The maturity index as a tool to facilitate the interpretation of changes in vitellogenin production and sex ratio in the Fish Sexual Development Test.

In July 2011, the Fish Sexual Development Test (FSDT) has officially been adopted as OECD test guideline 234 for the detection of endocrine disrupting chemicals (EDCs). Sex ratio and vitellogenin (VTG) induction are the mandatory endocrine endpoints within this test, whereas gonad staging is only included as an option. In the present study, five FSDTs with zebrafish (Danio rerio) were conducted with EDCs with different modes of action (17α-ethinylestradiol, dihydrotestosterone, 17β-trenbolone, prochloraz and 4-tert-pentylphenol). Results document that not only sex ratio and VTG production of the exposed fish were massively affected, but also gonad maturation. As a novel approach for the quantification of gonad maturation in zebrafish, the maturity index was developed to allow not only an improved assessment of dose-dependent EDC-related effects on gonad maturation, but also statistical analysis of histological data. VTG induction and maturity index showed an excellent correlation for all five EDCs tested. Most importantly, the maturity index often helped to find appropriate interpretations for results that seemed contradictory at first sight. Results show that histological analyses and their predictive power for population fitness are currently underestimated and should become a standard component in the evaluation of potential EDCs.