Identification and expression of piwil2 in turbot Scophthalmus maximus, with implications of the involvement in embryonic and gonadal development

Piwil2, a member of the Argonaute family, is involved in the biogenesis of PIWI-interacting RNAs (piRNAs) and plays an important role in regulating gametogenesis. In the present study, we identified turbot Scophthalmus maximus piwil2 gene, named Smpiwil2, which contained a PAZ domain and a PIWI domain. Sequence comparison, genomic structure and phylogenetic analyses showed that Smpiwil2 is homologous to that of teleosts and tetrapods. The Smpiwil2 transcript showed higher expression in the ovary than in the testis, demonstrating a sexually dimorphic gene expression pattern. In situ hybridization (ISH) showed that Smpiwil2 was expressed in the oogonia and all the stages of oocytes in the ovary as well as in spermatogonia and spermatocytes in the testis. Embryonic expression profile revealed that Smpiwil2 was maternally inherited, and its level was higher from the zygote to the blastula stage and subsequently decreased until hatching. Moreover, a CpG island was predicted to locate in the 5′-flanking region of Smpiwil2 gene, and its methylation levels detected by sodium bisulfite sequencing showed significant disparity between females and males, implying that the sexually dimorphic expression of Smpiwil2 might be regulated by methylation. These results indicated that Smpiwil2 had potentially vital functions in embryonic and gonadal development in this species. In addition, the temporal and sex differences in Smpiwil2 expression indicated that this gene may play different roles in gonadal development of different sexes.