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Relationship: 2349


A descriptive phrase which clearly defines the two KEs being considered and the sequential relationship between them (i.e., which is upstream, and which is downstream). More help

Agonism, Androgen receptor leads to Increased, Male Biased Sex Ratio

Upstream event
The causing Key Event (KE) in a Key Event Relationship (KER). More help
Downstream event
The responding Key Event (KE) in a Key Event Relationship (KER). More help

Key Event Relationship Overview

The utility of AOPs for regulatory application is defined, to a large extent, by the confidence and precision with which they facilitate extrapolation of data measured at low levels of biological organisation to predicted outcomes at higher levels of organisation and the extent to which they can link biological effect measurements to their specific causes.Within the AOP framework, the predictive relationships that facilitate extrapolation are represented by the KERs. Consequently, the overall WoE for an AOP is a reflection in part, of the level of confidence in the underlying series of KERs it encompasses. Therefore, describing the KERs in an AOP involves assembling and organising the types of information and evidence that defines the scientific basis for inferring the probable change in, or state of, a downstream KE from the known or measured state of an upstream KE. More help

AOPs Referencing Relationship

AOP Name Adjacency Weight of Evidence Quantitative Understanding Point of Contact Author Status OECD Status
Androgen receptor agonism leading to male-biased sex ratio non-adjacent Evgeniia Kazymova (send email) Open for citation & comment WPHA/WNT Endorsed

Taxonomic Applicability

Latin or common names of a species or broader taxonomic grouping (e.g., class, order, family) that help to define the biological applicability domain of the KER.In general, this will be dictated by the more restrictive of the two KEs being linked together by the KER.  More help
Term Scientific Term Evidence Link
zebrafish Danio rerio High NCBI
medaka Oryzias latipes Moderate NCBI
Chinook salmon Oncorhynchus tshawytscha Moderate NCBI
fathead minnow Pimephales promelas Moderate NCBI

Sex Applicability

An indication of the the relevant sex for this KER. More help
Sex Evidence
Unspecific High

Life Stage Applicability

An indication of the the relevant life stage(s) for this KER.  More help
Term Evidence
Development High

Key Event Relationship Description

Provides a concise overview of the information given below as well as addressing details that aren’t inherent in the description of the KEs themselves. More help

This key event relationship (KER) links  androgen receptor agonism in teleost fish during gonadogenesis to a male-biased sex ratio in a population. Sex determination in teleost fishes is highly plastic; it can be genetically or environmentally influenced. Species with environmentally-based sex determination in particular can be very sensitive to some steroid hormones during the period of differentiation. Exogenous hormones are of ecological concern because they have the potential to alter gonad development and sex differentiation. Activation of the androgen receptor (AR) by endogenous androgens plays a crucial role in normal sex differentiation, sexual maturation, and spermatogenesis in vertebrates and inappropriate signaling by exogenous AR agonists can disrupt theses processes. For example, studies have shown that during early development in some teleost species, exposure to androgenic steroids can induce complete gonadal sex inversion, resuting in increased differentiation to testis. This will result in a male-biased sex ratio in a population.

Evidence Collection Strategy

Include a description of the approach for identification and assembly of the evidence base for the KER. For evidence identification, include, for example, a description of the sources and dates of information consulted including expert knowledge, databases searched and associated search terms/strings.  Include also a description of study screening criteria and methodology, study quality assessment considerations, the data extraction strategy and links to any repositories/databases of relevant references.Tabular summaries and links to relevant supporting documentation are encouraged, wherever possible. More help

Evidence Supporting this KER

Addresses the scientific evidence supporting KERs in an AOP setting the stage for overall assessment of the AOP. More help

See below.

