Correlations between epidermal DNA synthesis and haemolymph ecdysteroid titre during the last larval instar of the tobacco hornworm, Manduca sexta

Abstract

During the fifth larval instar of Manduca sexta the commitment of the epidermis to the synthesis of pupal cuticle is presumably affected by a small increase in ecdysteroid titre when juvenile hormone levels are minimal. Two sequential rounds of DNA synthesis without an intervening mitosis occur at about this time, resulting in polyploidy of the epidermis. There is a definite temporal correlation between the first peak of ecdysone and the second round of DNA synthesis and indirect evidence has been presented which suggests that this small increase in ecdysteroid titre actually initiates the second period of DNA synthesis. Further, it appears that large doses of ecdysteroids do not elicit the same response as smaller doses at a specific developmental stage, indicating that the different physiological effects of ecdysteroids (reprogramming and apolysis) may be dependent upon the relative concentration of the hormone. Following mitosis which takes place on approximately day 6 of the last instar, the epidermis undergoes apolysis and secretes pupal cuticle, expressing the commitment made 4.5 days earlier. These results support the ‘quantal mitosis’ theory of cytodifferentiation since the covert differentiative event occurs during a period of DNA synthesis and since mitosis precedes the expression of that event.