Journal of Undergraduate Research
Volume 9, Issue 3 - Spring 2008

Starvation Reveals no Effect of Body Size on Pupal Hibernation in the Flesh Fly, Sarcophaga crassipalpis

Melanie C. Buskirk and Daniel A. Hahn

ABSTRACT

Many insects respond to harmful environmental conditions, such as the cold associated with winter, by entering a hibernation-like state termed diapause. Diapause is a physiologically distinct state of programmed developmental arrest initiated in response to stimuli signaling upcoming bad conditions. The flesh fly, Sarcophaga crassipalpis, will diapause in the pupal stage when exposed to short photoperiods, such as those found during fall and winter, as a larva. During larval development, diapause-destined individuals of S. crassipalpis accumulate greater nutrient reserves in the form of lipid and blood proteins than their non-diapausing counterparts. These additional reserves are thought to be critical for providing metabolic fuel for survival through the long non-feeding pupal diapause and as anabolic precursors for resuming development after the arrest period. The goal of our research was to determine how body size, which correlates with metabolic fuel loads, affected the probability of an individual entering diapause and the duration of the diapause period. We effectively used dietary restriction during larval life to produce pupae with a range of body sizes. We expected that small individuals with low reserves would abort the diapause program and develop directly, even when exposed to diapause-inducing conditions, allowing us to estimate the minimum larval weight threshold for entry into diapause. Furthermore, of the individuals that entered diapause, smaller pupae were expected to undergo a shorter diapause due to inadequate nutrient reserves. We found no evidence of an effect of body size on diapause, and, contrary to predictions, smaller individuals spent slightly longer within the pupal diapause program.

INTRODUCTION

Almost all organisms experience times when environmental conditions are unfavorable for growth or reproduction. Seasonal change, for example the onset of a cold winter, is among the most ubiquitous environmental challenges. Therefore, organisms have evolved a variety of strategies to deal with this adversity including dormancy, hibernation, and torpor. In insects, unfavorable environmental conditions typically induce a dormant state called diapause. Diapause is not just a simple arrest of growth and development, but it is a hormonally distinct alternate life history tactic wherein an insect will become dormant and live off of internal nutrient reserves without feeding for months (Tauber et al. 1986). In many insects, diapause is induced by short photoperiods and cool temperatures, such as those experienced during the fall and winter months, and it is thought that diapause evolved as a mechanism to survive adverse winter conditions (Denlinger 1972).

Although diapausing insects benefit from avoiding potentially lethal environmental conditions, diapause is an energy-intensive process in which no feeding occurs and insects are forced to live off of stored reserves for long periods of time. This leads to significant costs of diapause in many insect species. For example, the parasitoid wasp Asobara tabida experienced a significant reduction in fat reserves, dry weight, and fecundity as the length of the diapause period increased (Ellers and vanAlphen 2002). Likewise, a 60% loss of fitness in the mosquito, Wyeomyia smithii, occurred in the form of reduced survivorship to adulthood, fecundity, egg load, and adult life span as a result of completing the diapause program (Bradshaw et al. 1998). Therefore, insects that enter hibernal diapause trade an improved probability of surviving overwintering with significant costs to overall fitness (Ishihara and Shimada 1995).

The stress of diapause, coupled with the need to meet the energetic demands of post-diapause development, induces many insect species to increase their metabolic reserves.  For example, prior to diapause, larvae of the flesh fly Sarcophaga crassipalpis accumulated nearly twice the lipid and hemolymph protein reserves as their non-diapausing counterparts (Adedokun and Denlinger 1984; Rivers 1994). Similar fat sequestering by diapause-destined larvae has been documented in the pink bollworm Pectinophora gossypiella and European corn borer Ostrinia nubilalis, and in diapause-destined adults such as with the ironclad beetle Anatolica eremit, and the alfalfa weevil Hypera postica (Adkisson 1961; Beck and Hanec 1960; Edelman 1951; Bennett and Thomas 1964). In the extreme case, a five-fold increase in pre-diapause fat reserves has been documented in the common mosquito Culex pipiens (Buxton 1935).

