Control of DNA replication and spatial distribution of defined DNA sequences in salivary gland cells of Drosophila melanogaster

In dividing cells, each sequence replicates exactly once in each S-phase, but in cells with polytene chromosomes, some sequences may replicate more than once or fail to replicate during S-phase. Because of this differential replication, the control of replication in polytene cells must have some unusual features. Dennhöfer (1982a) has recently concluded that the total DNA content of the polytene cells of Drosophila salivary glands exactly doubles in each S-phase. This observation, along with previous studies demonstrating satellite underreplication in salivary gland cells, led us to consider the hypothesis that there is a doubling of DNA mechanism for the control of DNA replication in polytene cells. With this mechanism, a doubling of DNA content, rather than the replication of each sequence, would signal the end of a cycle of DNA replication. To test this hypothesis, we have reinvestigated the replication of several sequences (satellite, ribosomal, histone and telomere) in salivary gland cells using quantitative in situ hybridization. We find that underreplication of some sequences does occur. In addition we have repeated Dennhöfer's cytophotometric and labeling studies. In contrast to Dennhöfer, we find that the total DNA contents of nonreplicating nuclei do reflect this partial replication, in accord with Rudkin's (1969) result. We conclude that DNA replication in polytene cells is controlled by modifications of the mechanism operating in dividing cells, where control is sequence autonomous, and not by a doubling of DNA mechanism. — In situ hybridization to unbroken salivary gland nuclei reveals the distribution of specific sequences. As expected, satellite, histone and 5S sequences are usually in a single cluster. This rules out the possibility that sequences known to be underreplicated in chromosomal DNA exist as extrachromosomal copies. Telomere sequences are grouped into two to six clusters, as if the chromosome ends are partially but not completely paired in salivary gland nuclei.