Aop: 376


Each AOP should be given a descriptive title that takes the form “MIE leading to AO”. For example, “Aromatase inhibition [MIE] leading to reproductive dysfunction [AO]” or “Thyroperoxidase inhibition [MIE] leading to decreased cognitive function [AO]”. In cases where the MIE is unknown or undefined, the earliest known KE in the chain (i.e., furthest upstream) should be used in lieu of the MIE and it should be made clear that the stated event is a KE and not the MIE. More help

Androgen receptor agonism leading to male-biased sex ratio

Short name
A short name should also be provided that succinctly summarises the information from the title. This name should not exceed 90 characters. More help
AR agonism leading to male-biased sex ratio

Graphical Representation

A graphical summary of the AOP listing all the KEs in sequence, including the MIE (if known) and AO, and the pair-wise relationships (links or KERs) between those KEs should be provided. This is easily achieved using the standard box and arrow AOP diagram (see this page for example). The graphical summary is prepared and uploaded by the user (templates are available) and is often included as part of the proposal when AOP development projects are submitted to the OECD AOP Development Workplan. The graphical representation or AOP diagram provides a useful and concise overview of the KEs that are included in the AOP, and the sequence in which they are linked together. This can aid both the process of development, as well as review and use of the AOP (for more information please see page 19 of the Users' Handbook).If you already have a graphical representation of your AOP in electronic format, simple save it in a standard image format (e.g. jpeg, png) then click ‘Choose File’ under the “Graphical Representation” heading, which is part of the Summary of the AOP section, to select the file that you have just edited. Files must be in jpeg, jpg, gif, png, or bmp format. Click ‘Upload’ to upload the file. You should see the AOP page with the image displayed under the “Graphical Representation” heading. To remove a graphical representation file, click 'Remove' and then click 'OK.'  Your graphic should no longer be displayed on the AOP page. If you do not have a graphical representation of your AOP in electronic format, a template is available to assist you.  Under “Summary of the AOP”, under the “Graphical Representation” heading click on the link “Click to download template for graphical representation.” A Powerpoint template file should download via the default download mechanism for your browser. Click to open this file; it contains a Powerpoint template for an AOP diagram and instructions for editing and saving the diagram. Be sure to save the diagram as jpeg, jpg, gif, png, or bmp format. Once the diagram is edited to its final state, upload the image file as described above. More help


List the name and affiliation information of the individual(s)/organisation(s) that created/developed the AOP. In the context of the OECD AOP Development Workplan, this would typically be the individuals and organisation that submitted an AOP development proposal to the EAGMST. Significant contributors to the AOP should also be listed. A corresponding author with contact information may be provided here. This author does not need an account on the AOP-KB and can be distinct from the point of contact below. The list of authors will be included in any snapshot made from an AOP. More help

Kelvin J. Santana Rodriguez, Oak Ridge Institute for Science and Education, U.S. Environmental Protection Agency, Great Lakes Ecology Divison, Duluth, MN

Point of Contact

Indicate the point of contact for the AOP-KB entry itself. This person is responsible for managing the AOP entry in the AOP-KB and controls write access to the page by defining the contributors as described below. Clicking on the name will allow any wiki user to correspond with the point of contact via the email address associated with their user profile in the AOP-KB. This person can be the same as the corresponding author listed in the authors section but isn’t required to be. In cases where the individuals are different, the corresponding author would be the appropriate person to contact for scientific issues whereas the point of contact would be the appropriate person to contact about technical issues with the AOP-KB entry itself. Corresponding authors and the point of contact are encouraged to monitor comments on their AOPs and develop or coordinate responses as appropriate.  More help
Dan Villeneuve   (email point of contact)


List user names of all  authors contributing to or revising pages in the AOP-KB that are linked to the AOP description. This information is mainly used to control write access to the AOP page and is controlled by the Point of Contact.  More help
  • Dan Villeneuve
  • Kelvin Santana Rodriguez