Due to the significant energy demands of diapause, larger individuals with greater nutrient stores are typically more likely to survive the overwintering period. Similarly, small larvae that contain low nutrient reserves may abort the diapause program in favor of direct development. For instance, even in the presence of diapause-inducing conditions, undersized larvae of the blow fly Calliphora vicina failed to undergo diapause and instead became sexually mature adults (Saunders 1997). The cost of diapause is so great that there is strong evidence of a minimum weight threshold for diapause-destined individuals in the longicorn beetle, Psacothea hilaris. Within this species, light larvae abort diapause if they do not reach a weight threshold that is a 330% increase over that which is required for metamorphosis (Munyiri et al. 2004; Munyiri et al. 2003).  This preference for direct development in undersized individuals likely evolved as a survival program in nature, when the costs of overwintering were too great compared to completing development prior to winter.

Diapause can occur in various stages of development, including eggs, larvae, pupae, and adult. While the existence of a minimum viable weight has been substantially supported in two insect species that diapause as larvae, no studies report the effect of size on other life stages, such as pupae or adults. The pupal diapause of the flesh fly S. crassipalpis provides an excellent opportunity for studying the effect of body size on pupal diapause propensity and survival. Diapause in the flesh fly occurs when larvae are exposed to cooler temperatures and shorter photoperiods both as an embryo in the female uterus and early in the larval stage. While a directly developing individual will proceed from larval feeding to reproductively viable adult in 3-4 weeks, a fly that has entered the diapause program will arrest in the pupal stage for several months before adult development occurs (Denlinger 1972).

The intention of this study was to determine how size affects entering and completing diapause in pupae of S. crassipalpis. The pupal and adult body sizes of both diapausing and non-diapausing individuals are dictated by larval food intake (Nijhout 1981). Therefore, we used starvation of larvae as a tool to manipulate pupal and adult body size. Larvae were removed from food at regular intervals during development to induce a range of sizes at the completion of feeding. Specifically, our goals were to determine the minimum viable weight for pupariation, the minimum viable weight for diapause, and the effect of weight on diapause length. We expected that viable adult eclosion was to rely heavily on early life conditions such as larval weight at extirpation; therefore we predicted strong relationships between starvation and subsequent body size influencing key life stages. Due to the significant energy demands of diapause, the minimum viable weight for development through diapause was projected to be higher than that for direct developers, as demonstrated in previous studies of the species C. vicina and P. hilaris (Saunders 1997; Munyiri et al. 2004; Munyiri et al. 2003). Finally, similar reasoning leads us to expect that the energy reserves accumulated by larger individuals would promote a longer diapause in those pupae that weighed more at extirpation.

METHODS AND MATERIALS

Procedure

Larvae used in the following experiments were derived from a laboratory colony of S. crassipalpis Macquart. Adults were reared in 30 x 30 x 30 cm screened cages and provided water, sugar, dried milk, and liver ad libitum. The first group of adults was maintained with a long-day photoperiod of 15L : 9D (light : dark) at 25°C to promote direct development in their progeny (Denlinger 1972). Direct-developing larvae were then maintained at 20°C under the same light cycle. The second group of adults was held in a short-day length of 9L : 15D and 25°C, conditions known to promote diapause in their progeny. Larvae from short-day adults were subjected to 20°C and a short-day photoperiod from deposition until the conclusion of the experiment. All larvae were reared in aluminum foil packets placed inside a 500 mL plastic container with two mesh-covered windows for gas exchange. Approximately 1.5 cm of sawdust lined the bottom of the containers to provide a substrate for pupariation. Each packet contained 100 larvae and 50 + 0.9 g of homogenized beef liver as food.

To generate a continuous distribution of body sizes, two individuals were removed from each packet at 48, 57, 72, 81, 96, 105, 120, and 129 hours after larviposition. Individuals were washed in Ringer’s physiological saline, dried gently using a Kim Wipe®, and weighed to the nearest 0.1 mg. Larvae were placed individually into Petri dishes lined with filter paper to absorb excess moisture and returned to their original treatment temperature and light cycle until pupariation.

Individuals were weighed five days after pupariation, the time at which they had become fully formed pupae. Pupae were observed daily for adult eclosion. Eclosion state was categorized as "no eclosion" from the puparium, "partial eclosion" in which the individual broke through the operculum but died before becoming fully liberated, and "full" or eclosion which produced an adult with a normal appearance. Pupae failing to eclose after 160 days were evaluated to determine the life cycle point at which death occurred.

