Stimulation of sex pheromone production by head extract in Spodoptera littoralis at different times of the photoperiod.код для вставкиСкачать
Archives of Insect Biochemistry and Physiology 211-220 (1988) Stimulation of Sex Pheromone Production by Head Extract in Spodoptera littoralis at Different Times of the Photoperiod Teresa Martinez and Francisco Camps Department of Biological Organic Chemistry, Centro de Investigacidn y Desarroilo (C.S.I. C.), Barcelona, Spain Adult females of Spodoptera littoralis (Lepidoptera: Noctuidae) showed a cyclic pattern of sex pheromone production, and high titers of (Z,E)-9,11tetradecadienyl acetate, the major component of its pheromone blend, were only detected during scotophase. Maximal amounts of pheromone were extracted approximately 2 h into second scotophase. Decapitation before the beginning of darkness inhibited normal production of pheromone, and no calling behavior was observed. Injection of brain-suboesophageal ganglion (Br-SOG) homogenate at the onset of scotophase restored pheromone production in decapitated females to the levels characteristic of second scotophase. Pheromone biosynthesis was also stimulated in decapitated females during photophase. The response to Br-SOG homogenate injection was dose-dependent. The pheromonotropic activity of Br-SOC extract was the same when females were injected during photophase or scotophase. Key words: pheromone biosynthesis-activating neuropeptide, calling behavior, Lepidoptera INTRODUCTION The occurrence of circadian rhythms of sexual activity in Lepidoptera suggests that production and release of sex pheromone as well as calling behavior are well-regulated physiological events. Research on the control mechanism(s) of sex pheromone production in this order has only begun recently. Although in the leafroller moth, Platynota Acknowledgements: We thank Dr. Cemma Fabrias for the mass spectrometry analyses and her interest in this work and Dr. Jan A. Veenstra for valuable discussions and his help in the preparation of the manuscript. Financial support from C.A.I.C.Y.T. (grant 84/0087), C.S.I.C. (grant 85/263), and a postdoctoral fellowship (to T.M.) from the Spanish Ministry of Education and Science i s gratefully acknowledged. Received June 17,1988; accepted September 12,1988. Address reprint requests to Dr. Teresa Mart'tnez,Department of Biological Organic Chemistry (C.S.I.C.), Jordi Cirona Salgado 18-26,08034 Barcelona, Spain. 0 1988 Alan R. Liss, Inc. 212 Martinez and Camps stultana, exogenous treatment with the juvenile hormone analog ZR-512 was found to reduce pheromone titers in virgin females [l],no involvement of the corpora d a t a in the control of pheromone production has been reported in two other species studied, Heliothis zeu  and Lyrnantria dispur . Raina and Klun  provided the first experimental evidence that a hormonal factor from the head was involved in the control of sex pheromone production in females of H.zeu. These researchers indicated that the active factor was present in the brain both during photophase and scotophase, but it was only released into the hemolymph during the scotophase to stimulate pheromone production. Subsequently, the pheromonotropic factor was found to be a neuropeptide originating in the subesophageal ganglion .Similar pheromone biosynthesis-stimulating activity has recently been reported in head extracts of two other moth species [5,6]. On the other hand, a neural mechanism has been partially implicated in the control of pheromone production in L. dispur, based on experiments of ventral nerve cord transsection . In the present study, the control of sex pheromone production in Spodopteru littoralis, a widespread polyphagous pest, has been addressed. We have focused our attention on the possibility of stimulating pheromone biosynthesis during the photophase in a species that normally produces and releases its sex pheromone during darkness [7l, MATERIALS AND METHODS Insects S. littoralis were reared in the laboratory at 25 _+ 2°C with a 16L:8D photoregime. Larvae were fed an artificial diet , and adults were provided with a 10% aqueous sucrose solution. Pupae were sexed, and adults were kept in separate containers. Most animals emerged within 3 h before the onset of scotophase; these were the only ones used in the experiments. Pheromone Gland Extraction The ovipositors, containing the pheromone gland, were excised from females at the corresponding time and extracted individually (overnight, at 4°C) with 50 p1 dichloromethane containing 20 ng of dodecyl acetate as internal standard. Extracts were stored at -20°C until GLC* analysis. Gas-Liquid ChromatographicAnalyses Samples were carefully concentrated under nitrogen, and 1pl was injected into a capillary column. GLC was performed on an instrument equipped with a splitless injector and a flame ionization detector. The purge valve was *Abbreviations: Br-SOG = brain-subesophageal ganglion; GLC = gas-liquid chromatography; MS = mass spectrometry; Z9,Ell-I4:Ac = (Z,E)-9,1l-tetradecadienyl acetate; Ell-14Ac = (E)-11-tetradecenyl acetate; Z9-14Ac = (Z)-g-tetradecenyl acetate; Z11-14Ac = (Z)-11-tetradecenyl acetate. Stimulation of Sex Pheromone Production 213 opened 0.5 min after injection, and hydrogen (carrier gas) flow was 1.5 ccl min at 30°C. A SPB-5 (30 m X 0.25 mm i.d.) fused silica Supelco column was programmed from 120°C to 220°C at 5"Clmin after an initial delay of 4 min. A polar fused silica Supelco column SP-2340 (30 m x 0.25 mm i.d.) was also used in some experiments, since it allows a better separation of the minor components of the sex pheromone blend. This column was programmed from 80°C to 220°C at 5"Clmin after an intial delay of 10 min. Authentic standards were used to determine the retention times of the pheromone components: (Z,E)-9,ll-tetradecadienyl acetate, (Z)-9-tetradecenyl acetate, (E)-11-tetradecenyl acetate, and (Z)-11-tetradecenylacetate [9,10]. Some samples were also analyzed by GLC-MS to confirm their presence in the pheromone blend. Capillary GLC-MS with electron impact mode was performed on a Hewlett-Packard 5995 system, at 70 eV, coupled with a 5997C computer system. The columns mentioned above, at the indicated temperature programs, were used. The characteristic ions of each compound were detected. Throughout this study pheromone titers refer to (Z,E)-9,Stetradecadienyl acetate, the major component of S. liftoralis sex pheromone blend. Preparation and Injection of Br-SOG Homogenate Complexes of Br-SOG were dissected in 0.9% NaCl from male or female adults (- 24 h old). The optic lobes were excised, and the complexes were immediately transferred to a homogenizer stored on dry ice. Br-SOG was homogenized just before injection in Meyers and Miller's saline, containing 188.0 mM NaCl, 20.1 mM KC1, 9.0 mM CaC12, and 1.0 mM MgS04 Ill]. Two Br-SOG equivalents (5-pl volume) were injected into the abdomen of decapitated females briefly anesthetized with C02, using a 50-p1 Hamilton syringe. Injections were done at two times, either 10 h into the second photophase or close to the onset of the second scotophase. Decapitation in the latter case was always performed at least 20 min before darkness. Females were kept in normal rearing conditions after injection, and pheromone glands were excised and extracted 2 h later. Thus, in the first case the glands were removed from the animals during the photophase, and in the second case glands were removed approximately 2 h into scotophase (when peak pheromone titers were detected in intact virgin females). Dose-Response Groups of 55-60 Br-SOG complexes from - 24-h-old females were homogenized in saline to yield a final concentration of four Br-SOG in 5 pl. Dilutions were made containing 4, 2, 1, 0.5, and 0.25 Br-SOG equivalents in 5 p1. Each dose was injected into five second photophase females that had been previously decapitated. Controls were injected with 5 pl saline. The experiment was performed twice. Statistics Data presented in Table 1were analyzed by the parameter-free Wilcoxon two-sample test. 214 Martinez and Camps TABLE 1. Stimulation of Pheromone Biosynthesis in Decapitated Females of S. littoralis by Br-SOG HomogenateP Experiment Donor 1 2 3 ng Z9,Ell-l4:Ac/female, Photophase Saline Br-SOG Saline Female Female Male Male Female 9 Female Female Male Male Female Female 10 Male Scotophase Br-SOG Intact females * 4.0 f 1.4 (4) 20.3 1.5 (7) 6.9 & 1.5 (4) 33.6 f 3.3 (8) 9.8 f 0.7 (5) 43.0 f 5.5 (5) 24.3 f 2.5 (5) 31.6 f 5.8 (5) 2.8 f 0.6 (4) 29.2 f 3.4(5) 4 5 6 7 8 % + SEM (n) 3.9 f 0.8 (5) 26.7 f 2.4 (7) 4.2 + 1.5 (4) 39.4 f 2.1 (8) 2.7 f 0.3 (5) 26.4 & 1.1 (6) 2.5 + 0.3 (6) 18.0 f 1.3(6) 2.6 f 0.5 (5) 17.4 f 1.5(5) 6.4 f 0.9(5) 18.1 rt 2.7(5) 15.7 f 2.2(5) 20.4 3.2 (6) 22.6 + 2.7(6) 3.8 0.5 (6) 7.3 f 0.8 (6) 24.7 3.8 (6) 20.3 f 0.8 (6) 22.0 f 1.7(6) + * *Second-photophase or scotophase females were used, except in experiments 8 and 9, in which animals were 1day