Thymidine-3H 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-3H 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.
CHROMOSOMAL BASIS OF DOSAGE COMPENSATION IN DROSOPHILA : III. Early Completion of Replication by the Polytene X-Chromosome in Male: Further Evidence and Its Implications
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S. C. Lakhotia, A. S. Mukherjee; CHROMOSOMAL BASIS OF DOSAGE COMPENSATION IN DROSOPHILA : III. Early Completion of Replication by the Polytene X-Chromosome in Male: Further Evidence and Its Implications . J Cell Biol 1 October 1970; 47 (1): 18–33. doi: https://doi.org/10.1083/jcb.47.1.18
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