Parameters of codon-partitioned substitution rates, demographic functions, tMRCA and tree topologies were co-estimated from HA and NA gene datasets of reassortant and group 2 viruses separately in a BMCMC framework [18] using BEAST version 1.4.6 [65] . Substitution model HKY+C 4 with invariable site portion was used. Isolation dates were used to calibrate the molecular clock. Three clock models including strict clock, UCEN and UCLN relaxed clocks [21] were attempted independently, and the best-fit clock model was selected by comparing the BF calculated from their posterior distributions [20] . The Bayesian skyline plot [23] was used to estimate population dynamics, in terms of relative genetic diversity. Less complex parametric demographic models (constant size, exponential growth and logistic growth) were applied independently, and the best-fit models selected by BF tests were used to quantitatively estimate the growth rate and other demographic parameters. The BMCMC analyses contained 2610 8 states, with sampling every 1,000 states, and the first 10% of each chain was discarded as burn-in. Convergences and effective sample sizes of the estimates were checked using Tracer v1.4 [66] .