The segmentation generuntis needed to activateSex-lethal, a gene that controls sex determination and dosage compensation inDrosophila

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Abstract
Summary: InDrosophila, sex is determined by the relative number of X chromosomes to autosomal sets (X: A ratio). The amount of products from several X-linked genes, calledsisterlesselements, is used to indicate toSex-lethalthe relative number of X chromosomes present in the cell. In response to the X: A signal,Sex-lethalis activated in females but remains inactive in males, being responsible for the control of both sex determination and dosage compensation. Here we find that the X-linked segmentation generuntplays a role in this process. Reduced function of runt results in femalespecific lethality and sexual transformation of XX animals that are heterozygous forSxlorsisloss-of-function mutations. These interactions are suppressed bySxlMI, a mutation that constitutively expresses femaleSex-lethalfunctions, and occur at the time when the X: A signal determinesSex-lethalactivity. Moreover, the presence of a loss-of-functionruntmutation masculinizes triploid intersexes. On the other hand,runtduplications cause a reduction in male viability by ectopic activation ofSex-lethal. We conclude thatruntis needed for the initial step ofSex-lethalactivation, but does not have a major role as an X-counting element.