Incomplete dosage compensation in an evolving Drosophila sex chromosome.

Abstract

Cellular autoradiography was used to measure relative rates of chromosomal RNA synthesis and to examine the regulatory phenomenon of X-linked dosage compensation in D. miranda, a species containing 2 distinct, nonhomologous X chromosomes (X1 and X2). The X1 chromosome was found to be dosage-compensated, since the rate of RNA synthesis along the single X1 chromosome in males equaled that of both X1 chromosomes in females. Unlike other sex chromosomes that have been studied, the more recently evolved X2 heterochromosome exhibited regional differences in transcriptional activity when males and females were compared. The distal 10% of the X2 was not dosage-compensated, whereas the majority of an interior segment, representing 30% of the X2 chromosome''s length, was found to be dosage-compensated. The data are consistent with the idea that the evolution of X2 dosage compensation has paralleled the differentiation of the X2 sex chromosome. Gene rearrangement also seems to have accompanied the acquisition of a dosage-compensatory mechanism in the X2.