Figure 1.

Spontaneous fusion produces proliferating hybrids. (A) IMR90 cells (I0) resistant to puromycin (I0^P) or I0 cells transformed with E1A and resistant to hygromycin (IE^H) were cultured as indicated and stained with crystal violet to visualize the cells. (B) Co-culture of I0^P and IE^H cells results in cells resistant to both drugs. Cells were plated together, treated with PEG or left untreated, cultured for 20 d, and visualized as in A. (C and D) IE^H cells fuse to each other but not to themselves. I0^P and IE^H cells were dyed and cultured as indicated for 16 h and visualized by fluorescence microscopy. Heterokaryons (indicated by arrows in C and scored in D) contained both green and blue dyes and at least one green and one blue nucleus. (E) Tissue culture medium from IE^H cells induces cell fusion. Tissue culture medium conditioned by either I0^P or IE^H cells for 16 h was tested in the fusion assay (see Materials and methods). All experiments were performed at least three times. The data in D and E are from three independent experiments, and the error bars indicate SD.