James W. Erickson
Alternative developmental fates are often determine 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 Sxl. 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 expression only in the first 1 to 2 1/2 hours of development. After this time, Pe shuts off and Sxl is transcribed from a maintenance promoter, Pm, that is insensitive to X chromosome dose.
Genetic experiments have identified at least four elements on the X chromosome whose relative dose (one vs. two) is used to determine sex. These include the gene sisterless-a and -b, the gene runt, and Sxl itself. The sisterless genes and runt 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 HLH 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 the yeast two-hybrid system and biochemical analysis to identify novel molecules, and to characterize the protein/protein and protein/DNA interactions that regulate Sxlpe. In addition, we make extensive use of the genetic and molecular tools available in Drosophila to study the effects of alterations in regulatory gene dose or promoter sequence on Sxlpe activity. 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 determi nation should prove ideal for understanding how transcriptional regulators of different classes can cooperate to generate sharp threshold responses.