Factors that control focus morphology in cell cultures infected with Bryan strain Rous sarcoma virus (B-RSV) were examined. Cloning experiments with the roundc-cell (morph) and fusiform cell (morph) variants of RSV (Temin, 1960), confirmed that the two mutant viruses breed true and that these two distinct focus types were under the control of the virus genome.

Three subvariants of the morph focus type were described which failed to breed true in cloning experiments. It was concluded that this type of focus variation was not due to virus mutation.

Variation in focus morphology was observed when RSV was plated on chick embryo fibroblast cell lines from different sources, as reflected in the relative frequency of expression of the three morph focus types. These variations were exhibited between fibroblast cell lines of different genetic background, indicating that the cell genome had an influence on focus expression. Focus expression also varied in fibroblast cell lines from chickens of different ages. The strongest host influence on focus expression was observed with fibroblast cell lines derived from selected individual tissue of single embryos. The latter two factors (age and tissue origin) suggested that the differentiated state of the cells also influences RSV focus morphology.


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