DNA synthesis during the change to pupal commitment of Manduca epidermis

Cytophotometric and cell sorter analysis of Feulgen-stained nuclei of the dorsal abdominal epidermis of the tobacco hornworm Manduca sexta through the final larval instar showed that both 2C and 4C populations were present. A transient population of octaploid cells (30%) was seen on Day 3 at the time of the ecdysteroid-directed reprogramming of the cells for metamorphosis. This specific replication was found to occur in vitro, irrespective of whether the cells were incubated in 20-hydroxyecdysone to induce the change to pupal commitment or in hormone-free media during which they retain their commitment to make larval cuticle. Cytosine arabinoside (1 and 10 microg/ml) inhibited cell cycling during the time of the change of commitment but did not prevent the ecdysteroid-directed change. Thus, DNA synthesis is coincident with, but not necessary for, the genetic reprogramming of Manduca larval epidermis.