Development of a molecular biomarker for detecting intersex after exposure of male medaka fish to synthetic estrogen.

Although intersex of fish species has been widely reported in aquatic environments, there is no appropriate biomarker of the effects to assess the occurrence of intersex. In the present study, mRNA expression profiles of 14 genes, which are either involved in ovary development and maintenance or have relatively high mRNA transcription levels in ovarian gene expression profiles in fish species, were comprehensively evaluated in male Japanese medaka (Oryzias latipes) exposed to 17α-ethynylestradiol (EE2) to investigate their utility as indicators of the severity index of intersex. Of these 14 genes, mRNA expression of a novel gene, termed ovary structure protein 1 (OSP1) in the present study, showed female-like expression pattern with the highest transcription levels, and displayed the most significantly positive correlation with the severity index of intersex (r(2) =0.8215, p<0.0001). The full-length mRNA sequence of OSP1 is 802 bp, with an open reading frame (ORF) of 642 bp that encodes a 214-amino acid peptide. Reverse transcription polymerase chain reaction and in situ hybridization revealed that OSP1 was expressed exclusively in the ovaries, specifically in the oogonia and primary oocytes in ovaries and in testis with intersex. These results suggest that the analysis of mRNA expression of OSP1 can be used to indicate intersex in male medaka.