaller proportion of its total body
volume to the storage of energetic reserves than does An. gambiae, in
order to increase its capacity for water storage.
Energetic reserves changed little with male mosquito age; the only observed
difference was an age-related increase in glycogen in field-collected An.
gambiae s.s. It is unclear why this species' laboratory-reared
counterparts did not exhibit a similar increase in this resource with age. One
possibility is that sugar resources were so readily abundant to laboratory
males that this resource became saturated in their tissues early in life, and
simply could not increase further as they aged. Further analyses of male
mosquito resource use and energy budget in nature will help identify
additional proximate physiological markers of their survival and reproductive
success. We note that glycogen is used primarily to fuel mosquito flight
(Briegel et al., 2001). The
fact that this resource increased with age in the An. gambiae s.s.
field groups suggests that older males should be equally or even more capable
of swarming and dispersal than young males, and thus male reproductive fitness
may not decrease with age.
Our findings highlight the importance of validating laboratory-derived
estimates of insect physiology and fitness within a field-realistic context.
We have shown that indirect estimates of male mosquito fitness as obtained
from measurement of body size and energetic reserves vary between field and
laboratory populations, and not in a consistent direction (e.g. laboratory
mosquitoes do not always have higher or lower reserve levels than field
mosquitoes). Specifically, our findings suggest that if one is to release
laboratory-reared male mosquitoes of this stature (small and with lower lipid
reserves) the likelihood of surviving as long as their wild counterparts may
be reduced unless they can build up lipid reserves rapidly. We caution that
although the laboratory conditions employed in this study are generally
typical of An. gambiae laboratory rearing conditions, they do not
necessarily represent every permutation of them. Differences in temperature,
larval density and food provisioning in the laboratory have been shown to
impact adult Anopheles size and survival
(Lyimo and Takken, 1993;
Ng'habi et al., 2005;
Reisen and Emory, 1977). We
did not systematically evaluate the performance of mosquitoes reared under
different laboratory regimes to those in the wild, but rather those reared
under one set of conditions that we assume to be broadly representative of how
An. gambiae are reared in laboratory colonies throughout the world.
One slight discrepancy is that we chose to maintain mosquitoes from our
laboratory colony at ambient temperatures (28-30°C) which, although well
within the acceptable range of An. gambiae, is slightly higher than
the 27±1°C frequently reported in some laboratory colonies. Our
laboratory colony was deliberately maintained under the same ambient
conditions as our field populations, as this permitted assessment of the
relative performance of laboratory-reared and field mosquitoes under the same
thermal regime. It is possible that had we chosen to artificially manipulate
temperature and humidity conditions within the known acceptable range, we
might have found some combinations in which the apparent fitness deficit
between laboratory and field mosquitoes could have been reduced or reversed.
The task for those involved in field release trials of laboratory insects is
to identify if and what these conditions may be. What is clear from the
present study is that rearing conditions typical of most laboratory colonies
do not generate mosquitoes that are better provisioned than those in the
wild.
A final credo to these conclusions is that they have been reached by
considering only physiological determinants of survival. Equally as important
may be behavioral or genetic factors that alter the relative performance of
these phenotypes in nature, independently of the base differences in energetic
provisioning reported here. Previous control efforts based on releasing
laboratory-reared males suggest these behavioral factors would give an
advantage to field males (Benedict and
Robinson, 2003; Ferguson et
al., 2005), a similar conclusion to what we predict from
physiology. Clearly current insect rearing protocols need to be improved to
enhance the quality of males produced, to the point where they at least match,
if not exceed, the body size and energetic make-up of wild individuals.
Further studies to explore the intrinsic determinants of the mating success
and survival determinants in wild insects, especially those that are the
target of genetic control for disease control, are strongly encouraged.
|
Acknowledgments |
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References |
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