older. Pheromone titers of Br-SOG-injected females are in all cases significantly different from those of saline-injected controls ( P < 0.01). Differences between pheromone titers obtained after stimulation with Br-SOG homogenates from males and females are not statistically significant (experiments 4 and 10; P > 0.20), nor was a significant difference found between titers of females stimulated during photophase and scotophase (experiments 9 and 10; P > 0.20). RESULTS In order to be able to examine the stimulatory activity of Br-SOG homogenate on pheromone production, we first determined times of maximum and minimum pheromone titers in intact females. Analyses were performed during the first 5 days of adult life, and pheromone glands were removed and extracted the last hour of photophase and 2 h into scotophase. High pheromone titers were detected only during scotophase and peaked in the second scotophase (X = 28.6 ng per female). Comparatively low amounts were found 1h before the onset of darkness (X = 4.7 ng per female) (Fig. 1). In order to confirm the approximate time of maximum pheromone content in the gland, hourly determinations were made during the first part of second scotophase. A rapid increase followed during the first hour of darkness, and at 2-3 h after lights-off the highest amounts of (Z,E)-9,1l-tetradecadienyl acetate were extracted (X = 26.8 ng per female). From the third hour of scotophase pheromone titers began to decrease (Fig. 2). Three groups of experiments (summarized in Table 1) were performed with decapitated S. littoralis females to determine the pheromonotropic activity of Br-SOG homogenates from males and females. Stimulation of pheromone production was initially attempted at the time pheromone titer normally peaks in intact females. Injection of Br-SOG homogenate was close to the beginning of the second scotophase, and pheromone glands were excised 2 Stimulation of Sex Pheromone Production 215 30 25 20 15 U a u T r r 10 :N 5 '2 26 50 74 98 Female age ( h ) Fig. 1. Sex pheromone titers (expressed as ng of (Z,E)-9,11-tetradecadienyl acetate) in S. littoralis virgin females as a function of age. Pheromone glands were removed in the last hour of photophase and 2 h into scotophase. Stippled bars refer to scotophase (n = 10-15) and open ones to photophase (n = 8). Data represent means f SEM. Arrow indicates approximate time of emergence (2 h before darkness). h later. Decapitation before darkness inhibited the increase in pheromone titer characteristic of second-scotophase females. However, injection of two Br-SOG equivalents restored pheromone production in decapitated females. The average extractable amounts of (Z,E)-9, ll-tetradecadienyl acetate were similar (sometimes even higher) to those of intact second-scotophase females (Table 1, experiments 1-4). In one of these experiments, Br-SOG homogenates from male and female donors were tested on the same group of females (emerged simultaneously) in order to compare the pheromonotropic activity of Br-SOG in both sexes. No significant difference was found between pheromone titers obtained after stimulation with the two types of homogenate (Table 1, experiment 4; P > 0.20); a significant difference between BrSOG homogenates from males and females was also not found in a later experiment (Table 1, experiment 10; P > 0.20). In the next set of experiments a possible stimulatory effect of Br-SOG on pheromone production was investigated during photophase, when only low amounts of pheromone were detected in S. ZittomZis females. Injection of two Br-SOG equivalents into second-photophase decapitated females stimulated pheromone production. Mean pheromone titers were similar (or even higher) to those of intact second-scotophase females (Table 1, experiments 5-8). 216 Martinez and Camps 30 5 0 1 2 3 4 5 Second scotophase ( h1 Fig. 2. Sex pheromone titers (expressed as ng of (Z,E)-9,11-tetradecadienyl acetate) of virgin females during the second scotophase (first 5 hours). Determinations were made hourly from the beginning of darkness (0 h). Each point represents mean & SEM (n = 10). These results suggested that the active substance was as effective in photophase as in scotophase. In order to confirm this notion, stimulation with a single Br-SOG extract was attempted in the same group of females, during photophase and the following scotophase. This experiment was performed once with Br-SOG from male donors and twice with Br-SOG from females. No