Fig. 2. P4H1-null Dictyostelium cells fail to culminate and form spores. (A) A normal strain containing randomly integrated P4H1 disruption DNA (bsr⁺), strain P4H1a^- (HW288) and a pgtA^- strain (HW260) were starved on non-nutrient agar under standard overhead light in ambient O₂, and photographed after 19 and 23 hours. Normal cells culminate by 19 hours, but neither P4H1a^- nor an independent P4H1^- strain (HW289, not shown) culminated by 96 hours. Similar results were obtained in 24/30 trials (see text and panels C and E). (B) Culmination in different strains under ambient conditions was quantified by counting detergent-resistant spores. Spore numbers correlated with morphological appearance. Similar results were obtained in three or more trials, including P4H1^- strains HW288 and HW289. (C) Effect of NH₄Cl in MES buffer on strain Ax3 (normal) and P4H1a^- in a trial under ambient conditions in which P4H1a^- exhibited partial culmination breakthrough. (D) Effect of incubation in 40% O₂. Similar effects were observed in ten independent trials. (E) Interplay of the effects of O₂ and light on culmination, in Ax3 and P4H1a^- cells, based on percent spore numbers relative to values at 21% O₂, which were each within 20% of each other. P4H1a^- required a lower of O₂ than ambient (21%) to culminate in this trial, but nevertheless required 6% more than the parental strain Ax3.