Transgenic marine medaka (Oryzias melastigma): A sensitive sentinel for estrogenic pollutants

Pollution caused by endocrine disruptor (ED) (especially estrogenic EDs) has become one of the most serious anthropogenic threats to biodiversity and ecosystem. Our lab has generated a transgenic medaka, the O. melastigma, which can be used for biomonitoring of estrogenic activity in the marine and freshwater environment. Choriogenin genes, which encode precursors of the inner layer subunits of egg envelope, are regarded as sensitive biomarkers for estrogenic pollutants. We have shown that in the O. melastigma, expression of the choriogenin H (omChgH) appeared to be more sensitive to estrogen than the related gene member choriogenin L (omChgL). This observation highlighted the potential of using omChgH expression as a sensitive biomarker for estrogenic EDs in the developing O. melastigma embryos, juveniles and male adults. In our laboratory, a transgenic O. melastigma strain containing the reporter gene green fluorescence protein (GFP) which is regulated by the promoter region of the omChgH gene has been established. In this strain, GFP was expressed constitutively in the liver of mature female. Interestingly, the reporter gene expression could also be induced in the liver of embryos (initiated from stage 34), juveniles and male fish in response to 17B-estradiol (E2). By quantifying the intensity of the reporter gene expression, we found that low levels of E2 and 17ẞ-ethanylestradiol (EE2) could induce GFP expression in a dose-responsive manner in yolk-sac larvae of this strain. Moreover, this transgenic strain also responded to different estrogenic compounds such as estrone (E1), estriol (E3), 4- nonylphenol (NP), bisphenol A (BPA), genistein and ethinylestradiol 3-methyl ether. This is a novel observation as most of the published transgenic fish only responded to E2 but not estrogen-like substances. Furthermore, preliminary study found that water samples collected in the Hong Kong Victoria Harbor could induce GFP expression in the transgenic larvae. Our data indicate that this transgenic O. melastigma is a sensitive sentinel for biomonitoring of estrogenic EDs in filed samples. The wide salinity adaptability of this strain of medaka enables its application as a sentinel for both freshwater and marine environment.