The piRNA pathway of the mosquito, Aedes aegypti

The piRNA pathway is a dicer–independent pathway that, in Drosophila melanogaster, zebrafish and mice suppresses the activity of transposable elements thereby increasing genome stability.  In D. melanogaster three genes, piwi, aubergene and argonaute3 encode proteins that directly interact with small guide RNAs leading to the slicing of target RNAs, the majority of which emanate from transposable elements.  Aedes aegypti is significant vector of many RNA viruses, some of which are pathogenic to humans.  It has a large genome size of approximately 1,400 megabases of which almost half consists of transposable elements with many superfamilies of these being represented.  This is therefore in contrast to the much smaller D. melanogaster genome in which the abundance and diversity of transposable elements is considerably less.  The Ae. aegypti genome has six piwi genes and a single Ago3 gene with only the expression of the piwi2 gene being confined to the germ-line, the other genes being also expressed in somatic tissues.  While transposable element sequences are targeted by the piRNA pathway of Ae. aegypti, these account for approximately only 20% of all piRNAs, the majority going to other sequences, including genes of viral origin.  Our data suggest that the piRNA pathway in this mosquito has evolved in response to this insect’s evolutionary history of acquiring viral and transposable element sequences.