Replication in Drosophila chromosomes. Part VI. Relative heterochromatin content & autonomous endoreduplication cycles of hetero- & eu-chromatin in brain cells of Drosophila nasuta larvae

Abstract

Area of heterochromatic chromocentre in 6 hr old embryonic and in brain cells of late 3rd instar larvae of D. nasuta has been measured from Hoechst 33258 stained fluorescence preparations. The nuclear as well as the chromo-centre (cc) size varies widely in 3rd instar larval brain cells and in majority of them both these areas are considerably larger than the corresponding regions in embryonic cells. However the relative area of cc in a nucleus in larval brains is in general, lesser, although in some nuclei it is much larger, than in embryonic cells. Autoradiographic grain count data from preparations of late 3rd instar larval brains labelled chronically in vivo with ³H-thymidine (supplemented in food) from batching to late 3rd instar stage, show that the area of cc or non-chromocentre regions (ncc) is highly positively correlated with the degree of ³H-thymidine incorporation in respective areas. It is, therefore, suggested that different nuclei in larval brains have DNA content in excess of 2C/4C leves seen in embryonic cells. Since the relative cc area and the relative ³H-thymidine incorporation in cc region varies widely in different brain cells, it appears that different cells in brain of 3rd instar larvae of D. nasuta have varyingly disproportionate amount of hetero- and euchromatin. Late 3rd instar larval brains are labelled in vitro for a long period of 60 hr with ³H-thymidine and autoradiographic preparations are made after different periods (0, 24, 48 and 64 hr) of chase in 'cold' thymidine supplemented medium. In different samples, in addition to unlabelled and completely labelled nuclei, a significant proportion of interphase cells are seen in which either heterochromatin only or euchromatin only was labelled. It seems that the hetero- and euchromatin regions undergo temporally dissociated independent endore-duplication cycles in different brain cells of larvae.