Chikungunya virus populations experience diversity- dependent attenuation and purifying intra-vector selection in Californian Aedes aegypti mosquitoes

Differential Susceptibility of Aedes aegypti and Aedes albopictus Mosquitoes to Infection by Mayaro Virus Strains

Mayaro virus (MAYV) is an arthropod-borne virus (arbovirus) belonging to the family Togaviridae, genus Alphavirus. In recent years, the geographic distribution of MAYV may have expanded north from South and Central America into the Caribbean Islands. Although Haemagogus janthinomys is considered the main vector for MAYV, the virus has also been isolated from other mosquitoes, including Aedes aegypti, a widespread species that serves as the main vector for highly epidemic viruses. Given the possible expansion and outbreaks of MAYV in Latin America, it is possible that MAYV might be adapting to be efficiently transmitted by urban vectors. Therefore, to investigate this possibility, we evaluated the vector competence of Ae. aegypti and Ae. albopictus mosquitoes to transmit MAYV isolated during a year of low or high MAYV transmission. Adult Ae. aegypti and Ae. albopictus were orally infected with the MAYV strains, and the infection, dissemination, and transmission rates were calculated to evaluate their vector competence. Overall, we found higher infection, dissemination, and transmission rates in both Ae. aegypti and Ae. albopictus mosquitoes infected with the strain isolated during a MAYV outbreak, whereas low/no transmission was detected with the strain isolated during a year of low MAYV activity. Our results confirmed that both Ae. aegypti and Ae. albopictus are competent vectors for the emergent MAYV. Our data suggest that strains isolated during MAYV outbreaks might be better fit to infect and be transmitted by urban vectors, raising serious concern about the epidemic potential of MAYV.

Chikungunya virus produced by a persistently infected mosquito cell line comprises a shorter genome and is non-infectious to mammalian cells

Although RNA viruses have high mutation rates, host cells and organisms work as selective environments, maintaining the viability of virus populations by eliminating deleterious genotypes. In serial passages of RNA viruses in a single cell line, most of these selective bottlenecks are absent, with no virus circulation and replication in different tissues or host alternation. In this work, Aedes aegypti Aag-2 cells were accidentally infected with Chikungunya virus (CHIKV) and Mayaro virus (MAYV). After numerous passages to achieve infection persistency, the infectivity of these viruses was evaluated in Ae. albopictus C6/36 cells, African green monkey Vero cells and primary-cultured human fibroblasts. While these CHIKV and MAYV isolates were still infectious to mosquito cells, they lost their ability to infect mammalian cells. After genome sequencing, it was observed that CHIKV accumulated many nonsynonymous mutations and a significant deletion in the coding sequence of the hypervariable domain in the nsP3 gene. Since MAYV showed very low titres, it was not sequenced successfully. Persistently infected Aag-2 cells also accumulated high loads of short and recombinant CHIKV RNAs, which seemed to have been originated from virus-derived DNAs. In conclusion, the genome of this CHIKV isolate could guide mutagenesis strategies for the production of attenuated or non-infectious (to mammals) CHIKV vaccine candidates. Our results also reinforce that a paradox is expected during passages of cells persistently infected by RNA viruses: more loosening for the development of more diverse virus genotypes and more pressure for virus specialization to this constant cellular environment.

Rapid evolution of enhanced Zika virus virulence during direct vertebrate transmission chains

Zika virus (ZIKV) has the unusual capacity to circumvent natural alternating mosquito-human transmission and be directly transmitted human-to-human via sexual and vertical routes. The impact of direct transmission on ZIKV evolution and adaptation to vertebrate hosts is unknown. Here we show that molecularly barcoded ZIKV rapidly adapted to a mammalian host during direct transmission chains in mice, coincident with the emergence of an amino acid substitution previously shown to enhance virulence. In contrast, little to no adaptation of ZIKV to mice was observed following chains of direct transmission in mosquitoes or alternating host transmission. Detailed genetic analyses revealed that ZIKV evolution in mice was generally more convergent and subjected to more relaxed purifying selection than in mosquitoes or alternate passages. These findings suggest that prevention of direct human transmission chains may be paramount to resist gains in ZIKV virulence.Importance We used experimental evolution to model chains of direct and indirect Zika virus (ZIKV) transmission by serially passaging a synthetic swarm of molecularly barcoded ZIKV within and between mosquitoes and mice. We observed that direct mouse transmission chains facilitated a rapid increase in ZIKV replication and enhanced virulence in mice. These findings demonstrate that ZIKV is capable of rapid adaptation to a vertebrate host and indicate that direct human-to-human transmission could pose a greater threat to public health than currently realized.

Two Sides of a Coin: a Zika Virus Mutation Selected in Pregnant Rhesus Macaques Promotes Fetal Infection in Mice but at a Cost of Reduced Fitness in Nonpregnant Macaques and Diminished Transmissibility by Vectors

Although Zika virus infection of pregnant women can result in congenital Zika syndrome, the factors that cause the syndrome in some but not all infected mothers are still unclear. We identified a mutation that was present in some ZIKV genomes in experimentally inoculated pregnant rhesus macaques and their fetuses. Although we did not find an association between the presence of the mutation and fetal death, we performed additional studies with ZIKV with the mutation in nonpregnant macaques, pregnant mice, and mosquitoes. We observed that the mutation increased the ability of the virus to infect mouse fetuses but decreased its capacity to produce high levels of virus in the blood of nonpregnant macaques and to be transmitted by mosquitoes. This study shows that mutations in mosquito-borne viruses like ZIKV that increase fitness in pregnant vertebrates may not spread in outbreaks when they compromise transmission via mosquitoes and fitness in nonpregnant hosts.

