The abdominal epidermis of Calliphora Erythrocephala (diptera): I. polyteny and growth in the larval cells

The extremely rapid growth of feeding larvae of Calliphora is achieved with a hypertrophy of larval epidermal cells and concomitant polytene specialization of their nuclei. Within the epidermis of each abdominal segment are differentiated sense organs of the campaniform and a more complex - possibly chemosensory - kind in a constant pattern; there is no post-embryonic differentiation of new sense organs before metamorphosis, while the cuticle-secreting cell of these sense organs also becomes polytene. The polytene organization of epidermal nuclei differs from the classical giant chromosome structures of lower dipteran larval cells. Calliphora epidermal polytene chromosomes irregularly fragment into oligotene or smaller fibrils which, through apparently non-specific reassociations, form a nuclear reticulum of which the banded polytene regions are a part. This loose reticular organization is unlikely to reflect a condition of genetic activity. Photometric densitometry shows the period of polytene DNA replication in general epidermal nuclei is closely correlated with the period of larval growth. At hatching, epidermal nuclei are diploid, with the 2C DNA content. Through a series of endoreduplications, such that at any stage nuclei tend to fall into a series of DNA classes with a percentage falling in between, the largest polytene nuclei reach about 256C by the end of the feeding stage. During the feeding stage, whole mount autoradiography shows a very slow continuation of DNA synthesis in some epidermal nuclei; that is, after cessation of larval growth. DNA synthesis in cupologen nuclei of the campaniform organs ceases after secretion of the third instar cuticle at the end of the second instar. The polytene cycle responsible for nuclear DNA increase is discussed in the context of these data, and a functional correlation drawn for the rate of polytene increase in the epidermal nuclei and the cupologen nuclei. This correlation suggests active control of the cycle, and raises the question of the significance of the polytene cell cycle.