Figure 6.

Time-lapse fluorescence video microscopy of wild-type and cnm67Δ1 conjugations. Nuclei of haploid cells were labeled with a strongly fluorescent histone H4-GFPp (Hhf2-GFPp). (A) A nuclear fusion event of wild-type cells is followed by the first mitotic division of the zygote nucleus. Both daughter nuclei are correctly segregated between the zygote and the bud. Nuclear division is then followed by cytokinesis. (B) Conjugation of cnm67Δ1 cells. Nuclear migration (congression) before nuclear fusion and nuclear fusion occur as in wild-type cells in the central region of the zygote and also with about the same dynamics. The subsequent mitotic division, however, fails to segregate one of the daughter nuclei into the zygote bud. Presumably, congression is not affected because it is mediated by half-bridge–organized microtubules as in wild-type, whereas the first mitotic nuclear migration is impaired by the outer plaque defect of cnm67Δ1 cells. Cells that surrounded mating cells in panels A and B were removed electronically for clarity. Bars, 10 μm.