Data Analysis

Data analysis was performed using the JMP Statistical Software and figures were made in SigmaPlot. ANCOVA was used to analyze short day and long day weight relationships throughout all sampling periods, with a Bonferroni correction for eight post-hoc comparisons, with significance evaluated at p < 0.009. We defined a minimum viable weight for pupariation and for adult eclosion by inferring 50% survival thresholds derived from logistic regression.

RESULTS

Extirpation weight and time spent feeding

Removing larvae from their food at specific intervals allowed us to produce a wide range of body sizes in pupae and subsequently in adults (Fig. 1). Larval weight increased in a sigmoidal pattern with time spent feeding in both long day and short day treatments. There was an interaction between treatment and time of extirpation on body mass. Short day individuals were consistently lighter at all extirpation times up to 105 hours, after which weight did not differ between the two treatments (Fig. 2, Table 1).

Figure 1. Selective starvation of larvae yielded a broad range of pupal and adult body sizes.

 

Figure 2. Non-diapause individuals were heavier than diapause-destined individuals for most of the larval period, but diapause individuals caught up in mass by the end of larval development. Asterisks denote comparisons that were different in weight after a Bonferroni correction for multiple comparisons. Note that both groups overlay each other at the 120 h time point.

Extirpation weight and probability of pupariation

Larger larvae at extirpation were more likely to pupariate in both long day and short day treatments (Logistic fit_LD, χzz²=160.64, P<0.0001; Logistic fit_SD, χ²=232.77, P<0.001). The minimum viable weight, defined as the weight at which individuals have a 50% likelihood of pupariation, for larva in the long day treatment was 45.19 mg (33.20-56.29, 95% CI), and 44.73 mg (36.5-53.5, 95% CI) in the short day treatment. There was no difference in the minimum viable weight for pupariation between the two treatments, and these weights were 23.7% and 23.6% of the mean weight of the maximum weight sample. As an additional measure of minimum weight for pupariation we calculated the mean of the lightest 5% of individuals to successfully pupariate in each treatment. There was no difference in the lightest 5% weight in either treatment. The lightest 5% to pupariate in the long day treatment had a mean weight of 38.03mg (31.00-45.06, 95% CI) and the lightest 5% within the short day treatment had a weight of 44.25mg (40.80-48.74, 95% CI), which was 19.9% and 23.2% of maximum sample respectively.

Extirpation weight and diapause

Notably, the mean of the lowest 5% larval weight of those short day individuals that entered diapause was 59.86 mg (55.39-64.34, 95% CI), which was 31.6% of the maximum sample and significantly different from the mean of the lowest 5% for pupariation in the short day treatment. However, no effect of extirpation weight on the probability of entering diapause was found when analyzed with logistic regression, a more conservative statistical tool (χ²=1.564, P=0.2111). Therefore, we conclude that smaller individuals were equally as likely to enter diapause as their larger counterparts.

Extirpation weight and pupal weight

There was a strong linear relationship between larval weight at extirpation and pupal weight. The slopes and y-intercepts of the relationship did not differ between long day and short day treatments, suggesting that the association between larval size and pupal size is the same for diapause and direct developing individuals (Linear regression_LD F_1,242=11585.81, P<0.0001, R²=0.979, pupal wt = 1.0609272(larval wt)-0.468192; Linear regression_SDp F=14046.98, P<0.0001, R²=0.982, pupal wt=1.0528666(larval wt)-0.491556).

Extirpation weight and probability of survival to adulthood       

Although the MVW for pupariation was 45.19mg and 44.73mg for the long day and short-day treatments, respectively, smaller individuals that were able to pupariate were less likely to eclose than larger individuals in both treatments (Logistic fit_LD, χ²=201.38, P<0.0001, R²=0.5138; Logistic fit_SD, χ²=202.92, P<0.001, R²=0.4497). Within the long day treatment, the minimum viable weight for eclosion was 68.39 mg (56.96-79.28, 95% CI) and 79.46 mg (68.09-90.30, 95% CI) in the short day treatment, which is 35.9% and 41.9% of maximum weight sample respectively. Though the 95% confidence intervals overlapped for the minimum viable weight for eclosion, indicating no treatment effect, the short day larval mass is notably greater than the long day larval mass.   The mean larval mass of the lightest 5% to fully eclose was 60.45 mg (53.73-67.17, 95% CI) in the long day treatment and 58.35 mg (53.83-62.87, 95% CI) in the short day treatment, which was 31.7% and 30.8% of maximum sample weight, respectively.

