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An important endpoint in aquatic bioassays for potential endocrine disrupting chemicals (EDCs) is the gonadal phenotype of exposed fish, with special interest in intersex and sex-reversed individuals. Traditionally, the assessment of gonad phenotype is done via histology, which involves specialized and time-consuming techniques. The method detailed here increases the efficiency of the analysis by first determining the relative expression of four genes involved in gonad development/maintenance in Japanese medaka (Oryzias latipes), and then by using principal component analysis, assigning a phenotype to each gonad based upon the gene expression data. The gonad phenotype and the sexual genotype, which can be determined in medaka, can then be compared to assess potential adverse effects of exposure to endocrine disrupting chemicals.

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Japanese medaka (Oryzias latipes) has been widely used for the evaluation of the toxicity of endocrine active chemicals (EACs) and other chemicals as well as for monitoring the adverse effects of effluent discharges in relation to sexual development and function. It is useful for these evaluations for many reasons including the following: 1) it has a short life cycle facilitating studies extending over long phases of development and over multigenerations, 2) it is easy to rear, 3) male and female phenotypes can easily be distinguished on the basis of secondary sex characteristics, and 4) a genetic marker (DMY) is available for identifying the true genotypic sex. Several biomarkers have been found to be useful for identifying the effects of exposure to estrogenic and androgenic chemicals in medaka and they include increased levels of hepatic vitellogenin (VTG) and testis-ova induction in males for exposure to estrogenic chemicals, and decreased levels of hepatic VTG in females and an altered morphology of dorsal and anal fins and formation of papillae for androgenic chemicals. In this paper, we present a critical analysis of the use of medaka as a test species for studies of endocrine disruption and report on the use of sex-related genetic markers and alterations in gonadal development, including the induction of testis-ova formation, for assessing the disruptive effects of EACs. In this paper, we focus on some of the more recent studies and findings.

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Recently, concern regarding perchlorate contamination has arisen in many contexts. Perchlorate has many military, commercial, and domestic applications, and it has been found in milk, drinking and irrigation water, and produce. Perchlorate is harmful at low levels, yet it remains unregulated in the United States while the U.S. Environmental Protection Agency attempts to establish acceptable exposure levels. The present study investigated potential reproductive effects on vertebrates using a model fish species, the threespine stickleback (Gasterosteus aculeatus). Sticklebacks were raised from syngamy through sexual maturity in untreated water and in three target concentrations of sodium perchlorate-treated water. Perchlorate was found to interfere with the expression of nuptial coloration, courtship behavior, and normal sexual development. Genetic testing revealed that some females were masculinized to the extent that they produced both sperm and eggs, and histological analysis showed that these individuals had intersexual gonads (ovotestes) containing both oocytes and cells undergoing spermatogenesis. In vitro fertilizations revealed that those gametes were capable of self- and cross-fertilization. However, crosses using sperm derived from genetic females died either during the blastula phase or near the onset of organogenesis. Sperm derived from genetic males produced viable fry when crossed with eggs derived from genetic females from all treatments. To our knowledge, the present study provides the first evidence that perchlorate produces androgenic effects and is capable of inducing functional hermaphroditism in a nonhermaphroditic vertebrate.

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Pentachlorophenol (PCP) is widely used to control termites and protect wood from fungal-rot and wood-boring insects, and is often detected in the aquatic environment. Few studies have evaluated PCP as an environmental endocrine disruptor. In the present work, Japanese medaka (Oryzias latipes) was exposed to PCP for 28 days (F0 generation) with subsequent measurements of vitellogenin (VTG), hepatic 7-ethoxyresorufin-O-deethylase (EROD), and reproductive endpoints. Plasma VTG significantly increased in male fish treated with PCP concentrations lower than 200 microg/l and decreased in male and female animals exposed to 200 microg/l. Hepatic EROD from female fish increased when PCP exposure concentrations exceeded 20 microg/l, but decreased in the 200 microg/l PCP treatment group. Fecundity and mean fertility of female medaka decreased significantly in the second and third week following exposure concentrations greater than 100 microg/l, and testis-ova of male medaka was observed at PCP concentrations greater than 50 microg/l. Histological lesions of liver and kidney occurred when exposure concentrations exceeded 50 microg/l. In F1 generations, the hatching rates and time to hatch of offspring were significantly affected in fish exposed to 200 microg/l. These results indicated that PCP exposure caused responses consistent with estrogen and aryl hydrocarbon receptor activation as well as reproductive impairment at environmentally relevant concentrations.

