Evolutionary enhancement of Zika virus infectivity in Aedes aegypti mosquitoes

Zika virus (ZIKV) remained obscure until the recent explosive outbreaks in French Polynesia (2013-2014) and South America (2015-2016). Phylogenetic studies reveal that ZIKV has evolved into African and Asian lineages. The Asian lineage of ZIKV is responsible for the recent epidemics in the Americas. However, the underlying mechanisms through which ZIKV rapidly and explosively spread from Asia to the Americas are limited. We have recently shown that nonstructural protein 1 (NS1) facilitates flavivirus acquisition by mosquitoes from an infected mammalian host and subsequently enhances viral prevalence in mosquitoes. Here, we report that the antigenemia of NS1 determines ZIKV infectivity in its mosquito vector Aedes aegypti, which acquires ZIKV via a blood meal. Clinical isolates from the most recent outbreak in the Americas were much more infectious in mosquitoes than the FSS13025 strain, which was isolated in Cambodia in 2010. Further analyses showed that these epidemic strains have more robust NS1 antigenemia than the FSS13025 strain because of an alanine-to-valine amino acid substitution at the 188(th) residue in NS1. ZIKV infectivity was enhanced by this residue substitution in the ZIKV FSS13025 strain in mosquitoes that acquired ZIKV from a viremic Type I and II interferon receptor-deficient (ifnagr-/-) C57BL/6 (AG6) mouse. Our results reveal that ZIKV evolved to acquire a spontaneous mutation in its NS1 protein, resulting in increased antigenemia of the protein. Enhancement of NS1 antigenemia in infected hosts promotes ZIKV infectivity and prevalence in mosquitoes, which potentially facilitates transmission during the recent ZIKV epidemics.