You Song1 and Knut Erik Tollefsen1,2
1 Norwegian Institute for Water Research (NIVA), Section of Ecotoxicology and Risk Assessment, Gaustadalléen 21, N-0349 Oslo, Norway
2 Norwegian University of Life Sciences (NMBU), Faculty of Environmental Science and Technology, Department of Environmental Sciences (IMV). P.O. Box 5003, N-1432 Ås, Norway
Point of Contact
Knut Erik Tollefsen
- Knut Erik Tollefsen
- You Song
|Author status||OECD status||OECD project||SAAOP status|
|Under development: Not open for comment. Do not cite||Under Development|
This AOP was last modified on November 29, 2016 18:55
|Activation, Juvenile hormone receptor||September 16, 2017 10:17|
|Induction, Doublesex1 gene||September 16, 2017 10:17|
|Increased, Male offspring||December 03, 2016 16:37|
|Induction, Male reproductive tract||September 16, 2017 10:17|
|Decline, Population||December 03, 2016 16:33|
|Alteration, Food-web structures||December 03, 2016 16:37|
|Activation, Juvenile hormone receptor leads to Induction, Doublesex1 gene||December 03, 2016 16:38|
|Induction, Doublesex1 gene leads to Induction, Male reproductive tract||December 03, 2016 16:38|
|Induction, Male reproductive tract leads to Increased, Male offspring||December 03, 2016 16:38|
|Increased, Male offspring leads to Decline, Population||December 03, 2016 16:38|
|Decline, Population leads to Alteration, Food-web structures||December 03, 2016 16:38|
|fenoxycarb||November 29, 2016 18:42|
|DIOFENOLAN||November 29, 2016 18:42|
|pyriproxyfen||November 29, 2016 18:42|
|(7S)-Hydroprene||November 29, 2016 18:42|
|kinoprene||November 29, 2016 18:42|
Endocrine disruption (ED) associated with environmental sex determination (ESD) and reproduction effects has been frequently reported in various ecotoxicologcail studies with crustaceans, such as daphniids (reviewed in (LeBlanc and Medlock, 2015)). Daphniids normally go through parthenogenic reproduction cycles in which only female offspring are produced. When under environmental stress (e.g. chemical exposure, short length of daylight, food shortage and high population density), a switch of reproductive strategy from asexual to sexual reproduction occurs and male offspring are produced. As an indicator of ED, the induction of male offspring is used as an important endpoint in daphnia toxicity tests. It has been widely recognized that analogs of the crustacean juvenile hormone (JH), methyl farnesoate (MF) are able to bind and activate the crustacean JH receptor, methoprene tolerant (Met), thus inducing the putative sex determination genes, doublesex genes, and causing male offspring formation. The induction of male neonates is usually associated with reduced fecundity and population decline as adverse ED effects. The existing evidences from different studies currently support the causal relationships between the biological events occurred in daphniids after exposure to JH analogs and subsequently form an adverse outcome pathway (AOP) as proposed herein.
This optional section 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. Instructions To add background information, click Edit in the upper right hand menu on the AOP page. Under the “Background (optional)” field, a text editable form provides ability to edit the Background. Clicking ‘Update AOP’ will update these fields.
Summary of the AOP
Molecular Initiating Event
|Activation, Juvenile hormone receptor||Activation, Juvenile hormone receptor|
|Induction, Doublesex1 gene||Induction, Doublesex1 gene|
|Induction, Male reproductive tract||Induction, Male reproductive tract|
|Increased, Male offspring||Increased, Male offspring|
|Decline, Population||Decline, Population|
|Alteration, Food-web structures||Alteration, Food-web structures|
Relationships Between Two Key Events (Including MIEs and AOs)
|Activation, Juvenile hormone receptor leads to Induction, Doublesex1 gene||Directly leads to|
|Induction, Doublesex1 gene leads to Induction, Male reproductive tract||Directly leads to|
|Induction, Male reproductive tract leads to Increased, Male offspring||Directly leads to|
|Increased, Male offspring leads to Decline, Population||Directly leads to|
|Decline, Population leads to Alteration, Food-web structures||Indirectly leads to|
Life Stage Applicability
|Daphnia magna||Daphnia magna||NCBI|
|Daphnia pulex||Daphnia pulex||NCBI|
Graphical RepresentationClick to download graphical representation template
Overall Assessment of the AOP
This section addresses the relevant domain of applicability (i.e., in terms of taxa, sex, life stage, etc.) and weight of evidence for the overall hypothesised AOP (i.e., including the MIE, KEs and AO) as a basis to consider appropriate regulatory application (e.g., priority setting, testing strategies or risk assessment). It draws upon the evidence assembled for each KER as one of several components which contribute to relative confidence in supporting information for the entire hypothesised pathway. An important component in assessing confidence in supporting information as a basis to consider regulatory application of AOPs beyond that described in Section 6 is the essentiality of each of the key events as a component of the entire pathway. This is normally investigated in specifically-designed stop/reversibility studies or knockout models (i.e., those where a key event can be blocked or prevented). Assessment of the overall AOP also contributes to the identification of KEs for which confidence in the quantitative relationship with the AO is greatest (i.e., to facilitate determining the most sensitive predictor of the AO). Instructions To edit the “Overall Assessment 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 “Overall Assessment 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. The new text should appear under the “Overall Assessment of the AOP” section on the AOP page.
