An overview of host cell selection.

A virus with a new phenotype can arise from an apparently homogeneous population after a single passage by the application of a strong selection pressure. With less pressure, a variant population may arise gradually during repeated passage of the virus. Strong host selection has been observed when naturally occurring influenza virus is grown in embryonated hens' eggs whereas the natural virus grows apparently unrestricted in MDCK cells. However, repeated passage in MDCK cells can select for a virus more suited to replication in these cells and variants which resemble those selected in eggs have been observed in such passaged virus.