Figure 1. Threshold Response Model for Sex Determination.

Sxl is the direct target of the signals that control sex determination and is activated in response to a threshold concentration of X-linked gene products (in purple). The threshold level of X-linked gene products necessary for transcriptional activation of Sxl via the SxlPe promoter (indicated by the dotted line)—if it is to occur—must be accomplished by cellular blastoderm, because this developmental milestone is accompanied by a rapid degradation of XSE mRNAs and proteins. In diploids, this level is only reached in XX embryos. In XY embryos, the gene products produced from the single X chromosome do not reach the necessary threshold level, and Sxl remains off. In haploids, the cellular blastoderm stage is substantially delayed. Because of this delay, the XSE products produced from the single X chromosome are able to accumulate over a longer period of time, reaching the necessary threshold level for SxlPe activation.