Host-pathogen interaction: Aedes spp. mosquitoes and arboviruses

Arboviruses such as the flavivirus Zika, Dengue and Usutu viruses, and the alphavirus Chikungunya represent resurging or emerging public health threats worldwide. In Europe, arboviral risk is dependent on the presence of the competent vector Ae. albopictus, a mosquito that arrived from South East Asia only 30 years ago. At the moment, control of vector populations is the primary tool to prevent arboviral transmission because no drugs and limited vaccines are available for these diseases. Mosquito response to arboviral infection is a complex and evolving phenotype dependent primarily on the activity of RNAi pathways. Among RNAi, the piRNA pathway has been shown to have a significant role in mosquito immunity. However, the function of this pathway is still enigmatic because it was characterized only recently and it has been mostly studied in the model insect D. melanogaster, where it does not have antiviral activity, but functions in germ cells through small RNAs, called piRNAs, to silence TEs and preserve genome integrity. Recently, we discovered that the Ae. albopictus genome carries numerous integrations from viral genomes that are integrated mainly in loci producing piRNAs. On this basis, we hypothesized a functional link between the piRNA pathway and viral integrations. During my PhD, I developed a program with three specific aims to test the above-mentioned hypothesis. Briefly, I aimed to 1) characterize the genes of the piRNA pathway in Ae. albopictus 2) study the involvement of viral integrations in virus-host interactions in connection with the siRNA and piRNA pathways; 3) analyse Ae. albopictus fitness traits. The main questions I address in Aim 1 the piRNA pathway of Ae. albopictus concern the Piwi gene copy number, structure, variability, their protein architecture and expression profile, both throughout the mosquito development and following arboviral infection. I also set up experiments at different growth temperatures to analyze the effects on the expression profile of Piwi genes caused by temperature variations similar to the ones encountered in the newly invaded temperate regions. For aim 2 NIRVS and arboviral infection, I performed a comprehensive analysis of the expression profile of small RNAs whose sequences match the infecting viruses or NIRVS, in an attempt to shed light on the role of small RNAs of both viral and endogenous origin in the establishment of persistent infections. This analysis was done across carcasses and ovaries. For aim 3 Life history trains of Aedes albopictus, I carried out comparative analyses of fitness traits of two reference strains and three natural populations of Ae. albopictus paving the way for future studies on the analyses of the life history traits of this invasive mosquito species. Over-all results gained through my PhD activities will enhance our understanding of unexplored functions of the piRNA pathway beyond the model organism D. melanogaster, will give new insights into the evolution of the proteins of the piRNA pathway in Ae. albopictus and will provide novel insights on mosquito-vector interactions. As side projects I also contributed to the analyses of the Ae. albopictus genome, based on long-sequencing approaches and Hi-C data; I was involved in a project to test the biological function of NIRVS in Ae. aegypti and I contributed to population genetics analyses of Ae. albopictus populations in the context of studying the widespread of resistance to pyrethroid insecticides, one of the most-used tools of vector control.