Juvenile Hormone Involvement in Pupal Diapause of the Flesh Fly Sarcophaga Crassipalpis: Regulation of Infradian Cycles of O₂ Consumption

Pupal diapause is characterized by a shut-down of the brain-prothoracic gland system in saturniid silkmoths (Williams, 1946, 1947, 1952). This model for diapause is also applicable to flesh flies (Fraenkel & Hsiao, 1968; Ohtaki & Takahashi, 1972; Ž Zárek & Denlinger, 1975 ; Gibbs, 1967 ; Walker & Denlinger, 1980), but in addition, diapause in flesh flies is associated with a unique juvenile hormone (JH) profile (Walker & Denlinger, 1980). At puparium formation, flies destined for continuous development lack JH activity while flies programmed for pupal diapause show major pulses of JH activity having a periodicity of 24 h. In this study we extend our observations on the JH titre beyond the onset of diapause and suggest a link between infradian cycles of oxygen consumption (Denlinger, Willis & Fraenkel, 1972) and JH activity.

Cultures of Sarcophaga crassipalpis were reared as previously described (Denlinger, 1972). To induce pupal diapause, adult flies lacking a diapause history were maintained at 25 ± 1 °C, 12L: 12D (light: dark cycle), and their progeny were maintained at 20 ± 0·5 °C, 12L: 12D. With these conditions diapause incidence exceedea 99%. Pupal age was carefully defined by collecting newly formed puparia at hourly intervals.

Oxygen consumption of individual pupae was monitored at 25 °C using a Scholander respirometer (Mark Co., Brockton, MA). Diapausing pupae, 10–30 days after pupariation, were transferred to a temperature of 25 °C for at least 2 days before making the initial recording. Pupae were kept in the respirometer continuously and manometric measurements were recorded at 24-h intervals. The infradian cycles of O₂ consumption in this species have a periodicity of about 4 days at 25 °C (Denlinger et al. 1972).

JH was extracted from pooled, whole body homogenates (21 g, approx. 175 pupae) using the procedure of Hsiao & Hsiao (1977). Haemolymph JH determinations were based on 5 ml samples pooled from larvae at the time of pupariation (0 h) or collected 8 h after pupariation. Activity was determined using the Galleria wax wound bioassay (deWilde et al. 1968; deLoof & van de Veire, 1972) and scoring was based on the response of 10-12 pupae tested at each dilution. Activity in whole body extracts was expressed in Galleria units (GU) per gram fresh weight and activity in haemolymph samples as GUml^-1 haemolymph. One GU corresponds to 5 pg JHI.

The juvenile hormone analogue methoprene (ZR515) kindly provided by Zoecon Corporation (Palo Alto, CA) was applied directly to the head of diapausing pupae. Solvents could not be used as hormone carriers since many organic solvents are highly active in terminating diapause (Denlinger, Campbell & Bradfield, 1980). To deliver a dose of approximately 50 μg/pupa, 0·5 mg of the analogue was distributed among 10 pupae. Since solvents do not interfere with diapause when applied prior to pupariation, JHA applied to third instar larvae was dissolved in 5 μl acetone.

Pupae were neck ligated by placing a fine cotton thread around the neck and puncturing the head to permit tightening of the ligature. Remnants of the head were then cut off and remaining fluid was absorbed with a filter paper. In a similar manner, abdomen ligations were performed by puncturing the tip of the abdomen and tightening the ligature at mid-abdomen.

Previous observation of JH activity in diapausing flesh fly pupae was based on activity extracted from whole body homogenates (Walker & Denlinger, 1980). To determine whether such JH is merely sequestered within the corpora allata (contained within the ring gland) or is indeed released into the haemolymph, a 5 ml haemolymph sample from newly pupariated flies (0 h) programmed for pupal diapause was compared to haemolymph collected from flies 8h beyond pupariation. Whole body homogenates showed high activity (1000 GU g⁻¹) at 0 h and no detectable activity 8 h after pupariation (Walker & Denlinger, 1980). Activity in the haemolymph reflected asimilar pattern. Though some activity (10 GU ml⁻¹) was detected in pupae collected at 8 h, JH activity in haemolymph at 0 h was considerably higher (2600GU ml⁻¹).