Domain of Applicability
Life Stage Applicability This AOP applies to: juvenile (<24h old) exposed to the MF analogs during a chronic (>14d) exposure (Abe et al., 2014a); and sex-mature adult (>10d old) exposed to MF analogs in the middle of a reproduction cycle for at least three days (Abe et al., 2014a).
Taxonomic Applicability This AOP can be potentially applied to other crustacean species and insects. The ligand binding domain of Met is considered to be conserved across different daphnia species and insects (Miyakawa et al., 2013). The juvenile hormone III (methyl farnesoate) signaling pathway to exert hormonal actions are thought to be similar between daphnids and insects with only two amino acids being different, which enhances the responsiveness of the Met receptor to various juvenile hormone III analogs (Miyakawa et al., 2013).
This AOP applies to females.
Elaborate on the domains of applicability listed in the summary section above. Specifically, provide the literature supporting, or excluding, certain domains.
Essentiality of the Key Events
The support for essentiality for MiE is considered to be strong. It has been shown that MF and the MF analog pyriproxyfen activated the cloned Daphnia pulex Met when expressed with the mosquito SRC ortholog (LeBlanc et al., 2013). A two-hybrid in vitro screening bioassay has been developed using transfected Chinese hamster ovary (CHO) cells containing the transcriptional activity of Met/SRC for screening of Met activation by MF analogs (Miyakawa et al., 2013). The application of this screening assay by different studies supported that chemical with MF activities, including MF, juvenile hormone III, fenoxycarb (Miyakawa et al., 2013) and diofenolan (Abe et al., 2015a) were able to activate the Met receptor and initiate downstream transcriptional regulation in a concentration-dependent manner. Evidences generated from this screening assay quantitatively demonstrated the causal relationship between chemical exposure and transcriptional activation of Met.
Key Event Summary
The dsx1 gene has shown to be highly responsible for the regulation of male traits development in the embryos of D. magna after exposure to the MF analog fenoxycarb (Kato et al., 2011). The same study has also shown that lack of dsx1 gene expression in male embryos of D. magna resulted in all female phenotypes. A high number of studies have quantitatively assessed the effects of MF-like chemicals on the induction of male offspring in daphnids (Abe et al., 2015b; Abe et al., 2014b; Ginjupalli and Baldwin, 2013; Kim et al., 2011; Lampert et al., 2012; Matsumoto et al., 2008; Oda et al., 2007; Oda et al., 2005; Olmstead and LeBlanc, 2001a, 2003; Olmstead and LeBlanc, 2001b; Tatarazako et al., 2003; Wang et al., 2005). These studies strongly support the induction of male offspring as a results of MF-related ED effects. Therefore, it is evident that the support for essentiality of Met receptor activation by MF analogs leading to the induction of dsx1, male traits and male offspring is strong.
Provide an overall assessment of the essentiality for the key events in the AOP. Support calls for individual key events can be included in the molecular initiating event, key event, and adverse outcome tables above.
Weight of Evidence Summary
The overall weight of evidence (WoE) for this AOP is considered to be moderate. The WoE for each individual KER is summarized in the overview table above. The MiE of Met receptor activation leading to the induction of dsx1 gene is considered to be strong. It has been demonstrated in insects such as Drosophila that the direct target gene following the MF receptor activation was the sex lethal gene (sxl) (Zhang et al., 2014). The sxl protein as an RNA splicing factor then leads to the expression of the transformer (TRA) gene and protein to regulate the doublesex (dsx) genes (Kopp, 2012). However, studies have shown that the tra gene was not involved in the sex determination in daphnids (Kato et al., 2010). Therefore, intermediate targets between the Met receptor activation and the induction of dsx genes still need to be found. Nevertheless, the dsx1 gene has shown to be highly responsible for the regulation of male traits development in the embryos of D. magna after exposure to the MF analog fenoxycarb (Kato et al., 2011). The same study has also shown that lack of dsx1 gene expression in male embryos of D. magna resulted in all female phenotypes. Therefore, it is evident that the support for essentiality of Met receptor activation by MF analogs leading to the induction of dsx1 and male traits formation is strong. The induction of dsx1 genes and male traits development leading to the induction of male offspring is considered to be strong. A study by (Miyakawa et al., 2013) reported good correlation between the EC50s of male offspring induction and EC50s of Met receptor activation for MF-like chemicals, including MF, fenoxycarb, JH III and pyriproxyfen. The induction of male offspring leading to reduction of fecundity is considered to be moderate. A number of studies have shown that the reduction of fecundity often occurred with the increased male:female ratio of offspring in daphnia after exposure to MF analogs (Abe et al., 2015b; Abe et al., 2014b; Ginjupalli and Baldwin, 2013; Kim et al., 2011; Lampert et al., 2012; Matsumoto et al., 2008; Oda et al., 2007; Oda et al., 2005; Olmstead and LeBlanc, 2001a, 2003; Olmstead and LeBlanc, 2001b; Tatarazako et al., 2003; Wang et al., 2005). Although the underlying mechanism and quantitative causal relationship between male formation and reduced reproduction has not been well-characterized, these evidences support the hypothesis that reduction of fecundity may be caused by disrupted sex determination pathway. The reduction of fecundity and induction of male offspring leading to the decline of population size is considered to be weak. Although lack of direct experimental evidences, it may be deduced that the most efficient way of reproduction, parthenogenesis is disrupted, thus the population size in the next generations may decrease. The decline of population size leading to the disturbance of food-web structure is considered to be weak. Although lack of experimental evidences, it may deduced that the food-web structure may be disrupted due to the decline of daphnia population.
