Replication in Drosophila chromosomes. Part IV. Patterns of chromosomal replication in salivary gland polytene nuclei of Drosophila nasuta

Abstract

Replication in polytene nuclei, of late third instar larvae and early prepupae of D. nasuta has been studied by autoradiography of squash preparations of salivary glands following a 10 min in vitro pulse of ³H-thymidine. A reference photomap of polytene chromosomes of D. nasuta is also presented As described earlier in D. kikkawai [Roy and Lakhotia, Indian J exp Biol, 17 (I 979) 231], in D. nasuta also 3 Categories of early-S ³H-thymidine labelling patterns (low, mid, and heavy, interband type) are seen in high frequency in late larval stages but not in early prepupae when the conventional discontinuous (2D and I D) labelling patterns are predominant. In some of the low interband type labelled nuclei, only one puff site, 48A on 2R, is seen to be labelled and this particular puff also remains labelled with a much higher silver grain density in comparison with 4 other puff sites on 2R, in all labelling patterns. Most of the other puffs are unlabelled in late-S patterns. It is suggested that in polytene nuclei of late third instar larvae of D. nasuta, a polytene S-period is initiated by DNA synthesis only at the 48A puff, the other disperse regions (puffs and interbands) start replicating later in a sequential manner followed by initiation of replication of band regions, The 48A puff seems to have an unusually extended period of DNA synthesis and shows striking similarity with the E-11E puff of D. kikkawai (Roy and Lakhotia, 1979). The patterns of replication of X-.chromosome in male and female polytene nuclei of D. nasuta in the early stages of polytene-S period have also been examined. The observations suggest that in D. nasuta the replication of different sites on male X is initiated slightly later than some sites on autosornes or on female X's. However after this initial delay the male X becomes faster replicating from the heavy interband labelling stage onwards and as in other species, completes its replication cycle in late-S (ID type pattern) before many autosomal or female X chromosomal sites.