The role of 20-hydroxyecdysone in stimulating epidermal mitoses during the laval-pupal transformation of the tobacco hornworm, Manduca sexta

Temporal and regional changes in mitotic frequency were examined in the dorsal epidermis of the fourth and fifth abdominal segments of Manduca sexta during metamorphosis. Mitoses occurred only in the middle intrasegmental region, but not in the segmental margins. The mitoses began early on day 5 and rose to maximum of 2á6-4á6% about 10h later. When the integument from day 4 (wandering) larvae was cultured in Grace's medium containing 0.3 to 1µgml-1 20-hydroxyecdysone (20HE), the mitotic index increased with a peak at 18-24h exposure approximately equal to that found in situ. The level of 20HE required to initiate mitoses was similar to that found in vivo during the beginning of the prepupal rise in ecdysteroid and therefore is likely to be the signal for these cells to decrease their ploidy level of 4-32C to 2-8C at this time. The polyploid cells had larger mitotic figures and required a longer exposure to 20-hydroxyecdysone to initiate mitosis. Some multipolar mitotic figures were observed.