Re-evaluation of the function of the male specific lethal complex in Drosophila

A set of proteins and noncoding RNAs,referred to as the male specific lethal (MSL) complex,is present on the male X chromosome in　Drosophila and has been postulated to be responsible for dosage compensation of this chromosome - the up-regulation of its expression to be　equal to that of two X chromosomes in females.This hypothesis is evaluated in view of lesser known aspects of dosage compensation such as the　fact that metafemales with three X chromosomes also have equal expression to normal females,which would require a down-regulation of each　gene copy.Moreover,when this complex is ectopically expressed in females or specifically targeted to a reporter in males,there is no increase in　expression of the genes or targets with which it is associated.These observations are not consistent with the hypothesis that the MSL complex　conditions dosage compensation.A synthesis is described that can account for these observations.