Results
The most supported molecular clock model was the lognormal relaxed
molecular clock, with a Bayes factor with the next most supported model
(strict molecular clock) equal to 225, therefore we used this molecular
clock prior in all subsequent analyses. Under the SABDSKY model, most
parameter estimates were very similar, so we report them jointly unless
otherwise specified.
The mutation rate posterior median was 4.46 X 10-3 per
year (95% HPD: 3.72 X 10-3 - 5.36 X
10-3 per year), with a coefficient of variance of 0.5
(95% HPD: 0.28-0.9). The root of the tree was estimated to be in
1995.36 (95% HPD: 1995.09 – 1995.58) and the origin in 1995.23 (95%
HPD: 1994.92 – 1995.52). The sequence KT006745 (sampled in South
Australia in 2009) had a sampled ancestor posterior probability of
38.3% and it was always paired, when sampled ancestor, with the
sequence KF494930 (sampled in South Australia in 2010).
Across the four time intervals after 1995, the median estimatedR e had a slightly increasing trend until 2010
followed by a dramatic decline (Figure 1). However, based on the 95%
Highest Probability Density (HPD) intervals, no significant difference
was detected across the first three epochs in theR e estimate. The range of values forR e in the first five years after the release of
the virus was very low (0.76 – 1.4), successively increasing to 0.9 –
1.4 and then declining to below 1 after 2010 (0.2 – 0.9) (Figure 1).
The range of values for δ was 0.9 – 5.6 (median=2.6) and the sampling
proportion parameter was estimated to be 0.008 (0.0005 – 0.0277) in the
SABDSKY model with a common sampling proportion parameter across all the
epochs (after 1995), while δ was narrower (0.6 – 2.2, median=1.3) and
the sampling proportion showed a declining trend (Figure 2) in the
SABDSKY model with a variable sampling proportion parameter. Results
from the BDSKY analyses were similar to SABDSKY although with wider 95%
HPD (Figure 1), with the exception that δ estimate was much smaller
(0.5, 0.17 – 1.8) and the sampling proportion parameter had a much
wider 95% HPD (0.02 – 0.9) in the BDSKY model with a common sampling
proportion parameter across all the epochs as well as in the BDSKY model
with a variable sampling proportion parameter (Figure 2).
The BSP analysis, correctly detected the initial dramatic increase in
the virus population size at the
beginning of the initial outbreak (Figure 3), despite the paucity of
sequence data in the first few years since the release of the virus
(Figure S1). The analysis suggests a gradual increase in the virus
population size until 2007 and another substantial increase between 2007
and 2009. The virus population size then stabilised in the last five
years or so of the analysis. The results of the skyride analysis were
similar, and, although they had larger HPD intervals and the second
increase in the population size between 2007 and 2009 was not as evident
(Figure S2), these are not discussed further.
The analysis with the BDMM did not converge. We further attempted to
reduce the time interval for the estimation of R eand fixed δ across the different epochs to reduce the number of
parameters to be estimated with no success. The estimation of the
sampling proportion and type change events proved particularly difficult
and, even with runs in excess of 1.6 X 109 iterations
the latter parameter failed to converge.