Interrelationship between the states of chromosome compaction, replication and transcription patterns in the polytene-X chromosome ofDrosophila melanogaster

Abstract

We studied by autoradiography the replication and transcription patterns of polytene X chromosomes in different segmental aneuploid and hyperploid conditions in a constant diploid autosomal background. In all karyotypes, the patterns of X chromosome compaction and DNA synthesis were observed to be set strictly either at a “male-level” or a “female-level”. Addition of X-fragments to 1X progeny exceeding a certain limit (62% with proximal duplication) changed the pattern of chromatin compaction and replication of X chromosomes from male to female level, while similar additional X-fragments had no comparable effect on 2X hyperdiploids. In contrast, different levels of X-transcription per segment were observed in these aneuploids with variable X-load and in consequence, an equivalent amount of total X-transcription (relative to autosomal transcription) was visualized. These findings while favour a positive correlation between the state of chromosome compaction and replication patterns, are at odds with the idea that for an altered level of transcription, a modulation in the state of chromosome compaction and replication is an obligatory pre-requisite.