The earlier report of JH activity in diapause-destined flesh flies was limited to the first 4 days following puparium formation. Data included in Fig. 1 extend the results until day 12. JH activity is prevalent throughout this interval, and daily cycles noted during the first 4 days persist for several additional days. But, the activity patterns become much less precise in older samples. Since the samples consisted of a pool of approximately 175 pupae, the reduction in cycle precision could be caused by a gradual loss of synchrony among pupae, a genuine alteration of cycle periodicity, or a combination of these two events.

In flesh flies, O₂ is not consumed at a steady rate during diapause: days of high O₂ consumption occur with a periodicity of about 4 days in S. crassipalpis at 25 °C, and during other days O₂ consumption is barely detectable (Denlinger et al. 1972 and Fig. 3A). To determine if cycles of JH activity may be linked to the metabolic cycles, diapausing pupae were assayed for JH activity on different days of the O₂ consumption cycle. As shown in Fig. 2, JH activity was undetectable in pupae collected on the peak day of O₂ consumption but increased progressively toward the approach of the next peak.

The JH analogue ZR515 was applied topically to pupae on different days of the cycle to test its ability to alter the cycle. When applied on the day of peak O₂ consumption or 1 day later, JHA had little effect on the subsequent metabolic cycle (Table 1 and representative responses shown in Fig. 3B). By contrast, JHA application 2 or 3 days after an peak ( 1 or 2 days before the next anticipated peak) ended the cyclic pattern and caused pupae to shift to a sustained pattern of high metabolic activity (Fig. 3C).

JHA applied to third instar larvae just prior to pupariation did not alter the decision to enter diapause, but the pupae failed to exhibit the normal cycles of O₂ consumption (Table 1). In such pupae, O₂ consumption remained at a steady, high rate throughout the duration of diapause (Fig. 3D). The elevated metabolic rates elicited by JHA treatment were very similar to rates observed during a normal peaW (40–60 μlg⁻¹ h⁻¹).

If JH is involved in regulation of the M02 cycles, removal of the corpora allata, the site of JH synthesis, should eliminate the cycles. The corpora allata of fly pupae are contained within the ring gland and selective surgery is thus extremely difficult. As a less selective alternative to allatectomy, a neck ligature was used to produce a headless pupa. In a control group, a ligature was tied at mid-abdomen, and the posterior region, roughly equal to the head volume, was destroyed.

With both types of ligation, was initially high and then progressively decreased during the following 2–4 days (Fig. 4A, B). The abdomen-ligated pupae then reverted to a cyclic pattern (Fig. 4A, 90 % retained cyclic patterns, N = 10), although the cycles were more erratic and the metabolic rates were generally higher than among intact pupae (Fig. 3A). In contrast, the cyclic pattern was halted by neck ligation (Fig. 4B, 0 % retained cyclic patterns, N = 10). Though cycles were consistently eliminated in all pupae by neck ligation, the level at which was sustained was variable as shown by the two representative individuals in Fig. 4B: 6 of the 10 pupae maintained a low of 15–40 μl g⁻¹ h⁻¹ while the other 4 pupae stabilized at a much higher rate (100–150 μlig⁻¹ h⁻¹).

This study with flesh flies documents the presence of JH, not only at the very onset of pupal diapause (Walker & Denlinger, 1980), but also later in diapause. The precise 24-h cycles of JH activity noted at the onset of diapause, however, become much less distinct in older pupae. Since activity was determined using pooled samples, the apparent dampening of the cycle may represent individual pupae becoming less synchronous with time. Alternatively, this effect could be caused by the transition to a different JH activity pattern as diapause progresses. We suspect that both events are occurring.

Detection of JH activity in the haemolymph, as well as in whole body homogenates, implies that the hormone is not merely being retained within the corpora allata but is circulating and thus available to function physiologically. We suggest that JH is involved in regulating the metabolic cycles that persist throughout diapause.

