Groups of twelve guinea pigs were inoculated with 10 PFU of PR8-based viruses carrying avian M, NL09 M, chimeric NL09 M 7mut M, or chimeric avian M 9mut segments, as indicated. (A) Virus replication in nasal washings was measured by plaque titration at days 2, 4, 6, and 8 post-infection and the total area under the curve was calculated. The replication differences between viruses with NL09 M and NL09 M 7mutAv segments were not significant (P = 0.720). Differences between viruses encoding the NL09 M 7mut segment and the avian M 9mut segment trended towards, but were not statistically significant (P = 0.115). (B) Transmission to naïve cage mates that were exposed at 24 hpi. Neither chimeric M segment conferred efficient transmission (7/9 transmissions; 78%). Statistical significance of replication differences was determined using a two-tailed Student’s t-test. Statistical significance of transmission differences was determined using an unpaired two-tailed Student’s t-test. NL M and avian M data are identical to data presented in Fig 3 and are reproduced here for clarity.