Differential variations in the DNA of Drosophila melanogaster during development

Evidence is presented that during development there is active regulation of the information content of the genome of Drosophila melanogaster. This regulation involves a large fraction of the DNA sequences active in transcription. RNA/DNA hybrids formed under the conditions described were primarily with middle repetitive DNA sequences. RNA molecules transcribed from the highly repetitive centromeric DNA sequences or from the attached less repetitive "spacer' DNA (Kram et al., 1972) are not present in larval RNA. The hybrids formed were divided into (a) rRNA hybrids (formed by the two major rRNA species), and (b) non-ribosomal hybrids (formed by hnRNA and mRNA). There was a six-fold reduction in the production of non-ribosomal RNA/DNA hybrid formed with DNA of larvae collected 40 hours after hatching compared with that formed with DNA of eggs and adults. 90 hour larval DNA contained twice the proportion of these sequences present in 40 hour larval DNA. This was not due to the under-representation of the Y-chromosome in polytene tissues, nor could artifacts of extraction account for the differences. These findings are discussed in relation to the "one gene, one chromosome' concept, various anomolies in the behaviour of polytene chromosomes, intercalary heterochromatin and regulation of the content of the genome during development.