The piRNA pathway in Aedes aegypti – a potential tool for genetic control

Insect protection from viruses is typically carried out by the siRNA pathway which involves the degradation of foreign genetic material during an active infection. Recently, another class of small RNAs that form the piRNA pathway have been implicated in antiviral activity in Aedes aegypti. This is unusual as the piRNA pathway canonically functions to protect the germline from the effects of transposons, and hence tends to be limited to gonadal tissues. However, Aedes aegypti was discovered to express piRNAs somatically and the recent confirmation of the antiviral activity of piRNAs in the ovaries lends credence to this theory. Not much is known about how the associated effectors of this pathway, known as the PIWI class of proteins, participate in this antiviral activity. Using CRISPR-Cas9 we knocked out each PIWI protein one at a time and characterized the phenotypes of the resulting mosquitoes. Knockout of each PIWI protein led to a general reduction in viability and fertility of the G0 mosquitoes compared to the control. This suggests that the piRNA pathway may be involved in critical aspects of the development of the mosquito, though further work in vivo is necessary to tease apart the myriad potential functions. Why this pathway is involved in addition to the siRNA pathway is currently unknown, however, this additional protection could help explain the impressive capacity Aedes aegypti mosquitoes have for vectoring arboviruses. Further investigation of this pathway could result in valuable information on new targets for genetic control of this vector species.