Mode of sexual differentiation and its influence on the relative sensitivity of the fathead minnow and zebrafish in the fish sexual development test

Exogenous treatment of fish with natural sex hormones and their mimics has been shown to influencegonadal differentiation resulting in biased phenotypic sex-ratios. This has lead to the development ofthe Fish Sexual Development Test (FSDT) as a method for the detection of endocrine active chemicals.Proposed test organisms include the medaka, zebrafish (ZF) and stickleback, although the guideline alsoallows for inclusion of species such as the fathead minnow (FHM), provided the test duration allows forsufficient sexual differentiation. However, although the processes underlying sexual differentiation areknown to differ for each of these species, it is not known how, or if, these differences would influence theresults of the FSDT. In the experiments reported here, responses of the ZF and FHM to prochloraz, a sterolbiosynthesis inhibitor and androgen antagonist, were characterized and compared. Exposure to 320 g/Lof prochloraz, from embryo until 60 (ZF) or 95–125 (FHM) days post hatch inhibited somatic growth ofboth species, but while a negative impact on ZF larval survival was observed (LOEC 32 g/L) there was noevidence for an effect on FHM larval survival. Prochloraz influenced sexual differentiation in both speciesby decreasing the proportion of females (LOEC 100 g/L (ZF), 320 g/L (FHM)) and delaying completionof sexual differentiation; manifest as an increased incidence of ovotestis in the ZF (LOEC 100 g/L) and asan increased number of fish with undifferentiated gonads in the FHM (LOEC 320 g/L). However, whileexposure to 320 g/L prochloraz delayed maturation of the differentiated FHM testis, there was no sucheffect in the ZF. These results demonstrate that the different strategy of sexual differentiation in the ZFand FHM influences the profile of responses of their gonads to the masculinising effects of prochloraz,but does not affect their overall sensitivity.