significant difference was found between pheromone titers of females stimulated during photophase and those stimulated during scotophase (Table 1,experiments 9 and 10; P > 0.20). Stimulation of pheromone production during photophase was found to be dependent on the dose of Br-SOG homogenate injected (Fig. 3). The relative amounts of (Z,E)-9,1l-tetradecadienylacetate and minor components of the sex pheromone blend of S. littoralis were determined in intact second-scotophase females (2 h into scotophase) and compared with those in decapitated females injected with Br-SOG homogenate. Females stimulated at the onset of scotophase showed a pheromone blend with normal proportions of its components. However, when stimulation was performed during photophase, the ratio of (Z)-9-tetradecenyl acetate to (Z,E)-9,11-tetradecadienyl acetate was found to be higher than in intact second-scotophase females (Table 2). It is worth mentioning that after decapitation females exhibited repetitive abdominal movements with extrusion and retraction of the ovipositor; in Stimulation of Sex Pheromone Production 20 - 15 - 10 - 5 - 217 3 1 +/ I I I 1 I 0.25 0.5 1 2 4 # . rr S Br-SOG equivalents Fig. 3. Effect of Br-SOG homogenate injection into decapitated females on pheromone titers (expressed as ng of (Z,E)-9,11-tetradecadienyl actetate). Injections and pheromone gland excisions were performed during the second photophase. Each point represents mean SEM (n = 10). S, saline-injected females. TABLE 2. Relative Amounts (YO)of Pheromone Components in S. littoralis Females* Component Controlsa Br-SOG homogenate-injected females Scotophase Photophase Z11-14:Ac E11-14:Ac Z9-14:Ac Z9,Ell-l4:Ac 5.6 f 0.3 10.4 0.8 13.0 k 0.8 70.9 f 1.0 5.3 k 0.4 9.5 f 0.5 12.7 k 1.3 72.4 f 0.9 + 5.0 k 0.5 10.3 f 0.8 17.9 k 2.0 66.9 f 1.4 *Data represent means f SEM (n = 9). ahtact second-scotophase females. some females this behavior was accompanied by oviposition. However, animals were never observed to expose their pheromone gland or display a "calling posture." Thus, although injection of Br-SOG homogenate induced pheromone production in decapitated females, those animals were not observed to call. DISCUSSION Like other moth species, S . Zitturdis virgin females show a cyclic pattern of sex pheromone production. Only during the scotophase, when the females display a calling behavior, were high pheromone titers detected. Maximal amounts of pheromone (25-30 ng) were extracted during the second and 218 Martinez and Camps - third scotophases ( l-2-day-old females), and gland pheromone content peaked around 2 h into scotophase. In a recent study, Dunkelblum et al.  published some data on pheromone titers in S. littovalis females, indicating that 6-8 ng of (Z,E)-9,ll-tetradecadienyl acetate were the largest amounts extracted from l-3-day-old females. Although we can not rule out the possibility of strain differences as a partial explanation for the disparity between their results and ours, the extraction procedure (15 min in hexane) used by Dunkelblum et al. [7l is probably the main reason for the above differences. We found an average of 8 ng per female when following their procedure. However, if the same glands were subsequently extracted in dichloromethane (overnight, at 4"C), an average of 16.5 ng of additional pheromone per female was obtained. On the other hand, studies on pheromone titer in different moth species show that individual variation in the quantity of gland-extractable sex pheromone is frequently observed [l2,13]. We have also found some variability in the responsiveness to stimulation with Br-SOG homogenate. For example, when comparing the average pheromone titers obtained in the experiments presented in Table 1with mean titers in the dose-response experiment, the latter values are relatively low. However, in this type of experiment, we cannot rule out the possibility that some enzyme degradation of the active factor has occurred during the serial dilutions of the homogenate. A rapid loss of biological activity in crude Br-SOG preparations has been reported in Heliothis . The fact that pheromone production and sexual activity in S. littoralis appear to be restricted to a certain time of the photoperiodic cycle indicates that precise control mechanisms must be implicated. Decapitation of S. littoralis virgin females before the onset of the second scotophase was found to inhibit normal production of pheromone. Only reduced titers were detected in