X-chromosome transcription in Drosophila.

Duplications involving the X chromosome of Drosophila were used to produce individuals with 1.25, 1.50, 1.62 and 1.85 X chromosomes. Rates of transcription in polytene chromosomes were measured autoradiographically after pulse-labeling salivary glands with 3H-uridine. We conclude that: (1) the sume of all transcription occurring on the X elements is constant (relative to autosomal transcription) regardless of how much X-chromosome material is present; (2) this constancy is apparently achieved through a uniform reduction of the rate of synthesis over all X-chromosomal segments as the size of the duplication increases.--Thus, the transcription of a given segment of the X is dependent not just on the number of copies of that segment but also on the number of copies of other regions of the chromosome.