Limited impact of the siRNA pathway on transposable element expression in Aedes aegypti

Transposable elements (TEs) are DNA sequences that can change their position within a genome. In insects, small RNA pathways are central to the transcriptional and post-transcriptional regulation of TE expression. The Piwi-interacting RNA (piRNA) pathway is particularly important in germline tissues, where it silences TE transcripts via small RNAs of 24-30 nucleotides (nt) in length produced from genomic precursor transcripts as well as through a "ping-pong" amplification cycle. The small interfering RNA (siRNA) pathway helps restrict TE expression in somatic tissues via 21nt small RNAs produced from double-stranded RNA by the endonuclease Dicer2, which guide an RNA-induced silencing complex to degrade complementary RNAs. However, much of this knowledge comes from studies of the model insect Drosophila melanogaster. In the mosquito Aedes aegypti, a medically significant vector species, the siRNA pathway has mainly been investigated in connection with its antiviral role, leaving open whether it also regulates TE expression.

We investigated the expression of TEs and small RNAs in both somatic and gonadal tissues of a Dicer2 mutant line of Ae. aegypti and its wild-type counterpart. Our results show a modified pattern of TE expression and a decrease in TE-derived 21nt RNAs in the Dicer2 mutant, but no major shift of TE transcript abundance. The lack of a functional siRNA pathway also causes perturbations in piRNA ping-pong signatures and the expression of certain piRNA-associated genes, but without clear evidence for compensation by increased piRNA pathway activity.

The mosquito Ae. aegypti produces siRNAs derived from TEs but these lack a critical role in the regulation of TE expression both in somatic and in gonadal tissues.