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Relationship: 2350


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

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

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
fathead minnow Pimephales promelas Moderate NCBI
Oreochromis niloticus Oreochromis niloticus High 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
before or during gonadal sex differentiation 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

Prior to sex determination, many 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 (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 (Angelopoulou et al. 2012).

Cytochrome P450 aromatase (CYP19a1a) is the enzyme responsible for the conversion of T to E2 in gonadal tissues of vertebrates (Miller 1988; Simpson et al. 1994). Consequently, inhibition of CYPa191a expression/activity during gonadal differentiation can lead to an increased occurrence of testis. This can subsequently result in a male-biased sex ratio in the population of interest.

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

This key event relationship is highly plausible. If inhibition of aromatase (E2 production) overlaps with the critical period of sex differentiation in a susceptible species there will be an increase in the number of organisms developing testes, which would produce 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

As noted below, it is uncertain as to the full range of species this KER might be applicable due to susbstantial taxonomic variation in the role that steroid signaling plays in gonadal differentiation and, hence, the number of males in a population.

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.

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 to testis and, consequently, to a male phenotype 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

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 during the gonadal differentiation. This KER is not applicable to sexually differentiated adults. 


Because this KER occurs during differentiation, the relationship is relevant to animals with an undetermined (non-specific) sex.

Taxonomic Applicability 

Sequencing studies with mammalian, amphibian, reptile, bird, and fish species have shown that aromatase is well conserved among all vertebrates (Wilson et al. 2005; LaLone et al. 2018).

However, it is difficult to predict the biological domain of applicability of this KER based on phylogenetic characteristics. There is considerable within class variability, for example, among both fish and reptile species as to the role of aromatase expression and estrogen signaling in determining gonadal sex (Angelopoulou et al. 2012; Sarre et al. 2004). Thus susceptibility and relative sensitivities may vary considerably among species.


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.

Brown, A. R., Bickley, L. K., Le Page, G., Hosken, D. J., Paull, G. C., Hamilton, P. B., Owen, S. F., Robinson, J., Sharpe, A. D., & Tyler, C. R. (2011). Are toxicological responses in laboratory (inbred) zebrafish representative of those in outbred (wild) populations? - A case study with an endocrine disrupting chemical. Environmental science & technology, 45(9), 4166–4172.

Brown, A. R., Owen, S. F., Peters, J., Zhang, Y., Soffker, M., Paull, G. C., Hosken, D. J., Wahab, M. A., & Tyler, C. R. (2015). Climate change and pollution speed declines in zebrafish populations. Proceedings of the National Academy of Sciences of the United States of America, 112(11), E1237–E1246.

Fenske, M. & Segner, H. (2004). Aromatase modulation alters gonadal differentiation in developing zebrafish (Danio rerio). Aquatic toxicology (Amsterdam, Netherlands). 67. 105-26. DOI 10.1016/j.aquatox.2003.10.008.

Holbech, H., Kinnberg, K. L., Brande-Lavridsen, N., Bjerregaard, P., Petersen, G. I., Norrgren, L., Örn, S., Braunbeck, T., Baumann, L., Bomke, C., Dorgerloh, M., Bruns, E., Ruehl-Fehlert, C., Green, J. W., Springer, T. A., & Gourmelon, A. (2012). Comparison of zebrafish (Danio rerio) and fathead minnow (Pimephales promelas) as test species in the Fish Sexual Development Test (FSDT). Comparative Biochemistry and Physiology - C Toxicology and Pharmacology, 155(2), 407–415.

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.

Lau, E. S., Zhang, Z., Qin, M., & Ge, W. (2016). Knockout of Zebrafish Ovarian Aromatase Gene (cyp19a1a) by TALEN and CRISPR/Cas9 Leads to All-male Offspring Due to Failed Ovarian Differentiation. Scientific reports, 6, 37357.

Luzio, A., Matos, M., Santos, D., Fontaínhas-Fernandes, A. A., Monteiro, S. M., & Coimbra, A. M. (2016). Disruption of apoptosis pathways involved in zebrafish gonad differentiation by 17α-ethinylestradiol and fadrozole exposures. Aquatic toxicology (Amsterdam, Netherlands), 177, 269–284.

Luzio, A., Monteiro, S. M., Rocha, E., Fontaínhas-Fernandes, A. A., & Coimbra, A. M. (2016). Development and recovery of histopathological alterations in the gonads of zebrafish (Danio rerio) after single and combined exposure to endocrine disruptors (17α-ethinylestradiol and fadrozole). Aquatic toxicology (Amsterdam, Netherlands), 175, 90–105.

Luzio, A.,Monteiro, S., Garcia Santos, S., Rocha, E., Fontainhas-Fernandes, A.,& Coimbra, A. (2015). Zebrafish sex differentiation and gonad development after exposure to 17α-ethinylestradiol, fadrozole and their binary mixture: A stereological study. Aquatic Toxicology. 166. 83-95. DOI 10.1016/j.aquatox.2015.07.015.

Miller W. L. (1988). Molecular biology of steroid hormone synthesis. Endocrine reviews9(3), 295–318.

Muth-Köhne, E., Westphal-Settele, K., Brückner, J., Konradi, S., Schiller, V., Schäfers, C., Teigeler, M., & Fenske, M. (2016). Linking the response of endocrine regulated genes to adverse effects on sex differentiation improves comprehension of aromatase inhibition in a Fish Sexual Development Test. Aquatic toxicology (Amsterdam, Netherlands), 176, 116–127.

Payne, A. H., & Hales, D. B. (2004). Overview of steroidogenic enzymes in the pathway from cholesterol to active steroid hormones. Endocrine reviews25(6), 947–970.

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.

Simpson, E. R., Mahendroo, M. S., Means, G. D., Kilgore, M. W., Hinshelwood, M. M., Graham-Lorence, S., Amarneh, B., Ito, Y., Fisher, C. R., & Michael, M. D. (1994). Aromatase cytochrome P450, the enzyme responsible for estrogen biosynthesis. Endocrine reviews15(3), 342–355.

Thorpe, K. L., Marca Pereira, M. L., Schiffer, H., Burkhardt-Holm, P., Weber, K., & Wheeler, J. R. (2011). Mode of sexual differentiation and its influence on the relative sensitivity of the fathead minnow and zebrafish in the fish sexual development test. Aquatic Toxicology, 105(3–4), 412–420.

Thresher, R., Gurney, R., & Canning, M. (2011). Effects of lifetime chemical inhibition of aromatase on the sexual differentiation, sperm characteristics and fertility of medaka (Oryzias latipes) and zebrafish (Danio rerio). Aquatic toxicology (Amsterdam, Netherlands), 105(3-4), 355–360.

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.

Uchida, D., Yamashita, M., Kitano, T., & Iguchi, T. (2004). An aromatase inhibitor or high water temperature induce oocyte apoptosis and depletion of P450 aromatase activity in the gonads of genetic female zebrafish during sex-reversal. Comparative biochemistry and physiology. Part A, Molecular & integrative physiology, 137(1), 11–20.

Wilson, J. Y., McArthur, A. G., & Stegeman, J. J. (2005). Characterization of a cetacean aromatase (CYP19) and the phylogeny and functional conservation of vertebrate aromatase. General and comparative endocrinology140(1), 74–83.

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. Endocrinology, 158(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.