Could you please explain the figures? Slow Rates of WNV Evolution during Multipl

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Could you please explain the figures?

Slow Rates of WNV Evolution during Multiple Rounds of
Transmission
To enhance our predictions about virus evolution during
transmission, we extended our model through two rounds of
mosquito-to-bird transmission (Figure 5). The accumulation of
0.5 mutations per WNV transmission cycle again highlights
the slow rates of arbovirus evolution. Furthermore, between
each round of transmission, the frequency of synonymous mutations
greatly fluctuated (Figure 5B), and most of the nonsynonymous
mutations were removed (Figure 5A), indicating the strong
presence of genetic drift and purifying selection, respectively.
Others have documented that arbovirus variants generated
within the vertebrate can also be removed during transmission
to mosquitoes

Figure 5. Strong Purifying Selection and Genetic
Drift Shape West Nile Virus Populations
during Multiple Rounds of Transmission
(A) Two rounds of WNV transmission were performed
using Cx. quinquefasciatus mosquitoes 14
dpbf and chicks 2 dpi (n = 3 independent lineages).
Virus populations were sequenced from the bird
serum, but sufficient WNV could not be recovered
from all of the saliva collected from mosquitoes
involved in transmission. Genetic distance (mean
with 95% CI) was calculated from the input
bloodmeal virus and each infected bird.
(B) Frequencies of the consensus-changing variants
(n = 4, all synonymous) detected during the
two rounds of transmission were independently
tracked. Variants above the dotted line at frequency
0.5 are consensus changes.

Explanation / Answer

West Nile Virus is a viral infection typically spread by mosquitoes. Arboreal birds are the natural reservoirs of this virus and act as an amplifier host. Birds may become infected with West Nile Virus in multiple ways. The most common occurrence is through a mosquito vector. Over 40 species of mosquitoes have been found to carry West Nile Virus. The most important for the spread is the genus Culex, which mainly feeds on birds. If a mosquito feeds off an infected host with high enough viral load it will take up the virus into its gut. The virus will then multiply and spread to the rest of the mosquito’s body, including its salivary glands. The vector may then feed off another animal and transmit the virus to a new host.

Before we decipher the figure, lets first understand what synonymous and non-synonymous mutations are?

A synonymous mutation is a change in the DNA sequence that codes for amino acids in a protein sequence, but does not change the encoded amino acid. This is a silent mutation.

A nonsynonymous substitution is a nucleotide mutation that alters the amino acid sequence of a protein.

The figures depict slow rates of mutation observed in two rounds of viral transmission:

Fig A depicts at the initial stages the ratio of synonymous: non- synonymous: all mutations were observed to be the same this is during the input stage comparing this to the variants found in the input stage i.e. Fig B shows a constant rise in the synonymous mutations above the threshold of 0.5

Now the first stage of Mosquito to bird transmission showed a steady rise in the synonymous mutations along with the other types of mutation in Fig A and also on a simultaneous comparison of the same with Fig B the consensus variant was observed to be above 0.5 which is the threshold.

For the second stage of Mosquito to bird transmission the input stage was compared again and again a steady rise in the consensus variants was observed and hence it was more than 1 this time thus resulting in the peak in the Fig B corresponding to bird 2

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