Sex Determination and Threshold Responses in Development

Alternative developmental fates are often determined by small differences in the concentrations of signaling molecules. In many cases, cells respond to these signals within narrowly defined temporal windows and are unresponsive to the same signal molecules at other times in development. A number of aspects of Drosophila sex determination make it an ideal experimental system to study how strict temporal controls and small quantitative differences in protein concentration can elicit different developmental fates.

Sex is determined in Drosophila by the number of X chromosomes, with one X specifying male development and two specifying female. The dose of X chromosomes controls sex determination through its effects on the establishment promoter, SxlPe, of the regulatory gene Sex-lethal. Female development occurs as a consequence of Sxl being turned on in diplo-X animals while male development occurs in haplo-X animals because Sxl is left inactive. Although Sxl protein is required at all times to direct female differentiation, X chromosome dose affects Sxl expres ion only during a 30-40 min period in the pre-cellular embryo. After this time, Pe shuts off and Sxlis transcribed from a maintenance promoter, Pm, that operates in both sexes.

Genetic experiments have identified five elements on the X chromosome whose relative dose (one vs. two) is used to determine sex. These include the genes sisterlessA and -B, -C, runt, as well as Sxl itself. The sisA sisB and runt genes encode transcriptional activators of the bZIP, bHLH, and runt/AML class. The dose of these “counted” elements is measured with respect to a number of maternal and zygotically expressed proteins, some of which function as activators and some as inhibitors. We are studying the molecular interactions between the positively acting and inhibitory protein factors and their SxlPe promoter target. Our approach combines biochemistry with classical and molecular genetic analyses to identify novel molecules, and to characterize the protein/protein and protein/DNA interactions that regulate SxlPe. Given the ability to identify the key regulatory molecules, to study their expression, and to manipulate their levels and activity, in vitro, and in vivo; studies on Drosophila sex determination should prove ideal for understanding how transcriptional regulators of different classes can cooperate to generate sharp threshold responses.