Effects of extirpation weight on time spent in diapause

Within the short day treatment, a significant negative relationship was found between size and diapause duration, surprisingly indicating that smaller individuals spent longer in diapause than larger individuals (Fig. 3).

Figure 3. Smaller individuals spent less time in diapause than larger individuals.
y = -104.34 x + 97.50, R² = 0.11, F_1,203= 24.87, p<0.0001.

DISCUSSION

Diapause, the non-feeding developmental hibernation in insects, is energetically demanding and has been associated with fitness costs in numerous species of insects (Ishihara and Shimada 1995; Ellers and vanAlphen 2002; Saunders 1997; Munyiri et al. 2003; Munyiri et al. 2004). Therefore, we predicted a greater weight requirement among larvae destined for entry into the diapause developmental program than for direct development in S. crassipalpis, and we predicted that larger individuals would stay in diapause longer than smaller ones. We found no evidence for a minimum weight threshold for entering diapause, and, contrary to our predictions, we found that small individuals spent slightly longer in diapause than larger individuals.

Previous literature documents that diapause-destined individuals of S. crassipalpis accumulate almost double the fat and protein stores as non-diapausing larvae (Adedokun and Denlinger 1984; Rivers 1994). These reports appear to be in conflict with our findings that diapause-destined larvae were consistently lighter than their non-diapausing counterparts throughout the most significant growth period. One consideration may be that diapause-destined individuals did indeed accumulate greater nutrient reserves, but were smaller in somatic body size than direct-developing larvae of equal masses. Smaller size may be attributed to excess fat and protein accumulation at the cost of lean body tissue, such as flight muscles and reproductive tissue (Numata and Hidaka 1980; Loeb and Birnbaum 1981; Wolda and Denlinger 1984). Biochemical analysis of individuals from a companion study will allow us to determine if fat and protein content are elevated in diapause-destined individuals.

Although many studies have shown that diapause-destined individuals are heavier than their counterparts, this is not always the case. For example, lighter short-day larvae have been documented in a few species, such as the black swallowtail Papilio polyxenes and the field cricket Teleogryllus commodus (Blau 1981; Tanaka and Tsubaki 1984). Within Papilio spp., diapause-destined larvae are smaller because short photoperiods induced reduced feeding time and because no additional nutrient uptake occurs between the end of larval feeding and pupariation, larval weight corresponds very strongly with pupal weight (Tanaka and Tsubaki 1984).

Metamorphosis is an energy intensive process that occurs within the puparium (Munyiri et al.2003; Nelliot et al. 2006). Lighter individuals that have pupariated because they surpassed the minimum weight threshold can still have inadequate nutrient reserves to eclose as an adult. In our study, smaller larvae at extirpation were less likely to eclose as full adults. The minimum viable weight was approximately 42% of the average weight of fully fed individuals.

We predicted that a greater weight threshold would function to ensure that diapausing individuals had adequate reserves to survive the long and costly over-wintering period. Larvae with inadequate reserves to survive hibernal diapause were expected to abort the diapause program and resume direct development, with the idea that they may reproduce before the onset of adverse winter weather and that their progeny would diapause. Recent research shows a distinct weight threshold for entrance into this developmental strategy. Saunders (1997) reported that small larvae of the blow fly C. vicina subjected to diapause-inducing conditions will abort the diapause program and favor direct development. Similarly, smaller larvae of the longicorn beetle were more likely to abort the diapause program than large individuals (Munyiri et al.2003; Munyiri et al.2004). Our findings stand in contrast to previous literature, which supports the existence of a significantly greater weight requirement for over-wintering insects. We found no evidence of a weight threshold for pupal diapause in S. crassipalpis, and we suggest that more studies of body size thresholds for diapause be performed across species that diapause in different life stages.

