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Abstract--The feeding and gustatory responses to ecdysone and 20-hy- droxyecdysone were investigated in the silkworm, Bombyx mori. 20-Hydroxyecdysone ...
Journal of Chemical Ecology, Vol. 20, No. I, 1994

DIFFERENT FEEDING AND GUSTATORY RESPONSES TO ECDYSONE AND 20-HYDROXYECDYSONE BY LARVAE OF THE SILKWORM, Bombyx mori

Y O S H I A K I T A N A K A , * K I Y O S H I A S A O K A , and S A T O S H I T A K E D A Department of Insect Physiology and Behavior National Institute of Sericuttural and Entomological Science Tsukuba, lbaraki 305, Japan

(Received July 12, 1993; accepted September 8, 1993) Abstract--The feeding and gustatory responses to ecdysone and 20-hydroxyecdysone were investigated in the silkworm, Bombyx mori. 20-Hydroxyecdysone reduced feeding response strongly in fourth- and fifthinstar larvae, whereas ecdysone had no effect on feeding response. 20-Hydroxyecdysone stimulated the R receptor, the receptor to feeding deterrents, to a great degree. By contrast, ecdysone was much less effective for stimulating the R receptor. These results indicate that ecdysone and 20-hydroxyecdysone have different effects on feeding response due to different interactions with mouthpart chemoreceptors.

Key Words--Bombyx mori, Lepidoptem, Bombiridae, silkworm, larvae, ecdysone, 20-hydroxyecdysone, feeding behavior, electrophysiology, sensilla response, chemoreceptors. INTRODUCTION Ecdysone and 20-hydroxyecdysone are the two major insect molting hormones, but ecdysone is usually thought to be the precursor o f the active hormone, 20-hydroxyecdysone (Smith, 1985). H o w e v e r , different effects of the two ecdysteroids have been found in some cases (Oberlander, 1969; Clever et al., 1973; Quennedey et al., 1983; Robert et al., 1986; Perriere et al., 1993). Recently, we succeeded in inducing 11 larval ecdyses, i.e., seven additional larval ecdyses (ultranumerary larval ecdyses), with low mortality by the application o f ecdysone in the silkworm, B o m b y x mori (Tanaka and Takeda, 1993a), *To whom correspondence should be addressed. 125 O098-033t/9410tOO-Ot25507.0010~) 1994 Plenum Pubhshmg Corporation

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and found that dietary supplements of ecdysone and 20-hydroxyecdysone affect larval development differently (Tanaka and Takeda, 1993b). Our results suggest that ingested ecdysteroids affect endocrine organs differently (Tanaka and Takeda, 1993b), but further detailed study is needed to explore the different effect of the two ecdysteroids on development. 20-Hydroxyecdysone affects insect development by inhibiting feeding behavior (Ma, 1972; Schoonhoven and Derksen-Koppers, 1973; Jones and Firn, 1978), whereas the effect of ecdysone on feeding behavior has not been investigated in detail (Jones and Firn, 1978). It is possible that ecdysone has a different effect on feeding behavior from 20-hydroxyecdysone, because our preliminary results show that the amount of feces from larvae reared on the diet supplemented with ecdysone was apparently different from that of larvae reared on the diet supplemented with 20-hydroxyecdysone. In this study, we report on the different effects of ecdysone and 20-hydroxyecdysone on the feeding behavior of Bombyx mori.

METHODS AND MATE~ALS

Insect Rearing. The C145 x N140 race of Bombyx mori was used in all experiments. Larvae were reared on artificial diet (Yakult Co. Ltd.) at 25 + 1°C under a photoperiod of 12 hr light and 12 hr dark. Feeding Response. The larvae used in the experiment were freshly molted larvae (second to fifth instar) within 6 hr after ecdysis. Ten larvae were used in each experiment, and each experiment was replicated three times. The amount of diet actually eaten was estimated by the difference in the dry weight of the diet before and after testing. Chemicals. Ecdysone and 20-hydroxyecdysone (Sigma Chemical Company) were dissolved in 5 % ethanol and added to the diets during diet preparation. Concentration of ecdysteroids were expressed as parts per million (ppm) of dry matter. Electrophysiological Response. The larvae used in electrophysiological experiment were newly molted fifth instars. The chemoreceptor tested was the R receptor associated with one of the sensilla styloconica (called the Ss-II hair for easy reference) on the maxilla (Ishikawa, 1963). The electrophysiological methods used in the experiment were the same as described by Ishikawa (1963). The isolated head was fixed on an indifferent, platinum wire electrode. Stimuli were aqueous solutions contained in the stimulating recording electrodes. Stimulation and recording were started simultaneously when the electrode was slipped over the tip of the hair by means of a micromanipulator. The signal was amplified, observed on a cathode ray oscilloscope, and displayed on paper by thermal array recorder. The period of stim-

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ulation was 1 sec. Ecdysone and 20-hydroxyecdysone were dissolved in 2 x 10 -2 M NaC1 solution. Concentrations of ecdysone and 20-hydroxyecdysone were expressed as ppm in 2 × 10 -2 M NaC1 solution.

RESULTS

Feeding Response. The effects of ecdysone and 20-hydroxyecdysone on feeding behavior were investigated in fifth-instar larvae. Larvae were fed on a diet supplemented with 100 ppm ecdysone or 100 ppm 20-hydroxyecdysone for 36 hr following onset of feeding. The larvae fed on a diet supplemented with ecdysone entered larval apolysis (head capsule slippage occurred) within 36-48 hr, but the amount of diet eaten during the 24 hr following onset of feeding was not different from the control (Figure 1). By contrast, the larvae fed on a diet supplemented with 20-hydroxyecdysone did not enter apolysis. They did not begin spinning within 48 hr (data not shown) and continued to feed, but the amount of diet eaten was reduced compared with the control. Feeding activity was markedly diminished during the 12 hr following onset of feeding. The amount of diet eaten was reduced by 33% of the control. Feeding activity recovered to normal immediately following the first 12 hr. Feeding activity during the 12 hr following onset of feeding decreased in accordance with the concentration of 20-hydroxyecdysone (Figure 2). 20-Hydroxyecdysone at more than 50 ppm was effective in reducing feeding,

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