Exploring the coevolutionary potential of chikungunya virus and its Aedes mosquito vectors

SUMMARY Chikungunya virus (CHIKV) has become a global problem since it spread from the Indian Ocean region, around the globe. In 2014, this expansion included a major outbreak in Latin America and the Caribbean that has led to endemicity in several countries. The cost of the disease as a public health issue is substantial, given the chronic arthritis that tends to arise post-infection. Without a vaccine or drug treatment, vector control is the only solution for limiting the disease. CHIKV is transmitted by two species of mosquito, Aedes albopictus and Aedes aegypti, which have spread to inhabit much of the temperate and tropical regions of the world. Because it is very distantly related to its more ubiquitous cousin, dengue virus (DENV), mosquito populations have had little opportunity to evolve resistance to CHIKV. Since both DENV and CHIKV cause fitness reductions in mosquitoes, they have the potential to act as a selective force in populations where they occur. CHIKV’s potential for expanded impact is concerning, given that it appears better able to infect mosquitoes than DENV. In Aim 1, we take an experimental evolution approach to understand the relative potential for mosquitoes to evolve resistance to CHIKV compared to DENV. In the evolved lines we will use genome-wide association to identify candidate SNPs/genes that underpin virus resistance. Using efficient CRISPR-Cas9 methods developed specifically for Ae. aegypti, we can then test the functional involvement of top candidate genes in resistance. In Aim 2, we explore the functional importance of the suite of genetic variants that exist globally in a CHIKV envelope protein-encoding gene (E2). E2 mediates entry into mosquito tissues, and variation in the gene can affect vector transmissibility. In completing these aims, we will generate new tools and apply emerging tools in a new context including the use of improved fluorescent reporter viruses in mosquitoes rather than in vitro and the adaptation of the ‘Vectorchip’ vector competence system for use with CHIKV and Ae. albopictus. The key knowledge gaps we will address are (i) whether with the assistance of mosquito genetic diversity and resistance evolution if CHIKV transmission is likely to evolve to lower levels with time, (ii) a list of potential anti-CHIKV and shared anti- CHIKV and DENV mosquito genes to target in genetic modification approaches, and (iii) an understanding of how circulating E2 variants in CHIKV genome may shape mosquito infectivity and the global landscape of CHIKV epidemiology.