Chromosomal basis of dosage compensation in Drosophila. VII. DNA replication patterns of the puffs in the male and female larval polytene chromosomes

Abstract

A detailed analysis of the ³H-thymidine labelling pattern for a large fraction of the X (12A–20D) and a short section of the 4th (90C–94C) chromosome of Drosophila hydei with special emphasis on the 18 X-chromosomal puffs and 14 autosomal puffs is presented. Attempts to arrange the different labelling patterns into different ordered sequences lead to only one optimal sequence that fits with a continuous-to-discontinuous sequence of labelling, both at the level of whole chromosome sections as well as at the level of the 18 X-chromosomal and 14 autosomal puffs. This finding inplies that all genetically active sites start replication simultaneously and that each active locus may complete its replication according to the amount of DNA and/or the degree of puffing activity. The data suggest that the individual units behave autonomously. On the basis of the labelling frequency of the different non-puffed segments and puffing sites of the X and the 4th, it has been shown that (a) puff sites in most cases take a shorter time than non-puffed bands, and (b) the puff sites on the female X's require a longer period of replication as compared to the same puffs on the male X. These observations suggest that the asynchronous replication of the male X may be causally related to its hyperactivity. The data provide further evidence in support of piecemeal regulation of dosage compensation in Drosophila and reinstates the hyperactive male X model of dosage compensation.