Summary Culex pipiens densovirus (CpDV), a single stranded DNA virus, has been isolated from Culex pipiens mosquitoes but differs from other mosquito densoviruses in terms of genome structure and sequence identity. Its transmission from host to host, the nature of its interactions with both its host and host's endosymbiotic bacteria Wolbachia are not known. Here, we report the presence of CpDV in the ovaries and eggs of Cx. pipiens mosquitoes in close encounters with Wolbachia. In the ovaries, CpDV amount significantly differed between mosquito lines harbouring different strains of Wolbachia and these differences were not linked to variations in Wolbachia densities. CpDV was vertically transmitted in all laboratory lines to 17%–20% of the offspring. For some females, however, the vertical transmission reached 90%. Antibiotic treatment that cured the host from Wolbachia significantly decreased both CpDV quantity and vertical transmission suggesting an impact of host microbiota, including Wolbachia, on CpDV transmission. Overall our results show that CpDV is transmitted vertically via transovarian path along with Wolbachia with which it shares the same cells. Our results are primordial to understand the dynamics of densovirus infection, their persistence and spread in populations considering their potential use in the regulation of mosquito vector populations.

This contribution provides a window into the study of a densovirus called CpDV infecting Culex pipiens 33 mosquitoes. The originality of the study lies in the fact that CpDV, a DNA virus, was to date almost 34 unstudied, its interaction with its hosts and its transmission mode were unknown. Considering that all 35 Cx. pipiens are infected with Wolbachia, we studied the influence of this bacterial endosymbiont on 36 CpDV density and transmission. Although Wolbachia-RNA virus interactions, especially in non-native 37 Wolbachia-host associations have been widely studied, Wolbachia-DNA virus interactions were rarely 38 investigated. 40In this natural tripartite Cx. pipiens-Wolbachia-CpDV system: 41 1. We revealed the wide prevalence of CpDV, mostly at low densities, in Cx. pipiens laboratory lines. 42 2. We monitored CpDV presence in several cell types in ovaries including the oocytes where they co-43 exist with the bacterial endosymbiont Wolbachia.44 3. We demonstrated that CpDV density in ovaries was influenced by Wolbachia presence and strain 45 variations 46 4. We studied CpDV vertical transmission depending on Wolbachia presence and strain variations 47 48 We report for the first time the occurrence of widely distributed covert CpDV infections in Cx. pipiens 49 laboratory colonies. CpDV is maintained by both vertical and horizontal transmission and under the 50 influence of the vertically transmitted bacterial endosymbiont Wolbachia. Taken together our results 51 suggest that CpDV could be very prevalent in nature putatively influencing natural Cx. pipiens 52 populations' dynamics. 53 54 55 56 57 58 59 SUMMARY 60 Culex pipiens densovirus (CpDV), a single stranded DNA virus, has been isolated from Culex 61 pipiens mosquitoes but differs from other mosquito densoviruses in terms of genome structure 62 and sequence identity. Its transmission from host to host, the nature of its interactions with both 63 its host and host's endosymbiotic bacteria Wolbachia are not known. Here we report the presence 64 of CpDV in the ovaries and eggs of Cx. pipiens mosquitoes in close encounters with Wolbachia. In 65 the ovaries, CpDV quantity significantly differed between mosquito lines harboring different 66 strains of Wolbachia and these differences were not linked to variations in Wolbachia densities.67 CpDV was vertically transmitted in all laboratory lines to 17%-20% of the offspring. For some 68 females, however, the vertical transmission reached 90%. Antibiotic treatment that cured the host 69 from Wolbachia also significantly affected both CpDV quantity and vertical transmission 70 suggesting an impact of host microbiota, including Wolbachia, on CpDV transmission. Overall 71 our results show that CpDV is transmitted vertically via transovarian path along with Wolbachia 72 with which it shares the same cells. Our results are primordial to understand the dynamics of 73 densovirus infection, their persistence and spread in populations considering their potential use 74 in the regulation of mosquito vector populations. 75 76 77...