Fluctuations in ecdysteroid titre and development during the early stage of the last larval instar of trimoulter strains of the silkworm, Bombyx mori

Abstract

The correlation between slight fluctuations in haemolymph ecdysteroid titre and development of the larvae were explored in recessive and dominant trimoulter strains of the silkworm, Bombyx mori. In recessive trimoulters, the decision for precocious metamorphosis is reversible, and perfect supernumerary larval moults can be induced by the addition of 20-hydroxyecdysone to food sources during the early stage of the last (fourth) instar. Changes in haemolymph ecdysteroid titre show that a small peak occurs at 24 h. On the contrary, for dominant trimoulters, as well as for recessive trimoulters to which an imidazole compound (KK-42) is applied, or which are exposed to high temperature and moisture during the early stage of the penultimate (third) larval instar, the decision for precocious metamorphosis becomes irreversible in some tissues, and a perfect supernumerary larval moult can never be induced through 20-hydroxyecdysone feeding after the third ecdysis. Ecdysteroid titre levels remained low during the early stage of the last larval instar. We propose that the observed decrease after small increase in ecdysteroid titre in recessive trimoulters is related to changes in the developmental programme from larva to pupa. The mechanism by which the controlled fluctuations in ecdysteroid titres are regulated is discussed.