Estradiol-17 beta in Bombyx mori: possible significance and its effect on silk production

Although estrogen is well known as a vertebrate sex steroid, its presence i n insects, including Bombyx mori, raises questions about its precise role i n the physiology of insects. It was reported earlier that estradiol-17 beta (E-2) exerts a specific effect on silk-gland function in B. mori and that it may act in a nuclear-mediated way. To evaluate further the effect of E-2 on cocoon characters, larval growth and development, 1 mu g/g of E-2 was a pplied topically to the first and second day of fifth instar larvae. This r esulted in a significant enhancement of cocoon characters, such as cocoon s hell weight, silk filament length per cocoon, denier per filament and reela bility of the cocoons, without any adverse effect on fecundity and hatchabi lity. In the present study, E-2 levels in the haemolymph were quantified on different days of the fifth instar larvae and age-dependent changes in the endogenous E-2 titre have been demonstrated. These age-dependent variation s in E-2 content coincide with physiological events occurring during the fi fth instar. Such observations exclude the possibility of a dietary origin f or E-2, as a sudden and sharp rise of the E-2 level in the haemolymph was o bserved on the 10th day of the fifth instar, preceded by a small increase o n the ninth day after an eight-day feeding period. The increased level of e stradiol in the haemolymph of larvae treated topically with E-2 indicates e ffective penetration of this hormone through the larval cuticle. Moreover, similar patterns of alteration of E-2 levels on different days of the fifth instar in both control and treated groups suggests the existence of some i nternal metabolic pathway in the silkworm body to regulate the hormone titr e. Thus, the present investigation offers a system for investigating the un ique function of E-2 in B. mori and offers potential for improvement of sil k production. (C) 2000 Elsevier Science Ltd. All rights reserved.