Replication in Drosophila chromosomes. XI. Differences in temporal order of replication in two polytene cell types in Drosophila nasuta

Abstract

The temporal order of replication of specific sites in polytene chromosomes from salivary glands and gastric caeca of Drosophila nasuta larvae was compared using ³H-thymidine autoradiography. Labelling of different cytological regions in segments of chromosome 2R (section 47A to 49 C) and chromosome 3 (section 80 A to 82 C) was examined in detail in nuclei showing late S-period labelling (2D and 1 D types) in both cell types. The different labelling sites (22 on the 2R segment and 38 on the chromosome 3 segment) are cytologically similar in the two cell types. However, there are profound differences in the labelling frequencies of certain sites in polytene nuclei from salivary glands and gastric caeca during the late S-phase. This suggests that even though a comparable number of chromosomal replicating units operates in the two polytene cell types, the temporal order of completion of replication differs.