The status section is used to provide AOP-KB users with information concerning how actively the AOP page is being developed, what type of use or input the authors feel comfortable with given the current level of development, and whether it is part of the OECD AOP Development Workplan and has been reviewed and/or endorsed. “Author Status” is an author defined field that is designated by selecting one of several options from a drop-down menu (Table 3). The “Author Status” field should be changed by the point of contact, as appropriate, as AOP development proceeds. See page 22 of the User Handbook for definitions of selection options. More help
Author status OECD status OECD project SAAOP status
Open for citation & comment
This AOP was last modified on September 18, 2021 11:29
The date the AOP was last modified is automatically tracked by the AOP-KB. The date modified field can be used to evaluate how actively the page is under development and how recently the version within the AOP-Wiki has been updated compared to any snapshots that were generated. More help

Revision dates for related pages

Page Revision Date/Time
Agonism, Androgen receptor March 20, 2017 17:44
Increased, Differentiation to Testis September 15, 2020 14:01
Increased, Male Biased Sex Ratio August 07, 2020 18:17
Decrease, Population trajectory September 03, 2021 11:24
Agonism, Androgen receptor leads to Increased, Differentiation to Testis April 12, 2021 10:24
Agonism, Androgen receptor leads to Increased, Male Biased Sex Ratio April 12, 2021 09:31
Increased, Differentiation to Testis leads to Increased, Male Biased Sex Ratio August 23, 2021 11:55
Increased, Male Biased Sex Ratio leads to Decrease, Population trajectory September 17, 2020 06:34
17beta-Trenbolone November 29, 2016 18:42
Chemical:33664 17-Methyltestosterone March 23, 2021 13:34
5alpha-Dihydrotestosterone March 14, 2017 12:44
Methyldihydrotestosterone May 07, 2021 15:36
11-Keto-testosterone May 07, 2021 15:36


In the abstract section, authors should provide a concise and informative summation of the AOP under development that can stand-alone from the AOP page. Abstracts should typically be 200-400 words in length (similar to an abstract for a journal article). Suggested content for the abstract includes the following: The background/purpose for initiation of the AOP’s development (if there was a specific intent) A brief description of the MIE, AO, and/or major KEs that define the pathway A short summation of the overall WoE supporting the AOP and identification of major knowledge gaps (if any) If a brief statement about how the AOP may be applied (optional). The aim is to capture the highlights of the AOP and its potential scientific and regulatory relevance More help

This adverse outcome pathway links  androgen receptor agonism in teleost fish during gonadogenesis to male-biased sexual differentiation and successively, reduced population sustainability. Sex determination in teleost fishes is highly plastic; it can be genetically or environmentally driven or both depending on species. Species with environmentally-based sex determination in particular can be very sensitive to environmental pollutants during the period of differentiation. Exogenous hormones are of ecological concern because they have the potential to alter gonad development and sex differentiation. Androgens play a crucial role in sex differentiation, sexual maturation, and spermatogenesis in vertebrates and their modes of action are mediated via androgen receptors (ARs). Like many steroid hormones, androgens acts by entering the cell and forming a complex with a its hormone receptor allowing it to enter the nucleus where it can bind to specific short DNA sequences (Androgen Responsive Elements) and serve as transcription factors of androgen mediated genes involved in the male differentiation pathway. Many studies have shown that administration of androgens during early development in some teleost species can strongly bias sexual differentiation to testes. This can result in male biased sex ratios which, if sustained over multiple generations, can impact population fitness and long term viability.

Background (optional)

This optional subsection should be used to provide background information for AOP reviewers and users that is considered helpful in understanding the biology underlying the AOP and the motivation for its development. The background should NOT provide an overview of the AOP, its KEs or KERs, which are captured in more detail below. Examples of potential uses of the optional background section are listed on pages 24-25 of the User Handbook. More help

Summary of the AOP

This section is for information that describes the overall AOP. The information described in section 1 is entered on the upper portion of an AOP page within the AOP-Wiki. This is where some background information may be provided, the structure of the AOP is described, and the KEs and KERs are listed. More help


