Replication in Drosophila chromosomes. X. Two kinds of active replicons in salivary gland polytene nuclei and their relation to chromosomal replication patterns.

DNA fibre autoradiography of highly polytenized nuclei in salivary glands of Drosophila nasuta larvae reveals two distinct types of active replicons. Type I replicons are longer (mean size = 64 micrometers), have a very high rate of fork migration (average rate = 0.95 micrometer/min) and generally occur in large arrays often extending over several thousand micrometers. In contrast, the type II replicons are smaller (mean size = 20 micrometers), slow replicating (average rate = 0.07 micrometer/min) and occur in short arrays containing only a few closely spaced active replicons. Evidence is presented that type I replicons are active in the early S and type II in the late S. Observations on autoradiographic labelling of partially lysed polytene chromosomes provide evidence for a lack of temporal and spatial agreement in the activation of origin points in homologous regions of the lateral polytene strands; these observations also suggest local variations in levels of polyteny within a chromosome. On the basis of this and other available information on replication in polytene chromosomes the possible roles of the two replicon types in the generation of the different 3H-thymidine labelling patterns of polytene chromosomes are discussed.