Sex-specific heterochromatin: How does chromatin become male? Manasi Apte, Victoria Meller. Dept. of Biological Sciences, Wayne State University, Detroit, MI.

   Almost ~30% of the Drosophila genome is heterochromatic. Although relatively gene-poor, heterochromatic regions of Drosophila contain over 500 predicted genes. While heterochromatin is generally not considered to display sexual dimorphism, we have observed male-specific heterochromatic gene regulation. Interestingly, roX RNAs, critical components of the Male Specific Lethal (MSL) complex involved in dosage compensation, are required for full expression of heterochromatic genes at the autosomes. The MSL complex modifies X-chromatin to equalize the ratio of gene expression between the sex chromosomes and autosomes. We observed that the sex-specific role of roX RNAs in dosage compensation is genetically distinguishable from their role in regulating heterochromatic genes. Loss of roX RNAs results in down-regulation of the autosomal heterochromatic genes in males but not in females. Heterochromatic insertions that display position effect variegation (PEV) show de-repression in roX mutant males but not in females. We hypothesize that heterochromatin is different in male and female flies. These differences are expected to be under genetic control. To test genes in the conventional somatic sex-determination pathway for a role in establishing the heterochromatic sex, we are performing a targeted screen. The screen exploits a PEV reporter that is de-repressed in males, but not in females, upon loss of roX genes. PEV is examined in XX pseudo-males created by mutation of Sex-Lethal (Sxl), transformer2 (tra2) and other members of the canonical sex determination pathway. If PEV is sensitive to the loss of roX RNA, we conclude that heterochromatin has been masculinized. Our preliminary findings suggest that neither tra2 nor Sxl regulate sex of the heterochromatin. Further studies will focus on possible role of numerator elements and chromosome pairing status as possible signals that establish heterochromatic sex.