The early zygotic genes and microRNAs in the yellow fever mosquito Aedes aegypti  and the Asian malaria mosquito Anopheles stephensi

Mosquitoes are notorious vectors for multiple diseases like malaria, yellow fever and dengue fever. To manipulate gene expression in mosquito and spread desired genes among natural population for vector control, a thorough understanding of mosquito development and gene regulation is critical. Early embryogenesis is a rapid, complex yet crucial process in the very beginning of development. Previous research in other species indicated genes transcribed that early evolved fast and played essential roles. The study of mosquito early zygotic genes (EZGs) would offer unique insights into mosquito gene evolution as well as potential targets for mosquito control. In this study, I identified 61 pure EZGs (pEZGs) in mosquito Aedes aegypti. These pEZGs were enriched in architectures adapting to the rapid embryonic cell cycles and were over represented by domains or functions related to maternal zygotic transition. Phylogenetic analysis showed that pEZGs originated mainly from duplication, retrotransposition and de novo emergence. The comparison of pEZGs in Ae. aegypti with those in Drosophila revealed an interesting evolutionary paradox where the early zygotic genes turned over fast but the regulatory motif was conserved in two species. Curiously, the motif binding protein in Drosophila (zelda) seemed unable to initiate the earliest zygotic transcription in Ae. aegypti due to late temporal expression. The regulatory motif (VBRGGTA) found in Ae. aegypti pEZGs was shown necessary and sufficient for driving early zygotic gene expression by transient reporter assays and one motif-bearing promoter was tested with success in driving gene expression as early as 2-4h after egg laying in transgenic Ae. aegypti. This was the first characterized promoter with early zygotic but no maternal expression in Ae. aegypti that can be used for future genetic studies and mosquito control strategies.

As important gene regulators, miRNAs also play essential roles in early embryogenesis. The genome-wide predictions and systematic analysis of miRNAs in Ae. aegypti and Anopheles stephensi were conducted in this study. The first miRNA profiling in mosquito across all developmental stages was also performed to provide basis for future functional study. Several lineage-specific miRNAs were found highly expressed in embryos, indicating their special roles in the embryogenesis of mosquitoes.