Inducing conjugation and forming diploid zygotes. Diploids then undergo meiosis and sporulation, producing four haploid spores that, under adequate conditions, would germinate finishing the mating cycle. Most of these processes are controlled by signaling systems that detect nutritional changes in the environment and trigger the transition mitosis-meiosis through the conjugation/meiosis pathway. Thus, sexual differentiation in S. pombe is regulated by several signaling pathways, like the cAMP pathway, the MAPK pheromone signaling pathway, the TOR pathway, and the MAPK stress-responsive Sty1/Spc1 pathway. PKA negatively regulates mating, while the MAPKs Spk1 and Spc1/Sty1 positively regulate the mating process. TOR kinases, Tor1 and Tor2, exert positive and negative effects on mating, respectively. One of the mechanisms of Ste11 regulation is through the activity of the Spc1/Sty1 MAPK pathway. Upon nitrogen starvation, Atf1, a transcription factor regulated by Spc1/Sty1, activates Ste11 transcription and, therefore, mating capacity. Cells deficient in Spc1/Sty1 or Atf1 are not capable of arresting cell cycle in G1 upon nitrogen starvation and are, therefore, sterile. atf1+ mRNA levels, under certain stress conditions, like hydrogen peroxide treatment, are regulated by the activity of Csx1, an RNA binding protein with 3 RNA recognition motifs. Csx1 phosphorylation depends on Spc1/Sty1 activity, although the functional role of this phosphorylation remains unclear. We have noticed that Csx1-deficient cells may also have defects in mating. During the construction of strains containing different Csx1 alleles, we noticed that the mating efficiency of heterothallic Csx1 deficient strains was lower than wild type strains, indicating a possible role of Csx1 in sexual differentiation in fission yeast. To analyze the ability to mate of cells lacking Csx1, we observed sporulation in homothallic strains, h90 wild type and h90 csx1D. Both strains were plated in mating-inducing conditions and incubated at 24uC for two days. In these experiments, we did not observe any morphological difference between tetrads formed in csx1D and wild type strains. However, the number of zygotes and tetrads in cells lacking Csx1 appeared to be much lower than in wild type. To quantify mating efficiency we inoculated these strains in ME media. After 48 hours incubation at 24uC, the number of vegetative cells, zygotes and tetrads was measured, and the mating and sporulation ratio determined. As it is shown in Figure 1C, about 45�C55% of wild type cells mated in these conditions, while in csx1D cells, this ratio ranged between 4�C8%.