Origins of alphavirus-derived small RNAs in mosquitoes

Abstract

The continual transmission in nature of many arthropod-borne viruses depends on the establishment of a persistent, nonpathogenic infection in a mosquito vector. The importance of antiviral immunity directed by small RNAs in the mechanism by which alphaviruses establish a persistent, nonpathogenic infection in the mosquito vector has recently been demonstrated. The origin of the small RNAs central to this RNA silencing response has recently been the subject of debate. Here we briefly summarize what is known about the mechanism of small RNA-directed immunity in invertebrates, and discuss current models for the viral triggers of this response. Finally, we summarize evidence indicating that alphavirus double-stranded replicative intermediates trigger an exogenous-siRNA pathway in mosquitoes resulting in the biogenesis of virus-derived siRNAs.

Figure 1

Consistent patterns of viRNA biogenesis following ONNV infection of An. gambiae and high-throughput sequencing from two independent biological replicates. Small RNA libraries from female An. gambiae mosquitoes (4 days post injection with ONNV) were constructed, sequenced and analyzed on an Illumina Genome Analyzer as described in Myles et al. (2008). After removing non-coding RNAs, we obtained 4,242,423 and 4,258,690 usable reads from biological replicates 1 and 2, respectively. Small RNAs matching the ONNV genome (100% match) were plotted by length (A) or were mapped onto the viral genome (B). Numbers of total 21 nt viRNAs (Total) or unique-mapping 21 nt viRNAs (Unique) deriving from the 5′ 2/3 (non-structural region) or 3′ 1/3 (structural region) of the viral genome are indicated for each biological replicate.