Expression and Localization of <i>Piwil1</i> During Gonadal Development in <i>Monopterus albus</i>

PIWI proteins and Piwi‐interacting RNAs (piRNAs) are crucial for gametogenesis, embryogenesis, and stem cell maintenance in animals. The Piwil1 ( Piwi-like RNA-mediated gene silencing 1 ) gene has rapidly evolved in the protogynous hermaphrodite ricefield eel ( Monopterus albus ) compared to other teleosts. Notably, there is a significant reduction in DNA methylation levels and an increase in the expression levels of the Piwil1 gene in testes compared to those in ovaries. However, the role of Piwil1 in the gonadogenesis of ricefield eel remains unclear. This study elucidated the histomorphological characteristics of ricefield eel gonads at different developmental stages and analyzed the expression and localization of Piwil1 using qRT‐PCR, western blot, and immunofluorescence. Histological analysis showed that the female gonad contained oocytes at different developmental stages, the intersex gonad had both ovarian and testicular tissues, and the male gonad included spermatogenic cells and testicular lobules. Quantitative real‐time PCR and western blot indicated significantly higher Piwil1 expression in the ovary, intersexual gonad, and testis compared to other tissues, with levels increasing during gonadal differentiation and peaking in the testis. Immunofluorescence analysis revealed that the PIWIL1 protein was abundantly localized in the cytoplasm of immature oocytes, granulosa cells surrounding the oocytes, spermatogonia, and primary spermatocytes. Importantly, in both the ovary and testis, the expression of Piwil1 initially increased but then decreased at the mRNA and protein levels. These results suggest that Piwil1 might play a crucial role in gonadal development and gametogenesis in ricefield eels.