Growth and silk formation of silkworm larvae influenced by phytoecdysones

Abstract

The fourth and fifth instar larvae of the silkworm were reared on artificial diets containing ponasterone A, ecdysterone, and inokosterone. The growth of the larvae and their silk glands, fibroin-synthesizing activity, and silk formation have been investigated. With a diet containing ponasterone A, the fourth instar larvae grew slowly and only a few larvae could ecdyse, while the growth of the fifth instar larvae was disturbed and they died with a darkening of the skin. Ponasterone A also inhibited the growth of the silk glands during the fifth instar. In contrast, the other two phytoecdysones did not greatly influence larval growth. The fourth instar larvae grew rapidly and their ecdysis was advanced with a diet which contained 10 μg of inokosterone/1 g of dry diet. The diet which contained 5 μg of ecdysterone or 10 μg of inokosterone/1 g of dry diet accelerated maturation, while that containing 10 or 20 μg of ecdysterone, or 40 μg of inokosterone, delayed maturation of the fifth instar larvae. Only phytoecdysones caused a decrease in growth of the silk glands in the early half of the instar, and a large amount of phytoecdysones accelerated their growth during the last part of the fifth instar. The fibroin-synthesizing activity was levelled up by feeding ecdysterone and inokosterone, and inokosterone appreciably stimulated activity. Assay of in vitro fibroin synthesis showed that ponasterone A competed with ecdysterone in a stimulative action. Silk formation was much lower in larvae fed the diet containing 5 μg of ecdysterone or 10 μg of inokosterone/1 g of dry diet and was far greater in larvae fed the diet containing 40 μg of inokosterone than in the controls.