Dosage compensation of X-linked heat-shock puffs in Drosophila pseudoobscura

Four major puffs are inducible by heat shock in the larval salivary gland chromosomes of D. pseudoobscura. Two of these puffs are present at 23 and 39-40 on the right arm of the X chromosome and two are present at 53 and 58 on chromosome 2. By means of in situ hybridization, residual homologies were demonstrated between the puffs at 23 in D. pseudoobscura and at 63 C in D. melanogaster, and between the two chromosome 2 puffs of D. pseudoobscura and 87A and 87C of D. melanogaster. RNA synthesis was monitored as a function of 3H-uridine incorporation in the major heat-induced puffs of D. pseudoobscura and was found to be equivalent in males and females indicating dosage compensation of the two X-linked loci. The evoluation of the regulatory controls of these genes is discussed.