Biological Plausibility
Addresses the biological rationale for a connection between KEupstream and KEdownstream.  This field can also incorporate additional mechanistic details that help inform the relationship between KEs, this is useful when it is not practical/pragmatic to represent these details as separate KEs due to the difficulty or relative infrequency with which it is likely to be measured.   More help

The biological plausibility linking AR activation to a male-biased sex ratio in a population is very strong. Actions of androgens are mediated by the AR, a ligand-dependent transcription factors (Hossain et al., 2008). Steroidal androgens act by entering the cell and forming a complex with the AR, resulting in conformational change (Bohen et al., 1995; Pratt and Toft, 1997). The ligand-AR complex is translocated to the nucleus where it binds to specific short DNA sequences thereby activating transcripton of androgen regulated genes (Harbott et al., 2009). During sexual development, endogenous androgen can therefore induce the upregulation of many genes involved in the male developmental pathway, including gonad development/differentiation.

If the conditions that favor a male developmental pathway (in this case, exposure to AR agonsts) overlap with the critical period of sex differentiation in a given population, it is reasonable that more phenotypic males will be produced (Orn et al., 2003; Seki et al., 2004; Bogers et al., 2006; Morthorst et al., 2010; Baumann et al., 2014; Golan & Levavi-Sivian 2014). Therefore, androgen exposure for repeated or prolonged periods of time conceptually will result in a male-biased population.

Uncertainties and Inconsistencies
Addresses inconsistencies or uncertainties in the relationship including the identification of experimental details that may explain apparent deviations from the expected patterns of concordance. More help

Some studies with sexually undifferentiated channel catfish have demonstrated that oral administration of androgens (methyltestosterone, 17a-ethynyltestosterone, dihydrotestosterone) during development can produce all female populations (Goudie et al., 1983; Davis et al., 1990, 1992). In some instances this could be due to the use of aromatizable androgens such as methyltestosterone that can lead both to masculinization and feminization of fish (e.g., Piferrer et al. 1993), due to conversion of the androgen to its corresponding estrogen analogue (i.e., methylestradiol; Hornung et al. 2004 ). In the cases of non-aromatizable androgens (e.g., dihydrotestosterone) that have been reported to feminize fish exposed during early development, the mechanism underlying this is uncertain, but plausibly could involve activation of the estrogen receptor, which is known to interact with a variety of steroids, including androgens at comparatively high test concentrations.

Also, as noted below, it is uncertain as to the full range of species this key event relationship might be applicable due to susbtantial taxonomic variation in the role that steroid signaling plays in gonadal differentiation.

Known modulating factors

This table captures specific information on the MF, its properties, how it affects the KER and respective references.1.) What is the modulating factor? Name the factor for which solid evidence exists that it influences this KER. Examples: age, sex, genotype, diet 2.) Details of this modulating factor. Specify which features of this MF are relevant for this KER. Examples: a specific age range or a specific biological age (defined by...); a specific gene mutation or variant, a specific nutrient (deficit or surplus); a sex-specific homone; a certain threshold value (e.g. serum levels of a chemical above...) 3.) Description of how this modulating factor affects this KER. Describe the provable modification of the KER (also quantitatively, if known). Examples: increase or decrease of the magnitude of effect (by a factor of...); change of the time-course of the effect (onset delay by...); alteration of the probability of the effect; increase or decrease of the sensitivity of the downstream effect (by a factor of...) 4.) Provision of supporting scientific evidence for an effect of this MF on this KER. Give a list of references.  More help

There are almost certainly many factors that could modulate this KER, but a systematic description of these is not currently possible.

Modulating Factor (MF) MF Specification Effect(s) on the KER Reference(s)
Response-response Relationship
Provides sources of data that define the response-response relationships between the KEs.  More help

Not applicable.

Information regarding the approximate time-scale of the changes in KEdownstream relative to changes in KEupstream (i.e., do effects on KEdownstream lag those on KEupstream by seconds, minutes, hours, or days?). More help

The timeframe for differentiation of the bipotential gonad and subsequent phenotypic expression of sex is species-dependent occurring, for example, over the course of days to weeks in most fishes. However, this period of time could be substantially longer in long-lived species.