Molecular Initiating Events (MIE)
An MIE is a specialised KE that represents the beginning (point of interaction between a stressor and the biological system) of an AOP. More help
Key Events (KE)
This table summarises all of the KEs of the AOP. This table is populated in the AOP-Wiki as KEs are added to the AOP. Each table entry acts as a link to the individual KE description page.  More help
Adverse Outcomes (AO)
An AO is a specialised KE that represents the end (an adverse outcome of regulatory significance) of an AOP.  More help
Sequence Type Event ID Title Short name
1 MIE 25 Agonism, Androgen receptor Agonism, Androgen receptor
2 KE 1790 Increased, Differentiation to Testis Increased, Differentiation to Testis
3 KE 1791 Increased, Male Biased Sex Ratio Increased, Male Biased Sex Ratio
4 AO 360 Decrease, Population trajectory Decrease, Population trajectory

Relationships Between Two Key Events (Including MIEs and AOs)

This table summarises all of the KERs of the AOP and is populated in the AOP-Wiki as KERs are added to the AOP. Each table entry acts as a link to the individual KER description page.To add a key event relationship click on either Add relationship: events adjacent in sequence or Add relationship: events non-adjacent in sequence.For example, if the intended sequence of KEs for the AOP is [KE1 > KE2 > KE3 > KE4]; relationships between KE1 and KE2; KE2 and KE3; and KE3 and KE4 would be defined using the add relationship: events adjacent in sequence button.  Relationships between KE1 and KE3; KE2 and KE4; or KE1 and KE4, for example, should be created using the add relationship: events non-adjacent button. This helps to both organize the table with regard to which KERs define the main sequence of KEs and those that provide additional supporting evidence and aids computational analysis of AOP networks, where non-adjacent KERs can result in artifacts (see Villeneuve et al. 2018; DOI: 10.1002/etc.4124).After clicking either option, the user will be brought to a new page entitled ‘Add Relationship to AOP.’ To create a new relationship, select an upstream event and a downstream event from the drop down menus. The KER will automatically be designated as either adjacent or non-adjacent depending on the button selected. The fields “Evidence” and “Quantitative understanding” can be selected from the drop-down options at the time of creation of the relationship, or can be added later. See the Users Handbook, page 52 (Assess Evidence Supporting All KERs for guiding questions, etc.).  Click ‘Create [adjacent/non-adjacent] relationship.’  The new relationship should be listed on the AOP page under the heading “Relationships Between Two Key Events (Including MIEs and AOs)”. To edit a key event relationship, click ‘Edit’ next to the name of the relationship you wish to edit. The user will be directed to an Editing Relationship page where they can edit the Evidence, and Quantitative Understanding fields using the drop down menus. Once finished editing, click ‘Update [adjacent/non-adjacent] relationship’ to update these fields and return to the AOP page.To remove a key event relationship to an AOP page, under Summary of the AOP, next to “Relationships Between Two Key Events (Including MIEs and AOs)” click ‘Remove’ The relationship should no longer be listed on the AOP page under the heading “Relationships Between Two Key Events (Including MIEs and AOs)”. More help

Network View

The AOP-Wiki automatically generates a network view of the AOP. This network graphic is based on the information provided in the MIE, KEs, AO, KERs and WoE summary tables. The width of the edges representing the KERs is determined by its WoE confidence level, with thicker lines representing higher degrees of confidence. This network view also shows which KEs are shared with other AOPs. More help


The stressor field is a structured data field that can be used to annotate an AOP with standardised terms identifying stressors known to trigger the MIE/AOP. Most often these are chemical names selected from established chemical ontologies. However, depending on the information available, this could also refer to chemical categories (i.e., groups of chemicals with defined structural features known to trigger the MIE). It can also include non-chemical stressors such as genetic or environmental factors. Although AOPs themselves are not chemical or stressor-specific, linking to stressor terms known to be relevant to different AOPs can aid users in searching for AOPs that may be relevant to a given stressor. More help

Life Stage Applicability

Identify the life stage for which the KE is known to be applicable. More help
Life stage Evidence
Development High

Taxonomic Applicability

Latin or common names of a species or broader taxonomic grouping (e.g., class, order, family) can be selected. In many cases, individual species identified in these structured fields will be those for which the strongest evidence used in constructing the AOP was available in relation to this KE. More help
Term Scientific Term Evidence Link
zebra fish Danio rerio High NCBI
medaka Oryzias latipes Low NCBI
fathead minnow Pimephales promelas Low NCBI
channel catfish Ictalurus punctatus Low NCBI
Oreochromis niloticus Oreochromis niloticus Low NCBI
Chinook salmon Oncorhynchus tshawytscha Low NCBI

