Dosage compensation and intercalary heterochromatin in X chromosomes of Drosophila melanogaster

Regions of intercalary heterochromatin (IH) are dispersed in the euchromatic arms of polytene chromosomes and share the main properties of heterochromatin, namely chromosome constrictions resulting from DNA underreplication. These constrictions are frequent on the paired X chromosomes of females, but are practically absent from the single X chromosome of males. These sex-specific differences have been proposed to reflect the different levels of transcription and chromosome compaction due to dosage compensation, which in turn may affect the degree of underreplication in IH regions. To test this hypothesis, we induced dosage compensation in females by ectopic expression of MSL-2 protein. We then measured the extent of underreplication in IH regions by determining frequencies of constrictions, or by Southern blot analysis using a fragment of the ten(a) gene which is located in IH region 11A6-9. Females transheterozygous for Sxl(fhv1)/Sxl(f1) or carrying a constitutive msl-2 transgene are known to hypertranscribe their X chromosomes. In such females, both the frequency of constrictions and DNA underreplication were reduced. Suppression of underreplication occurs only when a complete functional MSL complex assembles on the X chromosomes. We also used three strains that carried constitutive transgenes of msl-2 with mutations in the 5' untranslated regions. These strains produced normal levels of SXL protein, but variable levels of MSL-2 protein. The SXL protein did not prevent the formation of an MSL complex in these transgenic females. We found that the extent of underreplication of tena DNA in IH region 11A6-9 negatively correlates with the amount of MSL complex.