Figure 1 Cell diameter before and after cell division and trivision. HaCaT, uveal melanoma (UM) and HeLa cells were subjected to time-lapse image microscopy. The diameter of mitotic cells was measured after their detachment and rounding up. Abbreviations: BD, before division; BT, before trivision; AD, after division; AT, after trivision; AT1, AT2, AT3, trivided daughter cells. In the three series of experiments (HaCaT, UM, HeLa) each set was tested relative to its own appropriate BD and AD control. Time-lapse microscopy: n = 10 for HaCaT and uveal melanoma (20 divided and 30 trivided cells), n = 5 for HeLa (10 divided and 15 trivided) cells. Statistical analysis: * = p < 0.05, ** = p < 0.01.

Figure 2 Cell fusion and first cell trivision of fused HeLa cell. Photographs of growing cells were taken every min by our custom-built video camera attached to the microscope and connected to the computer. A-H: Cells (f1 and f2) before fusion. I-L: Fusion of f1 and f2 cells. M-N: fused cell indicated by number 1. O-P: Trivision of cell #1 to 1a, 1b, 1c cells. As orientation black arrows point to the cells undergoing fusion, to the fused cell and to the trivided cells. Black number at the bottom of each frame shows the time of photography. Scale bar: 50 µm for each frame. Exposures were converted to videofilm by speeding up the projection to 25 exposures/s

Figure 3Time-lapse photography of fused HeLa cell undergoing second trivision. The same montage of time-lapse microscopy is presented an in Figure. 2. A-D) First trivision. E-I) Growth of trivided daughter cells. I-J) Fusion of the two smaller (1b, 1c) trivided cells. K-L) Growth of fused 1bc cell. M-O) Trivision of the fused 1bc cells. P) Growth of the tree daughter cells of the second trivision. Black arrows show the position of the fusion and the second trivision of the fused cell. Black number at the bottom of each frame indicates the time of photography. Scale bar: 50 µm, each frame.

Figure 4Time-lapse photography of HeLa cell undergoing trivision followed by division of its large daughter cell. A-F) Trivision (1a, 1b, 1c). G-P) Division of the larger daughter cell (1a) of trivision. Labels are the same as in Figure. 1. Scale bar: 50µm.

Figure 5 Schematic view of cell fusion and multipolar cell trivision in HeLa cells. Two cells in the cell culture (at least one of them hypodiploid) (a), attach (b), merge (c) and undergo complete cell fusion (< 4N) (d). Fusion is followed by growth (< 8N), first cell trivision, forming multipolar cells, one large (4N) and two small cells (2N and < 2N) (e). Large cell (4N) undergoes normal cell division (2N + 2N). The fusion of the two smaller cells leads to second trivision (f).

	HaCaT	UM	HeLa
Division (min)	11.6 ± 1.44 (n=10)	31.9 ± 5.50 (n=10)	22.3 ± 5.36 (n=5)
Trivision (min)	45.5 ± 1.0 (n=10, ** = p < 0.01)	43.9 ± 13.46 (n=10, p > 0.05)	42.8 ± 6.24 (n=5, ** = p < 0.01)

Table 1: Duration of trivisions relative to divisionsIn the three series of experiments (HaCaT, UM, HeLa) each set of trivision time was tested relative to its own appropriate division control. Time-lapse microscopy: n = 10 for HaCaT and uveal melanoma (20 divided and 30 trivided cells), n=5 for HeLa (10 divided and 15 trivided) cells. Statistical analysis: * = p < 0.05, ** = p < 0.01.