New methods for deep sequence analysis provide an opportunity to follow the emergence and dynamics of virus mutations in real time. Although viruses are commonly grown in cell culture 30 for research and for vaccine development, the cells used to grow the virus are often not derived from the same tissue or even the same host that the virus naturally replicates in. The selective pressures of culturing virus in vitro are still only partially understood. MDCK cells are the standard cell for growing influenza viruses yet are derived from the epithelium of the canine kidney and are also heterogenous. We passaged human H3N2, H1N1 pandemic, and canine However, the selection pressures that cell passaging imposes on viruses are often poorly 3 understood. We used deep sequence analysis to define, in detail, how three different influenza A viruses respond to passaging in different lineages of canine MDCK cells that are commonly used for their growth, as well as in variant cells engineered to express different forms of their cell surface receptor, sialic acid. This analysis revealed that most mutations occur in the HA 55 gene and few sequence changes in the virus population reached high proportions. This is relevant for understanding the selective pressures of virus growth in cell culture and how it shapes evolutionary patterns. 148 words