Sex Applicability

The authors must select from one of the following: Male, female, mixed, asexual, third gender, hermaphrodite, or unspecific. More help
Sex Evidence
Unspecific High

Overall Assessment of the AOP

This section addresses the relevant biological domain of applicability (i.e., in terms of taxa, sex, life stage, etc.) and WoE for the overall AOP as a basis to consider appropriate regulatory application (e.g., priority setting, testing strategies or risk assessment). The goal of the overall assessment is to provide a high level synthesis and overview of the relative confidence in the AOP and where the significant gaps or weaknesses are (if they exist). Users or readers can drill down into the finer details captured in the KE and KER descriptions, and/or associated summary tables, as appropriate to their needs.Assessment of the AOP is organised into a number of steps. Guidance on pages 59-62 of the User Handbook is available to facilitate assignment of categories of high, moderate, or low confidence for each consideration. While it is not necessary to repeat lengthy text that appears elsewhere in the AOP description (or related KE and KER descriptions), a brief explanation or rationale for the selection of high, moderate, or low confidence should be made. More help

See details below.

Domain of Applicability

The relevant biological domain(s) of applicability in terms of sex, life-stage, taxa, and other aspects of biological context are defined in this section. Biological domain of applicability is informed by the “Description” and “Biological Domain of Applicability” sections of each KE and KER description (see sections 2G and 3E for details). In essence the taxa/life-stage/sex applicability is defined based on the groups of organisms for which the measurements represented by the KEs can feasibly be measured and the functional and regulatory relationships represented by the KERs are operative.The relevant biological domain of applicability of the AOP as a whole will nearly always be defined based on the most narrowly restricted of its KEs and KERs. For example, if most of the KEs apply to either sex, but one is relevant to females only, the biological domain of applicability of the AOP as a whole would be limited to females. While much of the detail defining the domain of applicability may be found in the individual KE and KER descriptions, the rationale for defining the relevant biological domain of applicability of the overall AOP should be briefly summarised on the AOP page. More help

Life Stage

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


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


The taxonomic applicability of this AOP is the class Osteichthyes. However phylogenetic analysis has shown that the ar genes appear to be specific to jawed vertebrates (Gnathostomata), since no ar gene has been reported from Agnatha (Thornton 2001; Hossain et al 2008). Therefore, because all key events in the present AOP can be applicable to most non-mammalian vertebrates, it is probable that this AOP could be relevant to amphibians, reptiles and birds as well.  Though, the outcomes might differ due to species-specific differences. 

Essentiality of the Key Events

An important aspect of assessing an AOP is evaluating the essentiality of its KEs. The essentiality of KEs can only be assessed relative to the impact of manipulation of a given KE (e.g., experimentally blocking or exacerbating the event) on the downstream sequence of KEs defined for the AOP. Consequently evidence supporting essentiality is assembled on the AOP page, rather than on the independent KE pages that are meant to stand-alone as modular units without reference to other KEs in the sequence.The nature of experimental evidence that is relevant to assessing essentiality relates to the impact on downstream KEs and the AO if upstream KEs are prevented or modified. This includes: Direct evidence: directly measured experimental support that blocking or preventing a KE prevents or impacts downstream KEs in the pathway in the expected fashion. Indirect evidence: evidence that modulation or attenuation in the magnitude of impact on a specific KE (increased effect or decreased effect) is associated with corresponding changes (increases or decreases) in the magnitude or frequency of one or more downstream KEs.When assembling the support for essentiality of the KEs, authors should organise relevant data in a tabular format. The objective is to summarise briefly the nature and numbers of investigations in which the essentiality of KEs has been experimentally explored either directly or indirectly. See pages 50-51 in the User Handbook for further definitions and clarifications.  More help

Support for the essentiality of several of the Key Events in the AOP was provided mainly in combination of in vivo and in vitro studies of androgen receptor agonist and antagonist exposures during the critical period of sexual differentiation.