Although fetal death is now understood to be a severe outcome of congenital Zika syndrome, the role of viral genetics is still unclear. We sequenced Zika virus (ZIKV) from a rhesus macaque fetus that died after inoculation and identified a single intrahost substitution, M1404I, in the ZIKV polyprotein, located in nonstructural protein 2B (NS2B). Targeted sequencing flanking position 1404 in 9 additional macaque mothers and their fetuses identified M1404I at a subconsensus frequency in the majority (5 of 9, 56%) of animals and some of their fetuses. Despite its repeated presence in pregnant macaques, M1404I has occurred rarely in humans since 2015. Since the primary ZIKV transmission cycle is human-mosquito-human, mutations in one host must be retained in the alternate host to be perpetuated. We hypothesized that ZIKV I1404 increases viral fitness in nonpregnant macaques and pregnant mice but is less efficiently transmitted by vectors, explaining its low frequency in humans during outbreaks. By examining competitive fitness relative to that of ZIKV M1404, we observed that ZIKV I1404 produced lower viremias in nonpregnant macaques and was a weaker competitor in tissues. In pregnant wild-type mice, ZIKV I1404 increased the magnitude and rate of placental infection and conferred fetal infection, in contrast to ZIKV M1404, which was not detected in fetuses. Although infection and dissemination rates were not different, Aedes aegypti mosquitoes transmitted ZIKV I1404 more poorly than ZIKV M1404. Our data highlight the complexity of arbovirus mutation-fitness dynamics and suggest that intrahost ZIKV mutations capable of augmenting fitness in pregnant vertebrates may not necessarily spread efficiently via mosquitoes during epidemics.

IMPORTANCE Although Zika virus infection of pregnant women can result in congenital Zika syndrome, the factors that cause the syndrome in some but not all infected mothers are still unclear. We identified a mutation that was present in some ZIKV genomes in experimentally inoculated pregnant rhesus macaques and their fetuses. Although we did not find an association between the presence of the mutation and fetal death, we performed additional studies with ZIKV with the mutation in nonpregnant macaques, pregnant mice, and mosquitoes. We observed that the mutation increased the ability of the virus to infect mouse fetuses but decreased its capacity to produce high levels of virus in the blood of nonpregnant macaques and to be transmitted by mosquitoes. This study shows that mutations in mosquito-borne viruses like ZIKV that increase fitness in pregnant vertebrates may not spread in outbreaks when they compromise transmission via mosquitoes and fitness in nonpregnant hosts.

Impact of alphavirus 3'UTR plasticity on mosquito transmission

Alphaviruses such as chikungunya and western equine encephalitis viruses are important human pathogens transmitted by mosquitoes that have recently caused large epidemic and epizootic outbreaks. The epidemic potential of alphaviruses is often related to enhanced mosquito transmission. Tissue barriers and antiviral responses impose bottlenecks to viral populations in mosquitoes. Substitutions in the envelope proteins and the presence of repeated sequence elements (RSEs) in the 3'UTR of epidemic viruses were proposed to be specifically associated to efficient replication in mosquito vectors. Here, we discuss the molecular mechanisms that originated RSEs, the evolutionary forces that shape the 3'UTR of alphaviruses, and the significance of RSEs for mosquito transmission. Finally, the presence of RSEs in the 3'UTR of viral genomes appears as evolutionary trait associated to mosquito adaptation and emerges as a common feature among viruses from the alphavirus and flavivirus genera.

Distinct New York City Aedes albopictus Mosquito Populations Display Differences in Salivary Gland Protein D7 Diversity and Chikungunya Virus Replication

In an increasingly interconnected world, the exposure and subsequent spread of emergent viruses has become inevitable. This is particularly true for Aedes (Ae.) mosquito-vectored viruses, whose range has increased over the past decade from tropical to temperate regions. However, it is unclear if all populations of Ae. mosquitoes in temperate New York City are able to successfully replicate and transmit arboviruses. To answer this question, we reared Ae. albopictus mosquitoes living in a temperate climate from three locations in New York City. We first sequenced the salivary antiviral protein D7 from individual mosquitoes in each population and found single nucleotide variants that are both shared and unique for each Ae. albopictus population. We then fed each population chikungunya virus (CHIKV) via an artificial blood meal. All three mosquito populations could be infected with CHIKV, yet viral titers differed between populations at 7 days post infection. Moreover, we found that these mosquitoes could transmit CHIKV to mice, and that virus RNA reached the saliva as early as two days post infection. Upon sequencing of the saliva CHIKV genomic RNA, we found mutations at sites correlated with increased transmission and virulence. These studies show that NYC Ae. albopictus populations can be infected with and transmit CHIKV, CHIKV is able to evolve in these mosquitoes, and that host salivary factors display population-specific diversity. Taken together, these studies highlight the need to study how distinct mosquito populations control viral infections, both at the virus and host level.