Provide an overall summary of the weight of evidence based on the evaluations of the individual linkages from the Key Event Relationship pages.
The mechanistic evidences between the MiE of Met activation and induction of dsx1 gene have been clearly shown by various studies. However, the concentration-response relationship still needs to be obtained. A combination of the two-hybrid screening assay and quantitative real-time reverse transcriptase polymerase chain reaction (qPCR) would greatly facilitate the quantification of this KER. So this KER is considered to be moderate in terms of quantitative considerations. Demonstrations of the KER between the induction of dsx1 gene and male traits formation in the embryos using a combination of qPCR and in situ hybridization have been clearly provided (Kato et al., 2010). However, concentration-response relationship for this KER has not been well determined. So this KER is considered to be moderate in terms of quantitative considerations. The mechanism underlying the induction of male offspring leading to fecundity reduction has not been well characterized, and not quantitative assessment has been performed. So this KER is considered to be weak in terms of quantitative considerations. Linkages between the fecundity reduction, population decline and food-web disturbance have not been well studied. So these KERs are considered to be weak in terms of quantitative considerations.
Provide an overall discussion of the quantitative information available for this AOP. Support calls for the individual relationships can be included in the Key Event Relationship table above.
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. Detailing such considerations can aid the process of transforming narrative descriptions of AOPs into practical tools. In this context, it is necessarily beneficial to involve members of the regulatory risk assessment community on the development and assessment team. The Network view which is generated based on assessment of weight of evidence/degree of confidence in the hypothesized AOP taking into account the elements described in Section 7 provides a useful summary of relevant information as a basis to consider appropriate application in a regulatory context. Consideration of application needs then, to take into consideration the following rank ordered qualitative elements: Confidence in biological plausibility for each of the KERs Confidence in essentiality of the KEs Empirical support for each of the KERs and overall AOP The extent of weight of evidence/confidence in both these qualitative elements and that of the quantitative understanding for each of the KERs (e.g., is the MIE known, is quantitative understanding restricted to early or late key events) is also critical in determining appropriate application. For example, if the confidence and quantitative understanding of each KER in a hypothesised AOP are low and or low/moderate and the evidence for essentiality of KEs weak (Section 7), it might be considered as appropriate only for applications with less potential for impact (e.g., prioritisation, category formation for testing) versus those that have immediate implications potentially for risk management (e.g., in depth assessment). If confidence in quantitative understanding of late key events is high, this might be sufficient for an in depth assessment. The analysis supporting the Network view is also essential in identifying critical data gaps based on envisaged regulatory application. Instructions 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. The new text should appear under the “Considerations for Potential Applications of the AOP” section on the AOP page.
Abe, R., Toyota, K., Miyakawa, H., Watanabe, H., Oka, T., Miyagawa, S., Nishide, H., Uchiyama, I., Tollefsen, K.E., Iguchi, T., Tatarazako, N., 2014. Diofenolan induces male offspring production through binding to the juvenile hormone receptor in Daphnia magna. Aquatic toxicology 159C, 44-51.
Abe, R., Watanabe, H., Yamamuro, M., Iguchi, T., Tatarazako, N., 2014b. Establishment of a short-term, in vivo screening method for detecting chemicals with juvenile hormone activity using adult Daphnia magna. Journal of applied toxicology : JAT.
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Olmstead, A.W., LeBlanc, G.A., 2003. Insecticidal juvenile hormone analogs stimulate the production of male offspring in the crustacean Daphnia magna. Environmental health perspectives 111, 919-924.
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