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The Japanese medaka (Oryzias latipes) was used as an in vivo model to evaluate the effects of exposure to the synthetic estrogen 17alpha-ethinylestradiol (EE2) on reproductive behavior and reproductive success in fish. Exposures to EE2 began within 2 to 5 d posthatch and continued until medaka were sexually mature, between four and six months of age. Among male and female medaka exposed to EE2 at nominal concentrations of 0.2 and 2 ng/L, mating behavior and reproductive success were normal in reproductive trials. However, reproductive behavior (i.e., copulations) was suppressed in the treatment with 10 ng/L EE2. Among 19 males exposed to 10 ng/L EE2 and placed with unexposed females in reproductive trials, 16 males did not copulate and reproductive success was very low. None of the females exposed to 10 ng/L of EE2 participated in reproductive behavior with unexposed males. The reproductive trials in combination with a histological survey indicated that male fish with gonadal intersex (i.e., testis-ova) were still capable of reproductive behavior and could fertilize eggs. Even though females exposed to 10 ng/L EE2 had poor reproductive success, their ovaries showed normal development and oogenesis. These data are relevant to observations of intersex in feral fish populations. Although intersex in male fish may be an indicator of exposure to estrogenic compounds, it appears that the presence of oocytes in testicular tissue may not directly impact the reproductive capability of the male fish. However, it is clear that concentrations of endocrine disrupting chemicals (EDCs) that cause gonadal intersex are sufficient to reduce reproductive performance.

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The estrogenic isoflavone compound genistein recently has been found in the effluents of sewage treatment plants and pulp mills, and the related compound equol has been detected in the runoff from agricultural fields treated with hog manure. Waterborne exposures of Japanese medaka (Oryzias latipes) to equol from soon after hatch to approximately 100 days posthatch induced gonadal intersex (i.e., testis-ova) in males at incidences of 10 and 87% in equol treatments of 0.4 and 0.8 micro g/L, respectively. Exposure to the highest test concentration of genistein, 1,000 micro g/L, also caused a low incidence (i.e., 12%) of gonadal intersex in male medaka. The ovaries of female medaka from both equol and genistein treatments showed delayed oocyte maturation, atretic oocytes, an enlarged ovarian lumen, proliferation of somatic stromal tissue, and primordial germ cells; responses were concentration dependent. Alterations to externally visible secondary sex characteristics occurred in medaka exposed to both equol and genistein. In treatments with 1,000 micro g/L genistein, 72% of male medaka (as identified by the gonadal phenotype) showed feminized secondary sex characteristics. Gonadal intersex and alterations to secondary sex characteristics have been noted in several fish populations around the world. This laboratory study indicates that isoflavone compounds should be considered candidate estrogenic compounds that may be involved in the alteration of sexual development in feral fish populations.

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This study was focused on determining the effects of exposure to antiandrogens on the gonadal development of Japanese medaka (Oryzias latipes). Test compounds included the fungicide, vinclozolin and the clinical antiandrogen, cyproterone acetate. Newly hatched medaka were exposed to aqueous solutions of vinclozolin (2500 microg/l) and the vinclozolin fungicide formulation, Ronilan (1000 and 5000 microg/l) and cyproterone acetate (1 and 10 microg/l), for 3 months. Histological evaluation of the gonadal tissues of exposed fish indicated that the 5000 microg/l concentration of the vinclozolin formulation (Ronilan) induced a low incidence of intersex (i.e. testis-ova) and the 2500 microg/l concentration of vinclozolin-affected spermatogenesis in males. Also, the vinclozolin treatments induced moderate ovarian atresia. Cyproterone acetate also induced a low incidence of testis-ova, but in contrast to the vinclozolin treatment the amount of ovarian tissue in the testis-ova was equal to or greater than the amount of testicular tissue. In the cyproterone acetate treatments, both oogenesis and spermatogenesis were moderately inhibited at all test concentrations. The results of this study indicate that antiandrogens have the potential to alter testicular development and gametogenesis in fish. However, research is needed to determine the mechanisms by which antiandrogens affect fish.

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We elucidated the effects of bisphenol A (BPA) on the reproductive capacity (fecundity and fertility) and estrogenic response of adult Japanese medaka (Oryzias latipes) and studied the transgenerational effects (F, generation growth and sex) of this agent on the F1 offspring. Eight mating pairs of medaka were exposed to BPA for three weeks at a measured concentration of 837, 1,720, or 3,120 microg/L. Throughout the exposure period, we observed no reduction of fecundity and fertility in any treatment group and no reduction in survival and growth or any imbalance of the sex ratio of the offspring. However, histologic examination revealed testis-ova in 13% of the males in the 837-microg/L treatment group, 86% of those in the 1,720-microg/L group, and 50% of those that received 3,120 microg BPA/L. In addition, concentrations of hepatic vitellogenin were increased significantly in males treated with BPA at 3,120 microg/L (p = 0.0148). These results suggest that BPA is a weakly estrogenic compound that has the potential to alter sexual development and induce vitellogenin in male medaka but that has no adverse effect on reproductive success.

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From various environmental pollutants studied so far, specific effects on the reproductive system of small fish species Poecilia reticulata (guppy) and Oryzias latipes (medaka) were noted in the case of beta-hexachlorocyclohexane (induction of vitellogenesis and hermaphroditism, both indicative of estrogenic activity; 32 micrograms/l) and methyl mercury (impaired spermatogenesis; 1.8 micrograms/l). The latter effect was attributed to a disturbance of mitosis.

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