Discussion
Since TMUV was first reported in 2010, it had widely spread in China (Liu et al., 2013; Z. Xie et al., 2014; Yan et al., 2017; Yun et al., 2012; X. Zhou et al., 2016; Zhu et al., 2012). Many studies have focused on the mechanism of virus action and the establishment of detection methods (Bai et al., 2015; L. Chen et al., 2019; S. Chen et al., 2018; Cui et al., 2018; Fu, Ji, Liu, Dafallah, & Zhu, 2015; Hu et al., 2020; Jiang et al., 2012; Wu et al., 2016). In this study, we provide new insights into the origin, genetic diversity, and codon usage of TMUV. Compared with the previous studies, we provide better evidence for the origin, genotype diversity, and codon usage bias of TMUV (Dai et al., 2015; H. Zhou et al., 2015). We found that mosquito TMUV, which was isolated from 2015 to 2019, shared the most recent common ancestor with avian TMUV. Furthermore, we found a new potential clade that was isolated in the Shandong province; this new clade also showed high homology with the mosquito TMUV. Overall, the mosquito TMUV is the most likely origin of avian TMUV based on the currently available sequences. Mosquitoes are considered the main transmission hosts ofFlaviviridae . TMUV host jumps pose a major threat to the duck industry and public safety. According to previous research, TMUV virus has also been detected in humans (Cao et al., 2011).
Previously, Tao et al. used ML trees to type TMUV (Dai et al., 2015). In this study, we explored the phylogenetic history of TMUV isolated in China from 2010 to 2019. To identify the genotype of TMUV, three different methods (NJ, ML, and MCC) were used. All three methods showed that Chinese TMUV can be divided into three different clades (TMUV 1, TMUV 2, and TMUV 3). Additionally, within the TMUV 1 clade, 3 stable subclades were identified (TMUV 1a, TMUV 1b, and TMUV 1c), and in the TMUV 2 clade, 2 stable subclades were identified (TMUV 2a and TMUV 2b). Compared with the previous study (Dai et al., 2015; Ninvilai, Tunterak, Oraveerakul, Amonsin, & Thontiravong, 2019), the TMUV in the current study showed more genetic diversity. Nonetheless, in recent years, the rate of emergence of new genotypes has slowed, which is also supported by the Bayesian skyline plot analysis.
The codon usage bias can be used to understand the changes in codon usage and its influencing factors during the evolution of TMUV. Gene mutation pressure, secondary protein structure, natural selection pressure, and external environment were found to be the main factors causing codon bias (Bera et al., 2017; Hershberg & Petrov, 2008). In this study, ENC-plot analysis and neutrality plot analysis showed that mutation pressure played a decisive role in codon usage bias, which is similar to the results of previous research (H. Zhou et al., 2015). Notably, the preferred codons consisted of the optimal codons for human cells, which indicates that TMUV has the potential to infect humans after cross-species transmission; this is also supported by previous studies that have detected TMUV in humans (Tang et al., 2013; H. Zhou et al., 2015).
Cross-species transmission of viruses is a serious threat to public safety. There are already many such cases that have adversely affected public health, including SARS, MERS, SARS-CoV-2, and Zika virus infections (Dye & Gay, 2003; Lazear, Stringer, & de Silva, 2016; Verdoni et al., 2020; Zumla, Alagaili, Cotten, & Azhar, 2016). TMUV has the potential to spread across species and cause great damage. With the rapidly increasing population and increasing constant contact of humans with nature, there is an increasing possibility of an interaction between different hosts (avian to mosquito to human). Therefore, it is likely not a matter of if but when TMUV will cause a major infection outbreak in the human population.
In summary, we collected the sequences of TMUV that were reported in China over the past 10 years. We assigned the possible mosquito origin of TMUV and divided the TMUV into three clades. Furthermore, we raised the possibility that in future TMUV may infect people and cause serious consequences. Future research should focus on the following aspects: recombinant TMUV in animals, cross-species transmission of TMUV within the animal, and finally, establishing a detection network to prevent the sudden emergence of TMUV with high virulence that can infect humans. Overall, our study characterized TMUV spread in China and provided insights that may aid in future prevention and control of the virus spread.