We also predicted that smaller individuals would experience a shorter pupal diapause period. Typically, smaller individuals contain less metabolic reserves, which indicates a decreased ability to sustain a long non-feeding pupal stage. Contrary to expectations, lighter diapausing individuals spent slightly longer in the puparium. According to the regression analysis, the 176.6mg difference between the lightest and heaviest larvae indicates an increase of nearly three weeks in diapause for the smallest individuals. Perhaps lower metabolic rates in smaller individuals during diapause could have contributed to this difference, and this will be examined in future studies.

In the field, the diapause program in S. crassipalpis begins with pupal diapause, a programmed arrest of development starting early in the fall when temperatures remain favorable for development. Diapause is followed by a post-diapause quiescence within the puparium, which is maintained by temperatures being below the threshold for development. The post-diapause phase is broken in the spring by warmer temperatures, at which point adult differentiation occurs. This mechanism allows the synchronized emergence of adults, even when diapause is initiated at different points in the fall (Denlinger 2001).

While most individuals complete diapause, a previous study has shown that survival drops considerably during post-diapause quiescence (Hayward et al. 2005). Reduced survivorship during this period may be attributed to declining metabolic resources in the time period between the end of diapause and adult differentiation. Previous work in S. crassipalpis has shown that two-thirds of fat reserves are consumed during the early diapause period, leaving little for the post-diapause quiescence period and adult development (Adedokun and Denlinger 1985). Our study focused on the effects of body size on only the diapause period. Future work will focus on post-diapause quiescence and evaluate how size might affect survival through post-diapause quiescence. We predict that smaller individuals will show a decreased survival when forced to undergo diapause followed by the energetically-costly post-diapause quiescence period as in natural populations.  

