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The frequency of spontaneous transformation, defined as the acquisition of the ability to form colonies in soft agar (anchorage independence) was compared in rat fibroblasts of recently established lines possessing pseudodiploid to pseudotetraploid numbers of chromosomes. Transformation frequency appeared to be independent on the ploidy level. The effect of colcemid on the transformation frequency of pseudodiploid fibroblasts was also examined. Colcemid treatment resulted in an increased frequency of transformation. In addition, most of the transformed clones, either derived from colcemid-treated cells, or derived from untreated cells, were characterized by significant changes in chromosome number. These results cannot be easily reconciled with the hypothesis that the main cause of spontaneous transformation is the appearance of recessive mutations. They rather support the idea that this conversion is the consequence of alterations in the number of certain chromosomes.

The presence of cell surface fibronectin was examined by indirect immunofluorescence in 8 groups of related rat fibroblast lines expressing an in vitro transformed phenotype. The transformed cells were selected for anchorage independent growth either after X-ray treatment (X-ray transformed cells) or from control cultures (spontaneously transformed cells). All transformed fibroblasts of the latter class showed reduced expression of fibronectin at the cell surface, whereas most of the X-ray transformed derivatives exhibited a fibronectin-positive phenotype, like the untransformed parents. Moreover, from the fibronectin-negative spontaneously transformed cells, ouabain-resistant variants were isolated, the majority of which had regained the capacity to form an extracellular matrix of fibronectin. These results emphasize the variability in the properties of transformed cells and suggest that the properties of in vitro transformed cells may depend on the cause of transformation.