These genes generally have cell-autonomous activities and, consequently, are readily adaptable to a process that requires counting the number of chromosomes in each nucleus. Moreover, the apparent ligand, upd , corresponds to the X chromosome-counting element. Since Upd is secreted, it could potentially influence the counting process not only in the nucleus that produced the protein to begin with but also in adjacent nuclei. Supporting this possibility, it has been found that upd mutant cells can generate a normal pattern when adjacent to wild-type cells. For example, the signaling cascade may respond in a nonlinear fashion to variations in the dose of the ligand.
Metazoans use diverse and rapidly evolving mechanisms to determine sex.
The involvement of os in sex determination confirms that secreted ligands can function in cell-autonomous processes. This prediction was confirmed. Because only maternal rather than zygotic hop and Stat92E are likely to be relevant at the very early embryonic stage when Sxl Pe is activated, the maternal contribution of these two genes was eliminated by inducing homozygous mutant germline clones in mothers heterozygous for null alleles. The reduction was generally more uniform across the embryos than in the os experiment. Similar results were seen for Stat92E .