Replication in Drosophila chromosomes. Part II. Unusual replicative behavior of two puff sites in polytene nuclei of Drosophila kikkawai

Abstract

Replication in polytene nuclei of late 3rd instar larvae of D. kikkawai has been studied by autoradiography of squash preparadons of salivary glands following a 10 min in vitro pulse of ³H-thymidine. In addition to the usual continuous (all sites labelled) and discontinuous (condensed bands and chromocentre mainly labelled) types of autoradIographic labelling patterns, a variety of interband (IB) types of labelling patterns have been seen with a relatively high frequency. The disperse regions like puffs and interbands are chiefly labelled in inter band type of labelled patterns. Depending on the number and degree of labelling of these disperse regions, 3 major categories of IB patterns, viz. low (LIB), mid (MIB) and heavy (HIB) interband types have been recognized in D. kikkawai. The replication of 2 puff sites, 11E and 3C on arms E and C, respectively, of the reference photomap presented here, shows unusual features since these 2 sites were labelled in all kinds of labelling patterns. It is suggested that in polytene nuclei of late 3rd instar larvae of D. kikkawai the Initiation of replication at different early replicating sites (mainly disperse regions) of a nucleus is greatly asynchronous and some of the initiating sites (e.g. puffs E-11E and C-3C) continue to replicate till later Part of the 'S' period.