Genomic identification, rapid evolution, and expression of Argonaute genes in the tilapia, Oreochromis niloticus

Argonaute proteins are key components of the small RNA-induced silencing complex and have multiple roles in RNA-directed regulatory pathways. Argonaute genes can be divided into two subfamilies: the Ago (interacting with microRNA/small interfering RNA) and Piwi subfamilies (interacting with piwi-interacting RNAs (piRNAs)). In the present study, genome-wide analyses firstly yielded the identification of different members of Agos and Piwis in the tilapia, coelacanth, spotted gar, and elephant shark. The additional teleost Ago3b was generated following the fish-specific genome duplication event. Selective pressure analysis on Agos and Piwis between cichlids and other teleosts showed an accelerated evolution of Piwil1 in the cichlid lineages, and the positive selected sites were located in the region of PIWI domain, suggesting that these amino acid substitutions are adapt to targeted cleavage of messenger RNA (mRNA) in cichlids. Ago1 and Ago4 were detected at higher levels at 5 days after hatching (dah) in both ovaries and testes compared with other stages, supporting the previously reported requirement of Ago-mediated pathways to clear the maternal mRNAs during the early embryogenesis. The Piwis were abundantly expressed in tilapia testes, indicating their essential roles in male germline, especially in spermatogenesis. Notable expression of Piwis was also detected in skeletal muscle, indicating that piRNA pathway may not only be confined to development and maintenance of the germline but may also play important roles in somatic tissues. The expression of Piwil1 and Piwil2 was examined by quantitative PCR (qPCR) and in situ hybridization (ISH) to validate the spatial and temporal expression profiles. Taken together, these results present a thorough overview of tilapia Argonaute family and provide a new perspective on the evolution and function of this family in teleosts.