Role of the male specific lethal (msl) genes in modifying the effects of sex chromosomal dosage in Drosophila

Immunostaining of chromosomes shows that the male-specific lethal (MSL) pro teins are associated with all female chromosomes at a low level but are seq uestered to the X chromosome in males. Histone-4 Lys-16 acetylation follows a similar pattern in normal males and females, being higher on the X and l ower on the autosomes in males than in females. However, the staining patte rn of acetylation and the mof gene product, a putative histone acetylase, i n msl mutant males returns to a uniform genome-wide distribution as found i n females. Gene expression on the autosomes correlates with the level of hi stone-4 acetylation. With minor exceptions, the expression levels of X-link ed genes are maintained with either an increase or decrease of acetylation, suggesting that the MSL complex renders gene activity unresponsive to H4Ly s16 acetylation. Evidence was also found for the presence of nucleation sit es for association of the MSL proteins with the X chromosome rather than in dividual gene binding sequences. We suggest that sequestration of the MSL p roteins occurs in males to nullify on the autosomes and maintain on the X, an inverse effect produced by negatively acting dosage-dependent regulatory genes as a consequence of the evolution of the X/Y sex chromosomal system.