To date, work examining host adaptation has focused largely on the adaptive mutations themselves, and has to some extent ignored the implications the adaptive mutations observed may have on virulence. In addition, the two major tools used to study host adaptation in IAVs, serial passage in animal models and targeted mutation, have previously been used largely independently. This research is innovative because it utilizes both of these approaches, and examines an aspect of IAV biology that has been poorly studied. We have previously identified a number of adaptive polymerase mutations, and this proposal expands this work to demonstrate a link between adaptive mutations and changes in virulence. Additionally, this proposal expands on the serial passage approach to IAV host adaptation by pairing it with circular RNA sequencing technology, a novel approach to deep sequencing of RNAs that allows for the detection of extremely low frequency mutants by drastically reducing noise from PCR-induced mutations (Acevedo et al., 2014).…