Relationship of developmental stage to initiation of replication in polytene nuclei

The various types of autoradiographic patterns occurring in salivary gland nuclei ofD. melanogaster following a short pulse of tritiated thymidine have been described and the probable order of their sequence within a polytenic replication cycle assigned. On the basis of that assignment, the distribution of those patterns in the nuclei of salivary glands of 160 staged larvae and prepupae has been assessed and the following interpretations made: Throughout the third larval instar and prepupal period there is a continuing decrease in frequency of labeled nuclei. Within the larval period that decrease is reflected in a proportional decrease in label patterns characteristic of both initiation and propagation phases of the replication cycle. At the beginning of the prepupal period there is a rise in the proportion of labeled cells displaying initiation type patterns followed by an abrupt cessation of such patterns. It is proposed that termination of initiation of polytenic replication is amongst the physiologic manifestations of the induction of pupation.