Golan & Levavi-Sivian 2014 exposed genetically female Nile tilapia (Oreochromis niloticus) to 17-a-methyltestosterone (MT) and dihydrotestosterone (DHT), two well-known androgen receptor agonisms that induces the male differentiation pathway and a dose dependent male biased sex ratio (downstream key events). However, when these are combined with androgen antagonist Flutamide, the sex inversion potency of androgen is reduced in a dose-dependent manner. The decrease in sex inversion efficiency caused by flutamide is due to the direct blocking of the androgen binding to its androgen receptor. Therefore, this suggest that androgen receptor agonism ligand binding is required for the subsequent key events to occur.

Crowder et al., 2018 generated zebrafish with a mutation in the ar gene (aruab105/105) and the resulting mutants developed ovaries and displayed female secondary sexual characteristics. The small percentage of mutants that developed as males displayed female secondary sexual characteristics with structurally disorganized testes, and were unable to produce normal levels of sperm. This demonstrates that the AR is required for proper testis development and fertility. This supports the essentiality for the androgen receptor agonism for the testis differentiation pathway to proceed.

In a similar study with zebrafish, Yu et al. 2018 generate ar gene mutant line using CRISPR/Cas9 technology. The resulting showed that the number of female offspring was increased and the resulting ar-null males had female secondary sex characteristics and were infertile due to defective spermatogenesis. This study supports the essentiality for the ar agonism for the development of testis and subsequently a male biased sex ratio. 


Key Event



Agonism, Androgen


There is good evidence from a sex inversion treatment via the direct blocking of AR using androgen antagonist that support the specificity of androgen receptor agonism for the subsequent key events to occur.  

Differentiation to Testis


Biological plausibility provides strong support for the essentiality of this event for the subsequent key events to occur.

Male Biased Sex Ratio


Breeding females (and both sexes) are necessary for population sustainability. A male biased sex population suggests a reduced offspring production and consequentially reduced population sustainability.

Population Sustainability




Evidence Assessment

The biological plausibility, empirical support, and quantitative understanding from each KER in an AOP are assessed together.  Biological plausibility of each of the KERs in the AOP is the most influential consideration in assessing WoE or degree of confidence in an overall hypothesised AOP for potential regulatory application (Meek et al., 2014; 2014a). Empirical support entails consideration of experimental data in terms of the associations between KEs – namely dose-response concordance and temporal relationships between and across multiple KEs. It is examined most often in studies of dose-response/incidence and temporal relationships for stressors that impact the pathway. While less influential than biological plausibility of the KERs and essentiality of the KEs, empirical support can increase confidence in the relationships included in an AOP. For clarification on how to rate the given empirical support for a KER, as well as examples, see pages 53- 55 of the User Handbook.  More help

Biological Plausibility

The biological plausibility linking androgen receptor agonism through the increased differentiation to testis is very solid. Actions of androgens are mediated by the androgen receptor. The Androgen Receptor (AR) is part of the nuclear receptor superfamily. ARs are ligand-dependent transcription factors and contain a highly conserved DNA-binding domain (DBD) and a moderately well conserved ligand-binding domain (LBD) (Hossain et al., 2008). Steroid hormones such as androgens acts by entering the cell and forming a complex with a its hormone receptor. After ligand binding, AR monomers undergo conformational change, dissociate from chaperone proteins, dimerize, and bind coactivator proteins (Bohen et al., 1995; Pratt and Toft, 1997). After this, the complex is translocated to the nucleus where it can bind to specific short DNA sequences (Androgen Responsive Elements) and serve as transcription factors of androgen mediated genes (Harbott et al., 2009). During sexual development, endogenous androgen can therefore induce the upregulation of many genes involved in the male developmental pathway.

The direct link between increased differentiation to testis leading to a male biased sex ratio is also well supported by biological plausibility. If the conditions that favored a male producing phenotype (in this case, exposure to androgens) 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, persistence of androgen exposure for repeated or prolong periods of times within the habitat of given fish species, can result in a male-biased population. Empirical evidence supporting the direct link between male biased population and a reduced population sustainability in fish species is lacking. However, increasing or permanent biased sex ratios can definitely have significant effects in sustainable fish populations (Marty et al. 2017). A male-biased sex ratio already suggests that the number of breeding females is reduced. If the male-biased sex ratio persists and/or increases over time, the offspring production for such population could eventually decrease and consequently, population productivity would be reduced (Brown et al. 2015; Grayson et al. 2014).

