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

Although estrogen is well known as a vertebrate sex steroid, its presence in insects, including Bombyx mori, raises questions about its precise role in the physiology of insects. It was reported earlier that estradiol-17beta (E2) exerts a specific effect on silkgland function in B. mori and that it may act in a nuclear-mediated way. To evaluate further the effect of E2 on cocoon characters, larval growth and development, 1 microgram/g of E2 was applied topically to the first and second day of fifth instar larvae. This resulted in a significant enhancement of cocoon characters, such as cocoon shell weight, silk filament length per cocoon, denier per filament and reelability of the cocoons, without any adverse effect on fecundity and hatchability. In the present study, E2 levels in the haemolymph were quantified on different days of the fifth instar larvae and age-dependent changes in the endogenous E2 titre have been demonstrated. These age-dependent variations in E2 content coincide with physiological events occurring during the fifth instar. Such observations exclude the possibility of a dietary origin for E2, as a sudden and sharp rise of the E2 level in the haemolymph was observed on the 10th day of the fifth instar, preceded by a small increase on the ninth day after an eight-day feeding period. The increased level of estradiol in the haemolymph of larvae treated topically with E2 indicates effective penetration of this hormone through the larval cuticle. Moreover, similar patterns of alteration of E2 levels on different days of the fifth instar in both control and treated groups suggests the existence of some internal metabolic pathway in the silkworm body to regulate the hormone titre. Thus, the present investigation offers a system for investigating the unique function of E2 in B. mori and offers potential for improvement of silk production.