We used the Bayesian skyline plot (BSP) [23] to estimate the change of relative genetic diversity of the reassortant viruses and of the group 2 parental strain over time, as shown in Figure 3E -3H. For both the HA and NA datasets, the group 2 viruses consistently show a slow growth in relative genetic diversity over time which appears to follow a constant size or exponential growth model, whereas the reassortant viruses initially exhibited an abrupt rise in relative genetic diversity followed by stabilization, which visually resembles a logistic growth curve with two phases [24, 25] ( Figure 3E and 3F) . When the BSPs are superimposed upon the demographic results obtained under parametric growth models (i.e., constant, exponential and logistic growth; Figure S8 ), then a similar observation can also be made. However, BF tests (Table  S4) indicate there is insufficient statistical power to discriminate between the three parametric growth models (lnBF,2.99), suggesting a lack of strong demographic signal in these data. When the parametric demographic models were fitted to the data, the median estimates of growth rates for the reassortant datasets are generally higher than those estimated for the datasets of group 2 viruses (Table S1 ). However, the confidence intervals of some growth rate estimates are fairly large and overlapped among the reassortant and group 2 viral datasets.