Abstract
Argonaute2 (Ago2) is a rapidly evolving nuclease in the Drosophila melanogaster RNAi pathway that targets viruses and transposable elements in somatic tissues. Here we reconstruct the history of Ago2 duplications across the Drosophila obscura group, and use patterns of gene expression to infer new functional specialization. We show that some duplications are old, shared by the entire species group, and that losses may be common, including previously undetected losses in the lineage leading to D. pseudoobscura. We find that while the original (syntenic) gene copy has generally retained the ancestral ubiquitous expression pattern, most of the novel Ago2 paralogues have independently specialised to testis-specific expression. Using population genetic analyses, we show that most testis- specific paralogues have significantly lower genetic diversity than the genome-wide average. This suggests recent positive selection in three different species, and model-based analyses provide strong evidence of recent hard selective sweeps in or near four of the six D. pseudoobscura Ago2 paralogues. We speculate that the repeated evolution of testis-specificity in obscura group Ago2 genes, combined with their dynamic turnover and strong signatures of adaptive evolution, may be associated with highly derived roles in the suppression of transposable elements or meiotic drive. Our study highlights the lability of RNAi pathways, even within well-studied groups such as Drosophila, and suggests that strong selection may act quickly after duplication in RNAi pathways, potentially giving rise to new and unknown RNAi functions in non-model species.
