# Repeated duplication of Argonaute2 is associated with strong selection and testis specialization in Drosophila

## TL;DR

This study shows that repeated duplication of the Ago2 gene in Drosophila is linked to testis specialization and strong selection, suggesting new roles in RNAi pathways.

## Contribution

The paper reveals new insights into the evolutionary dynamics and functional specialization of Ago2 paralogues in Drosophila.

## Key findings

- Old Ago2 duplications are shared across the Drosophila obscura group, with some losses detected in D. pseudoobscura.
- Most novel Ago2 paralogues have evolved testis-specific expression, distinct from the ancestral ubiquitous pattern.
- Population genetic analyses indicate recent positive selection and selective sweeps in D. pseudoobscura Ago2 paralogues.

## Abstract

Argonaute2 (Ago2) is a rapidly evolving nuclease in the Drosophila melanogaster RNA interference (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 specialized 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.

All new sequences produced in this study have been submitted to Genbank as KX016642-KX016771.