Changes in ploidy level of epidermal cells during last larval instar of the tobacco hornworm, Manduca sexta.

The relative DNA content of Manduca sexta abdominal epidermal nuclei during the final larval instar was measured by cytophotometry of whole-mount preparations of the epidermis. In the middle intrasegmental region, epidermal cells showed a ploidy level of 4C to 32C on the day of ecdysis. During the subsequent period of feeding, the proportion of higher ploidy cells, such as 16C and 32C, increased. This situation remained until the day of apolysis preceding pupal cuticle formation when mitoses reduced the cells to 2C, 4C, 8C and 16C, except for the pupal pock-mark cells, which increased to 32C or 64C. Metaphase cells showed various ploidy levels, correlated with the size of their mitotic figures. By contrast, in the anterior and posterior margin of a segment where no mitoses occurred, the cells continued to increase in ploidy throughout the instar.