Concordance of Dose Response

The concentration-dependence of the key event responses with regard to the concentration of androgens has been established in vivo for some key events in the AOP. In general, effects on downstream key events occurred at concentrations equal to or greater than those at which upstream events occurred. Few studies that looked at multiple key events on this AOP were considered to be stronger support when evaluating the dose response relationship between key events. However, binding to the androgen receptor (the MIE) was not directly measured in any of the in vivo studies as this is often measured with in vitro studies. In fish, phenotypic masculinization of females has been used as an indirect measurement of in vivo androgen receptor agonism. However, in this case/aop, androgen receptor agonism is occurring prior to sexual differentiation and the resulting “phenotypic measurement” for the in vivo study is already considered as a separate downstream key event. Therefore, because support for the upstream key event is done in a different system (in vitro) than for the downstream key events (in vivo) , support for the dose concordance of the androgen receptor agonism (upstream) is done via in vitro studies that targeted the specific steroids used on the in vivo studies of the downstream key event.

  1. Concentration depended androgen receptor agonism (in vitro)
  • COS Whole Cell Binding Assay with fathead minnow AR (fhAR) were used in competitive binding experiments testing several natural and synthetic steroids, some of which are environmental contaminants, such as R188, 17B-trenbolone , and 17a-trenbolone. All showed a concentration dependent displacement of [3H]R1881 binding proving to be high affinity ligands for the fmAR. (Wilson et al., 2004).
    •  The synthetic steroids, R1881 and methytestosterone, had the highest affinities of all the chemicals tested with IC50 values of about 1.6 nM, followed by the synthetic steroids 17α- and 17β-trenbolone with IC50 values of about 8 and 16 nM, respectively.
    • Of the natural steroids, dihydrotestosterone was the strongest competitor with an IC50 of about 20 nM. The IC50 for the fish specific androgen, 11- ketotestosterone, was approximately 40 nM, followed by both testosterone and androstenedione at about 100 nM
  • Important to note that all of the above steroids tested were used in the in vivo studies that were selected to support this AOP demonstrating that all bound to the fhAR with a higher affinity than 11- ketotestosterone.
  1. Concentration dependent increased differentiation to testes:
  • Studies with Zebrafish (Danio rerio) exposed to 17β-trenbolone and Japanese medaka (Oryzias latipes) resulted in masculinization at different biological effect levels in a concentration-dependent manner as evidenced from a significantly increased maturity of testes (Orn et al., 2006; Morthorst et al., 2010; Baumann et al., 2014)) for some studies, this was determined ether by the abundance of spermatozoa and/or by the area of the testis.
  1. Concentration depended increased male biased sex ratio:
  • Zebrafish (Danio rerio) exposed to different concentrations of 17β-trenbolone and Dihydrotestosterone lead to increased number of males in a dose-dependent way (Orn et al., 2003; Morthorst et al., 2010; Baumann et al., 2013; Baumann et al., 2014)
  1. Concentration depended decline in population trajectory:
    • Modeled population trajectories show a concentration-dependent reduction in projected population size (Brown et al 2015, Miller et al. 2021?) based strongly on the ratio of male to female. Population-level effects have not been measured directly based on androgen receptor agonism. 

Dose Concordance Table

Temporal concordance



We are aware of no cases where the pattern of key events described was observed without also observing a significant impact on male sex ratios in teleost fish species. There are cases however, in which exposure to aromatizable androgens such as Methyltestosterone may lead to feminization of fish. This is due in part by excess aromatizable androgens available that can be converted to E2 and favor a female developmental pathway.

The adverse outcome is not specific to this AOP. Many of the key events included in this AOP overlap with AOPs linking other molecular initiating events during the period of development (ie. Aromatase inhibition, AOP 346) to male biased sex ratios.

