Meta-transcriptomic comparison of the RNA viromes of the mosquito vectors Culex pipiens and Culex torrentium in northern Europe

Abstract

There is mounting evidence that mosquitoes harbour an extensive diversity of insect-specific RNA viruses in addition to those important to human and animal health. However, because most studies of the mosquito virome have been conducted at lower latitudes there is a major knowledge gap on the genetic diversity, evolutionary history, and spread of RNA viruses sampled from mosquitoes in northern latitudes. Here, we determined and compared the RNA virome of two common northern Culex mosquito species, Cx. pipiens and Cx. torrentium, known vectors of West Nile virus and Sindbis virus, respectively, collected in south-central Sweden. Following bulk RNA-sequencing (meta-transcriptomics) of 12 libraries, comprising 120 specimens of Cx. pipiens and 150 specimens of Cx. torrentium, we identified 40 viruses (representing 14 virus families) of which 28 were novel based on phylogenetic analysis of the RNA-dependent RNA polymerase (RdRp) protein. Hence, we found similar levels of virome diversity as in mosquitoes sampled from the more biodiverse lower latitudes. Four libraries, all from Cx. torrentium, had a significantly higher abundance of viral reads, spanning [~]7- 36% of the total amount of reads. Many of these viruses were also related to those sampled on other continents, indicative of widespread global movement and/or long host-virus co-evolution. Importantly, although the two mosquito species investigated have overlapping geographical distributions and share many viruses, approximately one quarter of the viruses were only found at a specific location, such that geography must play an important role in shaping the diversity of RNA viruses in Culex mosquitoes.\n\nImportanceRNA viruses are found in all domains of life and all global habitats. However, the factors that determine virome composition and structure within and between organisms are largely unknown. Herein, we characterised RNA virus diversity in two common mosquito vector species, Culex pipiens and Culex torrentium, sampled from northern Europe. Our analysis revealed extensive viral diversity, including 28 novel viruses, and was comparable to the levels of diversity found in other temperate and tropical regions globally. Importantly, as well as harbouring RNA viruses that are closely related to other mosquito-derived viruses sampled in diverse global locations, we also described a number of viruses that are unique to specific sampling locations in Sweden. Hence, these data showed that geographical factors can play an important role in shaping virome structure even at local scales.