Male or Female? The Answer Depends on When You Ask

A key decision in the life of an organism is whether to be male or female. In Drosophila, each cell makes this choice independently of its neighbors such that diploid cells with one X chromosome (XY) are male and those with two chromosomes (XX) are female. In classic experiments carried out more than 80 years ago, Calvin Bridges made two important conclusions about how sex is determined in flies [1–3]. He showed that the Y chromosome is not a factor and suggested that sex is determined not simply by counting X chromosomes, but by calculating the ratio of X chromosomes to the number of sets of autosomes (known as the X:A ratio). The concept that sex is determined by a mechanism that evaluates the number of X chromosomes relative to autosomes was invoked to explain the observation that animals with two X chromosomes and three sets of autosomes (XX; AAA) develop as sexual mosaics rather than females. According to this model, animals with the same number of X chromosomes as autosome sets (ratio of 1) are female, animals with half as many X chromosomes as sets of autosomes are male (ratio of 0.5), and those with an intermediate ratio (XX; AAA; ratio of 0.67) are sexual mosaics. With the finding that haploid cells (X; A; ratio of 1) are female [4,5], the idea that sex is determined by the X:A ratio became enshrined in the literature. How is sex determined in molecular terms? In diploid cells, the choice to be male or female is made when the activity state of Sex-lethal (Sxl) is set during early embryogenesis (reviewed in [6–8]). The all or none character of Sxl activation depends on the expression levels of four X-encoded proteins: SISA, SCUTE, RUNT, and UNPAIRED. When expression levels reach a threshold, as in XX individuals, the three X-encoded transcription factors—SISA, SCUTE, and RUNT—stimulate transcription by binding the SxlPe promoter [9,10]. Activation is then reinforced through the Janus kinase pathway via Unpaired, an X-encoded secreted ligand [11,12]. Once active, the presence of the female-specific SXL RNA-binding protein directs all aspects of female development. In XY animals, the expression levels of the four X-linked gene products produced from the single X chromosome fail to reach the threshold concentration needed to activate Sxl, and male development ensues. The window of opportunity for Sxl activation is brief, ending at the cellular blastoderm stage, ~3.0 h after fertilization …