Uncertainties, inconsistencies, and data gaps

It’s important to note that the use of aromatizable instead of non-aromatizable androgens for sex reversals exposures can lead to excess aromatizable androgens being converted to E2 and inducing a female developmental pathway. Chinook salmon (Oncorhynchus tshawytscha) exposed to Methyltestosterone showed a concentration dependent increase in percentage of males where 100% males were obtained at doses 400 ug/L. but at higher concentrations, the percentage of males decreased and at 10,000ug/L. the percentage of males reduced to 79.4%. (Peferrer & Donaldson, 1993).

However, there have been similar cases to the latter but with the use of non-aromatizable androgens such as Dihydrotestosterone in which the primary cause of this feminization is not clear yet. Bogers et al. 2006 exposed Fathead minnow (Pimephales promelas) to Dihydrotestosterone where fish exposed to 0.1 μg/L all had developed testes with one fish showing mixed sex (testis–ova). Exposure to 0.32 μg/L resulted in 80% males and 20% mixed gonads. But at 1.0ug/L  the percentage of males reduced to 40% (40% female, 10% undifferentiated and 10% mixed gonad). Additionally, it has been reported that oral administration of non aromatizable androgen dihydrotestosterone to sexually undifferentiated Channel Catfish (Ictalurus punctatus) resulted in all female populations (Davis et al. 1992).

Quantitative Understanding

Some proof of concept examples to address the WoE considerations for AOPs quantitatively have recently been developed, based on the rank ordering of the relevant Bradford Hill considerations (i.e., biological plausibility, essentiality and empirical support) (Becker et al., 2017; Becker et al, 2015; Collier et al., 2016). Suggested quantitation of the various elements is expert derived, without collective consideration currently of appropriate reporting templates or formal expert engagement. Though not essential, developers may wish to assign comparative quantitative values to the extent of the supporting data based on the three critical Bradford Hill considerations for AOPs, as a basis to contribute to collective experience.Specific attention is also given to how precisely and accurately one can potentially predict an impact on KEdownstream based on some measurement of KEupstream. This is captured in the form of quantitative understanding calls for each KER. See pages 55-56 of the User Handbook for a review of quantitative understanding for KER's. More help

Based on the evidence reviewed during development of this AOP, quantitative understanding of the KERs is limited at this time.

Considerations for Potential Applications of the AOP (optional)

At their discretion, the developer may include in this section discussion of the potential applications of an AOP to support regulatory decision-making. This may include, for example, possible utility for test guideline development or refinement, development of integrated testing and assessment approaches, development of (Q)SARs / or chemical profilers to facilitate the grouping of chemicals for subsequent read-across, screening level hazard assessments or even risk assessment. While it is challenging to foresee all potential regulatory application of AOPs and any application will ultimately lie within the purview of regulatory agencies, potential applications may be apparent as the AOP is being developed, particularly if it was initiated with a particular application in mind. This optional section is intended to provide the developer with an opportunity to suggest potential regulatory applications and describe his or her rationale.To edit the “Considerations for Potential Applications of the AOP” section, on an AOP page, in the upper right hand menu, click ‘Edit.’ This brings you to a page entitled, “Editing AOP.” Scroll down to the “Considerations for Potential Applications of the AOP” section, where a text entry box allows you to submit text. In the upper right hand menu, click ‘Update AOP’ to save your changes and return to the AOP page or 'Update and continue' to continue editing AOP text sections.  The new text should appear under the “Considerations for Potential Applications of the AOP” section on the AOP page. More help

Sex ratios can be a useful endpoint in risk and hazard assessment of chemicals. In July 2011, the Fish Sexual Development Test (FSDT) has officially been adopted as OECD test guideline no. 234 for the detection of EDCs within the OECD conceptual framework at level 4 (OECD, 2011b). The Fish Sexual Development Test covers endocrine disruption during the developmental period of sexual differentiation of particularly zebrafish and uses gonadal differentiation and sex ratio as endocrine disruption-associated endpoints. Therefore, this AOP can provide additional support to the use of alternative measurements in this type of tests when screening for sex steroid hormones or any other type of EDC affecting androgen receptors.


List the bibliographic references to original papers, books or other documents used to support the AOP. 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.

Crowder, C. M., Lassiter, C. S., & Gorelick, D. A. (2018). Nuclear Androgen Receptor Regulates Testes Organization and Oocyte Maturation in Zebrafish. Endocrinology, 159(2), 980–993.

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