A comprehensive study of the changes in ecdysteroid levels during the feeding phase of fifth instar larvae of the silkworm, Bombyx mori (Lepidoptera: Bombycidae)

Bela Keshan,Bembem Thounaojam,Sanathoibi D Kh

doi:10.14411/eje.2015.088

Abstract

The present study describes changes in haemolymph ecdysteroid levels during the feeding phase of fifth instar larvae of Bombyx mori. A small and sharp peak in ecdysteroid level was recorded on day 4 when larvae had already reached the critical weight (2.76 ± 0.11 g) for metamorphosis. Food deprivation of larvae from day 4 resulted in an increase in the ecdysteroid level. Injection of insulin in both 1 day (before reaching critical weight) and 4 day (after reaching critical weight) old fed larvae resulted in an increase in the ecdysteroid level. However, when deprived of food only the haemolymph ecdysteroid levels of 4 day old larvae respond to an injection of insulin. The hormone level was also increased by treating 1day old larvae with a juvenile hormone analogue, but again, like insulin, only the fed larvae were affected by this treatment. Neck- and thorax-ligated 4 day old larvae had decreased ecdysteroid levels compared to both fed and food deprived larvae. However, a significant increase in ecdysteroid level was recorded in both neck- and thorax-ligated larvae injected with insulin. The injection of 20-hydroxyecdysone into 4 day old larvae resulted in an increase in the level of this substance in the haemolymph of fed and food deprived larvae, but the level was significantly higher in neck- and thorax-ligated larvae. The data, thus, indicates that ecdysteroid levels vary both in relation to the critical weight of the larvae and its nutritional status, and insulin might also play an important role in regulating the level of ecdysteroid.

Highlights

The growth and development of an insect is under the control of various intrinsic and extrinsic factors such as hormones and nutrition (Riddiford, 1994; Nijhout, 2003; Riddiford et al, 2003; Dubrovsky, 2005; Mirth & Riddiford, 2007; Truman & Riddiford, 2007; Layalle et al, 2008; Nijhout et al, 2014)
The classical idea is that the synthesis and release of the insect steroid hormone, ecdysteroid, by the prothoracic gland (PTG), are governed by the prothoracicotropic hormone (PTTH), a neuropeptide produced by the neurosecretory cells (NSCs) in the brain (Bollenbacher & Granger, 1985; Gilbert et al, 2002; Rybczynski, 2005; Truman, 2006; Marchal et al, 2010)
This study indicates that in B. mori, the ecdysteroid levels fluctuate under both fed and food deprived condition s during the fifth larval instar and in relation to the critical weight of the larvae, and that both Juvenile hormone (JH) and insulin play a role in regulating the ecdysteroid level

Summary

Introduction

The growth and development of an insect is under the control of various intrinsic and extrinsic factors such as hormones and nutrition (Riddiford, 1994; Nijhout, 2003; Riddiford et al, 2003; Dubrovsky, 2005; Mirth & Riddiford, 2007; Truman & Riddiford, 2007; Layalle et al, 2008; Nijhout et al, 2014). The stimulation of ecdysteroidogenesis in the PTG by insulin/insulin-like growth peptides/insulin signaling is reported in other insects, such as Rhodnius prolixus and D. melanogaster (Vafopoulou & Steel, 1997; Caldwell et al, 2005; Colombani et al, 2005; Mirth et al., 2005). The gut tissues are a source of ecdysiotropic peptides (see review by Marchal et al, 2010)

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Conclusion