Known Feedforward/Feedback loops influencing this KER
Define whether there are known positive or negative feedback mechanisms involved and what is understood about their time-course and homeostatic limits. More help

None known.

Domain of Applicability

A free-text section of the KER description that the developers can use to explain their rationale for the taxonomic, life stage, or sex applicability structured terms. More help

Life Stage

The life stage applicable to this KER is developing embryos and juveniles prior to- or during the gonadal developmental stage. This KER is not applicable to sexually differentiated adults.


The molecular initiating event for this KER occurs prior to gonad differentiation. Therefore, this AOP is only applicable to sexually undifferentiated individuals.


Most evidence for this KER is derived from fish in the class Osteichthyes. Both phylogenetic analysis and evaluation of protein sequence conservation via SeqAPASS ( has shown that the structure of the AR is well conserved among most vertebrates (e.g., LaLone et al. 2018). This KER is not expected to apply to mammals, birds, or other vertebrates with genetic sex determination. However, it may be applicable to fishes, amphibians, and reptiles with environmentally-dependent sex determination, although outcomes may differ across physiologically different taxa. The present KER is not considered relevant to Agnathans since the AR appears not to be present in jawless fishes (Thornton 2001; Hossain et al 2008).


List of the literature that was cited for this KER description. More help

Baumann, L., Holbech, H., Keiter, S., Kinnberg, K. L., Knörr, S., Nagel, T., & Braunbeck, T. (2013). The maturity index as a tool to facilitate the interpretation of changes in vitellogenin production and sex ratio in the Fish Sexual Development Test. Aquatic toxicology (Amsterdam, Netherlands), 128-129, 34–42.

Baumann, L., Knörr, S., Keiter, S., Nagel, T., Rehberger, K., Volz, S., Oberrauch, S., Schiller, V., Fenske, M., Holbech, H., Segner, H., & Braunbeck, T. (2014). Persistence of endocrine disruption in zebrafish (Danio rerio) after discontinued exposure to the androgen 17β-trenbolone. Environmental toxicology and chemistry, 33(11), 2488–2496.

Bogers, R., De Vries-Buitenweg, S., Van Gils, M., Baltussen, E., Hargreaves, A., van de Waart, B., De Roode, D., Legler, J., & Murk, A. (2006). Development of chronic tests for endocrine active chemicals. Part 2: an extended fish early-life stage test with an androgenic chemical in the fathead minnow (Pimephales promelas). Aquatic toxicology (Amsterdam, Netherlands), 80(2), 119–130.

Bohen, S. P., Kralli, A., & Yamamoto, K. R. (1995). Hold 'em and fold 'em: chaperones and signal transduction. Science (New York, N.Y.)268(5215), 1303–1304.

Davis, K. B., Goudie, C. A., Simco, B. A., Tiersch, T. R., & Carmichael, G. J. (1992). Influence of dihydrotestosterone on sex determination in channel catfish and blue catfish: period of developmental sensitivity. General and comparative endocrinology, 86(1), 147–151.

Davis, K. B., Simco, B. A., Goudie, C. A., Parker, N. C., Cauldwell, W., & Snellgrove, R. (1990). Hormonal sex manipulation and evidence for female homogamety in channel catfish. General and comparative endocrinology, 78(2), 218–223.

Galvez, J., Mazik, P., Phelps, R., Mulvaney, D. (1995) Masculinization of Channel Catfish Ictalurus punctatus by Oral Administration of Trenbolone Acetate. World Aquaculture Society, 26(4), 378-383.

Goudie, C., Redner, B., Simco, B. Davis, K. (1983), Feminization of Channel Catfish by Oral Administration of Steroid Sex Hormones. Transactions of the American Fisheries Society, 112: 670-672.<670:FOCCBO>2.0.CO;2

Harbott, L. K., Burmeister, S. S., White, R. B., Vagell, M., & Fernald, R. D. (2007). Androgen receptors in a cichlid fish, Astatotilapia burtoni: structure, localization, and expression levels. The Journal of comparative neurology, 504(1), 57–73.