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REFERENCES

1. Adedokun, T.A. and D.L. Denlinger. 1984. Metabolic reserves associated with pupal diapause in the flesh fly Sarcophaga crassipalpis. Journal of Insect Physiology. 31 (3): 229-233.
2. Adkisson, P.L. 1961. Effect of larval diet on the seasonal occurrence of diapause in the pink bollworm. Journal of Economic Entomology. 54(6): 1107-1112.
3. Asencot M. and Y. Lensky. 1976. Effect of Sugars and Juvenile-Hormone on Differentiation of female honeybee larvae (Apis-Mellifera-L) to Queens. Life Sciences. 18 (7): 693-700.
4. Beck, S.D. and W. Hanec. 1960. Diapause in the European corn borer, Pyrausta nubilalis. Journal of Insect Physiology.  4(4): 304-318.
5. Bennett, S.E. and C.A. Thomas. 1964. Lipid content in the alfalfa weevil as related to seasonal activity. Journal of Economic Entomology. 57(2): 237-239.
6. Blau, W.S. 1981. Life history variation in the black swallowtail butterfly. Oecologia. 48(1): 116-122.
7. Bradshaw, W.E., P.A. Armbruster, and C.M. Holzapfel. 1998. Fitness consequences of hibernal Diapause in the Pitcher-Plant Mosquito, Wyeomyia smithii. Ecology. 79 (4): 1458-1462.
8. Buxton, P.A. 1935. Changes in the composition of adult Culex pipens during hibernation. Parasitology. 27(2): 263-265.
9. Denlinger, D.L. 1972. Induction and termination of pupal diapause in Sarcophaga. Biology Bull. 142: 11-24.
10. Denlinger, D.L. 2001. “Interrupted development: the impact of temperature on insect diapause.” Environment and Animal Development: genes, life histories, and plasticity. Ed. D. Atkinson and M. Thorndyke. BIOS Scientific Publishers Ltd, Oxford. 234-250.
11. Edelman, I.M. 1951. [The influence of low temperature on beetles of the family Tenebrionidae.] Ent. Obozr. 31(3-4): 374-385. (In Russian with English abstract)
12. Ellers, J. and  J.J.M. vanAlphen. 2002. A trade-off between diapause duration and fitness in female parasitoids. Ecological Entomology. 27: 279-284.
13. Fordyce, J.A. 2006. The evolutionary consequences of ecological interactions mediated through phenotypic plasticity. Journal of Experimental Biology. 209: 2377-2383.
14. Hayward, S.A.L., S.C. Pavlides, S.P. Tammariello, J.P. Rinehart, and D.L. Denlinger. 2005. Temporal expression patterns of diapause-associated genes in flesh fly pupae from the onset of diapause through post-diapause quiescence. Journal of Insect Physiology.  51 (6): 631-640.
15. Ishihara, M. and M. Shimada. 1995. Trade-off in allocation of metabolic reserves:
    effects of diapause on egg production and adult longevity in a multivotine bruchid, Kytorhinus sharpianus. Functional Ecology. 9: 618-624.
16. Loeb, M.J. and M.J. Birnbaum. 1981. The relationship of hemolymph osmotic pressure to sperogenesis in the tobacco budworm, Heliothis virescens. International Journal of Invertebrate Reproduction. 4(2): 67-80.
17. Michimae, H. 2006. Differentiated phenotypic plasticity in larvae of the cannibalistic salamander Hynobius retardatus. Behavioral Ecology and Sociobiology. 60 (2): 205-211.
18. Moczek, A.P. 2003. The behavioral ecology of threshold evolution in a polyphenic beetle. Behavioral Ecology. 14 (6): 841-854.
19. Moczek, A. P. 2005. The Evolution and Development of Novel Traits, or How Beetles got Their Horns. BioScience. 55 (11) 937-951.
20. Moehrlin, G.S. and S.A. Juliano. 1998. Plasticity of insect reproduction: testing models of flexible and fixed development in response to different growth rates. Oecologia. 115: 492-500.
21. Munyiri F.N., W. Asano, Y. Shintani, and Y. Ishikawa. 2003. Threshold weight for starvation-triggered metamorphosis in the yellow-spotted longicorn beetle, Psacothea hilaris. Applied Entomology and Zoology. 38(4): 509-515.
22. Munyiri, F.N., Y. Shintani, and Y. Ishikawa. 2004. Evidence for the presence of a threshold weight for entering diapause in the yellow-spotted longicorn bettle, Psacothea hilaris. Journal of Insect Physiology. 50: 295-301.
23. Nelliot, A., N. Bond, and D.K. Hoshizaki. 2006. Fat-Body Remodeling in Drosophila melanogaster. Genesis. 44:396–400.
24. Nijhout H.F. and C.M. Williams. 1974. Control of moulting and Metamorphosis in the Tobacco Hornworm, Manduca sexta (L.): Growth of the Last-Instar Larva and the Decision to Pupariate. Journal Exp. Biol. 61: 481-491.
25. Nijhout H.F. 1981. Physiological control of molting in insects. American Zoology. 21: 631-640.
26. Numata, H. and T. Hidaka. 1980. Development of male sex cells in the swallowtail, Papilio xuthus, in relation to pupal diapause. Applied Entomology and Zoology. 15(2): 151-158.
27. Nylin S. and K. Gotthard. 1998. Plasticity in life-history traits. Annual Review Of Entomology. 43: 63-83.
28. Rivers, D.B. and D.L. Denlinger. 1994. Redirection of metabolism in the flesh fly, Sarcohphaga-bullata, following envenomation by the ectoparasitoid Nasonia-vitripennis and correlation of metabolic effects with the diapause status of the host. Journal of Insect Physiology. 40 (3): 207-215.
29. Saunders D.S. 1997. Under-sized larvae from short-day adults of the blow-fly, Caliiphora vicina, side-step the diapause programme. Physiological Entomology 22: 249-255.
30. Tanaka K. and Y. Tsubaki. 1984. Seasonal dimorphism, growth and food consumption in the swallowtail butterfly Papilio xuthus. Kontyu 46(1): 135-151.
31. Tauber, M. J., C.A. Tauber and S. Masaki. 1986. Seasonal adaptations of insects. Oxford University Press, New York.
    Wheeler, D. E. 1991. The developmental basis of worker caste polymorphism in ants. The American Naturalist. 138 (5): 1218-1238.
32. Wolda, H. and D.L. Denlinger. 1984. Diapause in a large aggregation of a tropical beetle. Ecological Entomology. 9(2): 217-230.

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