PHYSIOLOGY OF INSECT DIAPAUSE. XIII. DNA SYNTHESIS DURING THE METAMORPHOSIS OF THE CECROPIA SILKWORM

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Abstract
DNA synthesis at stages in the life-cycle of the Cecropia and Cynthia silkworms was examined by radioautographic survey of the incorporation of tritiated thymidine. In sections of pupae previously injected with tritiated thymidine, incorporation of the nucleotide into DNA, as well as cell division, was observed in several tissues as late as one week after pupation. In diapausing pupae, incorporation of thymidine occurred in spermatogonia, hemocytes, a few midgut regenerative cells, and a few cells of the testicular walls. Neither incorporation nor mitotic figures were observed in diapausing epidermal tissues. Large epidermal injury increased DNA synthesis in the blood cells of diapausing pupae and induced incorporation in a few epidermal cells in the immediate vicinity of the wound. Epidermal cells outside the wound periphery did not incorporate thymidine after injury. A generalized incorporation was observed in synchrony with the termination of pupal diapause. The first cells to show increased incorporation were the regenerative cells of the midgut, followed by cells of the epidermis, muscles, nerves, tracheoblasts, and ultimately, the fat body. The incorportion in the epidermis precedes and then accompanies the extensive cell division associated with adult differentiation. Lack of DNA synthesis in the epidermal tissues of injured diapausing pupae appears to be the first biochemical or metabolic criterion so far recognized that permits one to distinguish between an injured pupa and a developing adult.