In flesh flies, the rate of O₂ consumption during diapause is not constant (Denlinger et al. 1972). Days of high O₂ consumption are separated by several days in which O₂ consumption is barely detectable. At 25 °C, the periodicity of S. crassipalpis is about 4 days, but at lower temperatures the periodicity is greater. At lower temperatures, it is also apparent that cycle periodicity changes during diapause : early in diapause, peaks of activity are close together; in mid-diapause, the periodicity is increased; and as the end of diapause is approached, the cycles again become closer together. By focusing on the 4-day cycles observed at 25 °C, we find a close correlation between JH activity and phase of the O₂ consumption cycle. We suggest that JH activity progressively increases during the phase of low O₂ consumption, reaches a critical threshold, and initiates a rise in O₂ consumption (Fig. 5). During the peak, JH activity declines sharply and cannot be detected by bioassay techniques.

Diapausing pupae are highly sensitive to JH late in the cycle. If a pupa is supplemented with exogenous JH shortly before an peak, the rises to a normal peak but fails to return to the base level. By contrast, application of JHA during an peak or 1 day later fails to elicit an effect, suggesting that at this time the hormone is either degraded very rapidly or JH receptors are not present. When JHA is applied before pupariation, the cycles never appear. Pupae are locked into a sustained pattern of high (40–60 μlg⁻¹ h⁻¹). While this rate is comparable to that observed on days of peaks, it remains considerable lower than the nadir (150μlg⁻¹ h⁻¹) of non-diapausing pupae (Denlinger et al. 1972).

The high stimulated by JHA may account for the efficacy of JHA in shortening diapause. When applied to third instar larvae, JHA can reduce the length of diapause by half (Denlinger, 1981). This suggestion, however, implies that diapause termination is hastened by the utilization of a finite energy reserve. Though such a mechanism is possible, its existence has not yet been demonstrated. The presence of JH during diapause may also produce a covert, cumulative effect on the other neuroendocrine centres that would eventually result in activation of the brain-prothoracic gland system. This, too, could account for the shortening of diapause observed when pupae are supplemented with extra JH.

Corpora allata and corpora cardiaca have been implicated in control of the metabolic rate in several species. Allatectomy lowers in adults of Calliphora erythrocephala (Thomsen, 1949), Leucophaea maderae (Sagesser, 1960), and Pyrrhocoris apterus (Sláma, 1964), but in diapausing prepupae of Monema flavescens, allatectomy elevates (Takeda, 1978). InP. apterus, is further depressed when the corpus cardiacum is also removed (Sláma, 1964). Allatectomy fails to alter in Blaberus discoidalis, but extirpation of the corpora cardiaca significantly lowers (Keeley tar & Friedman, 1967). In the Blaberus cockroaches, the corpora cardiaca are the source of a neurohormone that enhances respiratory capacity of fat body mitochondria by stimulating synthesis of cytochromes aa₃+b (Keeley, 1981). Many effects of hormones on respiration may not be direct. The response we observe in flesh flies could be secondary to another metabolic change elicited by JH.

The difficulty of extirpating corpora allata from the ring gland of fly pupae precludes the surgical manipulations that are possible in some other species. But, from our head ligation experiments it is clear that the metabolic cycles are dependent upon a cephalic regulatory mechanism. Headless pupae become acyclic. In contrast, cycles persisted when a portion of the abdomen was ligated. The level of O₂ uptake observed in pupae following head ligation may be a function of the phase of the cycle at the time of ligation, but this possibility has not been tested.

Whether JH is involved in the pupal diapause of other insects remains unclear. Histological evidence suggests that the corpora allata of Mimas tiliae remain active during pupal diapause (Highnam, 1958), and slight JH activity is detectable in Hyalophora cecropia early in diapause (Gilbert & Schneiderman, 1961). By contrast, there is no evidence to suggest a role for the corpora allata during the pupal diapause of Manduca sexta (Bradfield & Denlinger, 1980). The cycles are not unique to the pupal diapause of Sarcophaga but have also been reported in several species of Lepidoptera (Crozier, 1979). Such species are perhaps most likely to share a similar regulatory mechanism utilizing JH during diapause. As with larval diapause (Chippendale, 1977; Beck, 1980), the role of JH in pupal diapause may be highly variable among different species.

This research was supported in part by the Science and Education Administration of the U.S. Department of Agriculture under Grant No. 8000233 from the Competitive Research Grants Office.