Hornung, M. W., Jensen, K. M., Korte, J. J., Kahl, M. D., Durhan, E. J., Denny, J. S., Henry, T. R., & Ankley, G. T. (2004). Mechanistic basis for estrogenic effects in fathead minnow (Pimephales promelas) following exposure to the androgen 17alpha-methyltestosterone: conversion of 17alpha-methyltestosterone to 17alpha-methylestradiol. Aquatic toxicology (Amsterdam, Netherlands)66(1), 15–23.

Hossain, M. S., Larsson, A., Scherbak, N., Olsson, P. E., & Orban, L. (2008). Zebrafish androgen receptor: isolation, molecular, and biochemical characterization. Biology of reproduction, 78(2), 361–369.

LaLone, C.A., D.L. Villeneuve, J.A. Doering, B.R. Blackwell, T.R. Transue, C.W. Simmons, J. Swintek, S.J. Degitz, A.J. Williams and G.T. Ankley. 2018. Evidence for cross-species extrapolation of mammalian-based high-throughput screening assay results. Environ. Sci. Technol. 52, 13960-13971.

Larsen, M. G., & Baatrup, E. (2010). Functional behavior and reproduction in androgenic sex reversed zebrafish (Danio rerio). Environmental toxicology and chemistry, 29(8), 1828–1833.

Morthorst, J. E., Holbech, H., & Bjerregaard, P. (2010). Trenbolone causes irreversible masculinization of zebrafish at environmentally relevant concentrations. Aquatic toxicology (Amsterdam, Netherlands), 98(4), 336–343.

Pandian, T.J. & Sheela S.G. 1995. Hormonal induction of sex reversal in fish. Aquaculture 138, 1-22.

Pratt, W. B., & Toft, D. O. (1997). Steroid receptor interactions with heat shock protein and immunophilin chaperones. Endocrine reviews18(3), 306–360.

Örn, S., Holbech, H., & Norrgren, L. (2016). Sexual disruption in zebrafish (Danio rerio) exposed to mixtures of 17α-ethinylestradiol and 17β-trenbolone. Environmental toxicology and pharmacology, 41, 225–231.

Orn, S., Holbech, H., Madsen, T. H., Norrgren, L., & Petersen, G. I. (2003). Gonad development and vitellogenin production in zebrafish (Danio rerio) exposed to ethinylestradiol and methyltestosterone. Aquatic toxicology (Amsterdam, Netherlands), 65(4), 397–411.

Orn, S., Yamani, S., & Norrgren, L. (2006). Comparison of vitellogenin induction, sex ratio, and gonad morphology between zebrafish and Japanese medaka after exposure to 17alpha-ethinylestradiol and 17beta-trenbolone. Archives of environmental contamination and toxicology, 51(2), 237–243.

Piferrer, F., Baker, I. J., & Donaldson, E. M. (1993). Effects of natural, synthetic, aromatizable, and nonaromatizable androgens in inducing male sex differentiation in genotypic female chinook salmon (Oncorhynchus tshawytscha). General and comparative endocrinology, 91(1), 59–65.

Seki, M., Yokota, H., Matsubara, H., Maeda, M., Tadokoro, H., & Kobayashi, K. (2004). Fish full life-cycle testing for androgen methyltestosterone on medaka (Oryzias latipes). Environmental toxicology and chemistry, 23(3), 774–781.

Shi, W. J., Jiang, Y. X., Huang, G. Y., Zhao, J. L., Zhang, J. N., Liu, Y. S., Xie, L. T., & Ying, G. G. (2018). Dydrogesterone Causes Male Bias and Accelerates Sperm Maturation in Zebrafish ( Danio rerio). Environmental science & technology, 52(15), 8903–8911.

Thornton J. W. (2001). Evolution of vertebrate steroid receptors from an ancestral estrogen receptor by ligand exploitation and serial genome expansions. Proceedings of the National Academy of Sciences of the United States of America, 98(10), 5671–5676.