| Summary: | Chromobox homolog 2 (CBX2), a key member of the polycomb group (PcG) family, is essential for gonadal development in mammals. A functional deficiency or genetic mutation in <i>cbx2</i> can lead to sex reversal in mice and humans. However, little is known about the function of <i>cbx2</i> in gonadal development in fish. In this study, the <i>cbx2</i> gene was identified in medaka, which is a model species for the study of gonadal development in fish. Transcription of <i>cbx2</i> was abundant in the gonads, with testicular levels relatively higher than ovarian levels. In situ hybridization (ISH) revealed that <i>cbx2</i> mRNA was predominately localized in spermatogonia and spermatocytes, and was also observed in oocytes at stages I, II, and III. Furthermore, <i>cbx2</i> and <i>vasa</i> (a marker gene) were co-localized in germ cells by fluorescent in situ hybridization (FISH). After <i>cbx2</i> knockdown in the gonads by RNA interference (RNAi), the sex-related genes, including <i>sox9</i> and <i>foxl2,</i> were influenced. These results suggest that <i>cbx2</i> not only plays a positive role in spermatogenesis and oogenesis but is also involved in gonadal differentiation through regulating the expression levels of sex-related genes in fish.
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