A novel gonad-specific Argonaute 4 serves as a defense against transposons in the black tiger shrimp Penaeus monodon

• A cDNA encoding a novel Argonaute of Penaeus monodon (PmAgo4) was cloned.
• PmAgo4 is structurally related to AGO subfamily but expresses only in the gonad.
• PmAgo4 expression is not strongly responsive to dsRNA or YHV challenge.
• PmAgo4-knockdown shrimp show dramatic derepression of transposons.
• PmAgo4 therefore plays a role as a defense against transposons in shrimp germline.

Argonaute is a key protein of the small-RNA guided gene regulation process. The Argonaute family is generally divided into two subfamilies; AGO and PIWI. In this study, a cDNA encoding a novel type of Argonaute (PmAgo4) in the black tiger shrimp Penaeus monodon was identified and characterized. PmAgo4 cDNA contained an open reading frame of 2433 nucleotides that can be translated into a deduced amino acid with the conserved PAZ and PIWI domains. PmAgo4 was phylogenetically clustered with the AGO subfamily while exhibited a gonad-specific expression pattern similar to that of proteins in the PIWI subfamily. The expression of PmAgo4 did not change significantly in response to either double-stranded RNA or yellow head virus injection suggesting that PmAgo4 may not be the main AGO proteins that play a role in dsRNA-mediated gene silencing or antiviral defense. Interestingly, PmAgo4 appeared to participate in the control of transposons since the activation of both DNA transposon and retrotransposon was detected in the testis of PmAgo4-knockdown shrimp. Our study thus provided the first evidence for an unusual type of the AGO proteins that was predominantly expressed in shrimp gonad and implication of its role in protecting the shrimp genome against an invasion of transposons.