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Relationship: 2145


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

Reduction, E2 Synthesis by the undifferentiated gonad leads to Increased, Differentiation to Testis

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
Aromatase inhibition leads to male-biased sex ratio via impacts on gonad differentiation adjacent Moderate Kelvin Santana Rodriguez (send email) Under Development: Contributions and Comments Welcome 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
Oreochromis niloticus Oreochromis niloticus Moderate NCBI

Sex Applicability

An indication of the the relevant sex for this KER. More help
Sex Evidence
Mixed Moderate

Life Stage Applicability

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

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

Prior to sex determination, vertebrates have a bipotential gonad that can develop into testis or ovary depending on genetic makeup (genetic sex determination), environmental conditions during development (environmental sex determination) or a combination of both (Graves et al. 2010; Trukhina et al. 2013).

A key variable influencing gonad differentiation is the production of sex steroids such as 17ß-estradiol (E2) and testosterone (T). In many vertebrates, including a variety of fish species, the "default" gonadal sex is male, with the presence of E2 (or perhaps the relative relationship between E2 and T production/levels) controlling the alternative path to development of ovaries.

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

Among the different forms of estrogens, E2 is considered the most fundamental to gonad differentiation in most vertebrates, as it is responsible for inducing and maintaining ovarian development (Bondesson et al., 2015; Li et al., 2019). Estrogens bind to estrogen receptors (ER), that regulate the transcription of estrogen-responsive genes necessary for proper gonad development of for a female pathway (Guiguen et al., 2010; Gorelick et al., 2011). However, reductions in E2 biosynthesis during the critical period of sexual differentiation of the bipotential gonad would logically lead to decreased E2 signaling necessary for ovarian development, thereby leading to morphological development of testis. Therefore, it is plausible that E2 reduction in the undifferentiated gonad at the onset of sexual differentiation promotes the preferential occurrence of testis. 

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

Even for vertebrate classes known to be subject to environmental sex determination, the relative importance of genetic versus environmental factors in terms of influencing local production of steroids by the bipotential gonad is not well characterized, nor readily predicted based on phylogeny (Angelopoulou et al. 2012, Sarre et al. 2004). Consequently, both the occurrence and importance of this relationship may vary considerably among species.

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

Various environmental and genetic factors are known to influence differentiation of the bipotential gonad. However, quantitative understanding of this relationship is inadequate to precisely define the effect of such factors on the concentrations of E2 required to support differentiation to testis versus ovary, particularly in a manner that could be generalized across multiple species.

Response-response Relationship
Provides sources of data that define the response-response relationships between the KEs.  More help

There are not sufficient data to support derivation of a generalizable relationship between levels of E2 in differentiating gonad tissue and development to a testis phenotype.

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 is species-dependent occurring, for example, over the course of days to weeks in most fishes.

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

Undefined at present.

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 upstream event in for this KER is associated with the undifferentiated bipotential gonad. Therefore, this relationship is relevant to early life-stages prior to sexual development/differentation.


Because the upstream event in this relationship pertains to the undifferentiated gonad, the sex applicability of this relationship is non-specific.

Taxonomic applicability

This relationship is most applicable to vertebrates subject to environmental sex determination. The relevance to species with predominantly genetic sex determination is less clear, likely depending on species-specific plasticity.


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

Angelopoulou, R., Lavranos, G., & Manolakou, P. (2012). Sex determination strategies in 2012: towards a common regulatory model?. Reproductive biology and endocrinology : RB&E10, 13.


Bondesson, M., Hao, R., Lin, C. Y., Williams, C., & Gustafsson, J. Å. (2015). Estrogen receptor signaling during vertebrate development. Biochimica et biophysica acta1849(2), 142–151.


D'Cotta, H., Fostier, A., Guiguen, Y., Govoroun, M., & Baroiller, J. F. (2001). Aromatase plays a key role during normal and temperature-induced sex differentiation of tilapia Oreochromis niloticus. Molecular reproduction and development59(3), 265–276.


Gorelick, D. A., & Halpern, M. E. (2011). Visualization of estrogen receptor transcriptional activation in zebrafish. Endocrinology, 152(7), 2690–2703.


Guiguen, Y., Fostier, A., Piferrer, F., & Chang, C. F. (2010). Ovarian aromatase and estrogens: a pivotal role for gonadal sex differentiation and sex change in fish. General and comparative endocrinology, 165(3), 352–366.


Marshall Graves, J. A., & Peichel, C. L. (2010). Are homologies in vertebrate sex determination due to shared ancestry or to limited options?. Genome biology11(4), 205.

Rashid, H., Kitano, H., Lee, K. H., Nii, S., Shigematsu, T., Kadomura, K., Yamaguchi, A., & Matsuyama, M. (2007). Fugu (Takifugu rubripes) sexual differentiation: CYP19 regulation and aromatase inhibitor induced testicular development. Sexual development : genetics, molecular biology, evolution, endocrinology, embryology, and pathology of sex determination and differentiation, 1(5), 311–322.

Ruksana, S., Pandit, N. P., & Nakamura, M. (2010). Efficacy of exemestane, a new generation of aromatase inhibitor, on sex differentiation in a gonochoristic fish. Comparative biochemistry and physiology. Toxicology & pharmacology : CBP, 152(1), 69–74.

Sarre, S. D., Georges, A., & Quinn, A. (2004). The ends of a continuum: genetic and temperature-dependent sex determination in reptiles. BioEssays : news and reviews in molecular, cellular and developmental biology26(6), 639–645.


Trukhina, A. V., Lukina, N. A., Wackerow-Kouzova, N. D., & Smirnov, A. F. (2013). The variety of vertebrate mechanisms of sex determination. BioMed research international, 2013, 587460.


Yin, Y., Tang, H., Liu, Y., Chen, Y., Li, G., Liu, X., & Lin, H. (2017). Targeted Disruption of Aromatase Reveals Dual Functions of cyp19a1a During Sex Differentiation in Zebrafish. Endocrinology158(9), 3030–3041.

Zhang, Xianbo & Li, Mengru & Ma, He & Liu, Xingyong & Shi, Hongjuan & Li, Minghui & Wang, Deshou. (2017). Mutation of foxl2 or cyp19a1a Results in Female to Male Sex Reversal in XX Nile Tilapia. Endocrinology. 158. 10.1210/en.2017-00127.