title: Chromosomal basis of dosage compensation in Drosophila. 3. Early completion of replication by the polytene X-chromosome in male: further evidence and its implications.
abstract: Thymidine-(3)H labeling patterns on the X (section 1 A to 12 E of Bridges' map) and 2 R (section 56 F to 60 F of Bridges' map) segments in the salivary gland chromosomes of Drosophila melanogaster have been analyzed in male and female separately. The observed patterns fit, with a few exceptions, in a continuous to discontinuous labeling sequence. In nuclei with similar labeling patterns on the 2R segment in both sexes, the number of labeled sites on the X in male is always less than in female X's. The labeling frequency of the different sites on the male X is considerably lower than those on the female X's, while the sites on the 2R segment have very similar frequency in the two sexes. The rate of thymidine-(3)H incorporation (as judged by visual grain counting) is relatively higher in male X than in female X's. It is concluded that the model sequence of replication in polytene chromosomes follows a continuous to discontinuous labeling sequence, and that the single X in male completes its replication earlier than either the autosomes in male or the X's in female. This asynchronous and faster rate of replication by the polytene X-chromosome in male substantiates the hypothesis of hyperactivity of the single X in male as the chromosomal basis of dosage compensation in Drosophila.
mesh_terms: D000818:Animals; D001345:Autoradiography; D002467:Cell Nucleus / Q000378:metabolism; D002874:Chromosome Mapping; D003582:Cytogenetics; D004247:DNA / Q000096:biosynthesis; D004261:DNA Replication; D004330:Drosophila / Q000166:cytology*; D005260:Female; D006651:Histocytochemistry; D007814:Larva / Q000166:cytology; D008297:Male; D012469:Salivary Glands / Q000166:cytology; D012730:Sex Chromosomes / Q000378:metabolism*; D013936:Thymidine / Q000378:metabolism; D014316:Tritium
chemical_list: D014316:Tritium; D004247:DNA; D013936:Thymidine