The anterior expression of tsl is confine to border cells. These are specialized follicle cells that exhibit behaviors quite different from the vast majority of follicle cells. Instead of forming a cell layer around the oocyte as do other follicle cells, border cells remain at the very anterior tip of the egg chamber. These border cells will later migrate to the oocyte-nurse cell boundary, where they associate with other cells to form a hollow cone that allows sperm cell passage. The special circumstance that results in tsl expression by posterior follicle cells has not yet been characterized.
New information creates new complexities and contradictions to await possible explanation.
A second potential ligand of Torso has recently been characterized. It is not well understood how this signal is generated, although it is known to depend on the activity of many genes such as torso-like and trunk . To further investigate the mechanism involved in the local activation of the Tor receptor, the normal expression of the Tsl protein was altered by generating females in which the tsl gene is expressed in the oocyte under the control of the tor promoter, rather than in the ovarian follicle cells. Analysis of the phenotypes generated by this hybrid gene and its interactions with mutations in other genes in the pathway has enabled dissection of the mechanism of Tor receptor activation and a more precise definition of the role of the different genes acting in this process. Uniform expression of tsl in the oocyte is able to trigger unrestricted activation of the Tor receptor.