those females 2 h after lights-off. Calling behavior was also inhibited after decapitation, although females showed repetitive abdominal movements with extrusion of the ovipositor. These findings are in agreement with the observations reported by Tang et al. , who indicated that females of L. dispar did not call normally after transsection of the ventral nerve cord. In the moths P. stultana [l] and Chilo suppressalis , calling was also inhibited by decapitation. However, while some decapitated S. littoralis females were observed to oviposit, in P. stultana oviposition did not occur [l]. Injection of Br-SOG homogenate induced pheromone production in decapitated females, but no evidence of calling was ever observed in those animals. These results indicate that calling behavior in S. littoralis females is not controlled by the same mechanism as pheromone production. An intact nervous connection of the brain and/or the subesophageal ganglion with the terminal abdominal ganglion appears to be essential for calling behavior in Hyalophora cecropia  and Manduca sexta . Raina and Klun  reported that a factor from the head is involved in the control of sex pheromone production in H. zea. These researchers demonstrated that injection of crude Br-SOG extracts into neck-ligated females caused a significant increase in pheromone titers when the animals were injected during the scotophase. At that time, the stimulatory activity was Stimulation of Sex Pheromone Production 219 also found in female hemolymph. If, as their data strongly suggest, a neurohormone regulates sex pheromone biosynthesis, then one may expect that it can also stimulate pheromone biosynthesis during the photophase. However, Raina et al.  and Raina and Menn  have recently indicated that pheromone production in H. zed was not stimulated when Br-SOG homogenate was injected into ligated females during photophase, and they have suggested the existence of some additional control mechanism. The results reported here demonstrate that pheromone production in S. littoralis females can also be stimulated during photophase. Similar responsiveness in decapitated females is achieved after injection of Br-SOG homogenate during photophase and scotophase, as evidenced by the amounts of pheromone extracted at both times. Preliminary results indicate that stimulation of pheromone production during the light period can also be achieved in another moth species, Thaumetopoea pityocampa (Notodontidae), and thus suggest that S. littoralis is not an exception in this respect. A normal pheromone blend was produced in S. littoralis females after injection of T. pityocumpa Br-SOG homogenate (unpublished observations). These findings are in agreement with the indication that the pheromone biosynthesis-activating neuropeptide does not regulate pheromone specificity . Average pheromone amounts found in decapitated control females during scotophase were usually a little higher than those of controls extracted in photophase (Table 1, experiments 9 and 10; P < 0.01). Since decapitations were always performed during photophase, these findings suggest that in S. liftoralis the active factor may start to be released into the hemolymph a little earlier than the lights-off signal. On the other hand, the reduced pheromone titers found in these decapitated females, when compared with the amounts extracted from intact second-scotophase animals, would be in agreement with the hypothesis that a continuous presence of the active factor is required to maintain pheromone production [2,5]. The results reported in the present study support the existence of a hormonal control of pheromone production in S. littomlis, although they do not rule out the possibility that a neural mechanism can mediate the response. Furthermore, the ability to stimulate pheromone biosynthesis in S. littoralis females during the photophase suggests that the necessary cellular machinery is available and can probably be activated at any time of the photoperiodic cycle in this species. LITERATURE CITED 1. Webster RP, Carde RT: The effects of mating, exogenous juvenile hormone and a juvenile hormone analogue on pheromone titre, calling and oviposition in the omnivorous leafroller moth (Plufynofustultunu). J Insect Physiol30, 113 (1984). 2. Raina AK, Klun JA: Brain factor control of sex pheromone production in the female of the corn earworm moth. Science 225, 531 (1984). 3. 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