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Key Event Title
Increase, Premature molting
Key Event Components
|ecdysis, chitin-based cuticle||decreased|
Key Event Overview
AOPs Including This Key Event
|AOP Name||Role of event in AOP||Point of Contact||Author Status||OECD Status|
|SAM depletion leading to population decline (2)||KeyEvent||You Song (send email)||Under development: Not open for comment. Do not cite|
|SAM depletion leading to population decline (1)||KeyEvent||You Song (send email)||Under development: Not open for comment. Do not cite|
|Chitinase inhibition leading to mortality||KeyEvent||Simon Schmid (send email)||Under development: Not open for comment. Do not cite||Under Development|
|Chitobiase inhibition leading to mortality||KeyEvent||Simon Schmid (send email)||Under development: Not open for comment. Do not cite||Under Development|
|CHS-1 inhibition leading to mortality||KeyEvent||Simon Schmid (send email)||Open for citation & comment||EAGMST Approved|
|SUR binding leading to mortality||KeyEvent||Simon Schmid (send email)||Under development: Not open for comment. Do not cite||Under Development|
Key Event Description
This key event is measured on the level of the individual. In order to grow and develop, arthropods need to shed their exoskeleton periodically (molting) (Heming 2018). During molting, the newly secreted cuticle is subject to mechanical stress associated and therefore needs to possess enough structural and functional integrity. The ecdysis motor program, which constitutes the behavioral part of the cuticle shedding requires the newly secreted cuticle to possess a certain strength to support for muscular force in order to shed the old cuticle (Ewer 2005). Cuticular integrity is also important after ecdysis, as insects and crustaceans expand their new cuticle by increasing internal pressure by swallowing air and water, respectively. This happens in order to expand and provide stability to the new cuticle until it is hardened (tanned) (Clarke 1957; Lee 1961; Dall et al. 1978; deFur et al. 1985). If arthropods are not able to molt properly, the organism will eventually die. Premature molting describes the unsuccessful molting where the organism is not able to shed the old cuticle, but also other effects related to molting in an immature stage where the new cuticle is not mature enough for the molt, such as rupture of the new cuticle and associated desiccation, deformities, higher susceptibility to pathogens or impaired locomotion. Specific effects observed are animals stuck in their exuviae (Wang et al., 2019), and if molting can be completed despite an immature cuticle, animals might be smaller and die at subsequent molts (Arakawa et al., 2008; Chen et al., 2008; Mohammed et al., 2017).
How It Is Measured or Detected
Premature molting can be determined by observation. No standardized tests for the endpoint of molting exist to date. However, during an OECD 202 Daphnia sp. Acute immobilization test (OECD 2004), the cumulative number of molts can be assessed as an additional endpoint. Molting can also be assessed during a OECD 211 Daphnia sp. Reproduction test (OECD 2012), as proposed previously (OECD 2003). One could even prolong the test to 96h to get a clearer result of this endpoint. Additionally, one could apply histopathological methods to monitor the maturity of the newly synthesized cuticle (e.g. thickness of procuticle).
Domain of Applicability
Taxonomic: Effect data for the occurrence of this KE exist from Pieris brassicae and Lucilia cuprina. However, all arthropods undergo molting, so it is highly likely that this KE is applicable to the whole phylum of arthropods.
Life stage: This KE is applicable for organisms that undergo molting in order to grow and develop, namely larval stages of insects and all life stages of crustaceans and arachnids.
Sex: This KE is applicable to all sexes.
Chemical: Substances known to induce premature molting are of the family of pyrimidine nucleosides (e.g. polyoxin D and nikkomycin Z) (Gijswijt et al. 1979; Tellam et al. 2000; Arakawa et al. 2008).
Evidence for Perturbation by Stressor
Arakawa T, Yukuhiro F, Noda H. 2008. Insecticidal effect of a fungicide containing polyoxin B on the larvae of Bombyx mori (Lepidoptera: Bombycidae), Mamestra brassicae, Mythimna separata, and Spodoptera litura (Lepidoptera: Noctuidae). Appl Entomol Zool. 43(2):173–181. doi:10.1303/aez.2008.173.
Chen, X.; Tian, H.; Zou, L.; Tang, B.; Hu, J.; Zhang, W. Disruption of Spodoptera Exigua Larval Development by Silencing Chitin Synthase Gene A with RNA Interference. Bull. Entomol. Res. 2008, 98 (6), 613–619. https://doi.org/10.1017/S0007485308005932.
Clarke KU. 1957. On the Increase in Linear Size During Growth in Locusta Migratoria L. Proc R Entomol Soc London Ser A, Gen Entomol. 32(1– 3):35–39. doi:10.1111/j.1365-3032.1957.tb00361.x.
Dall W, Smith DM, Press B. 1978. Water uptake at ecdysis in the western rock lobster. J Exp Mar Bio Ecol. 35(1960). doi:10.1016/0022- 0981(78)90074-6.
deFur PL, Mangum CP, McMahon BR. 1985. Cardiovascular and Ventilatory Changes During Ecdysis in the Blue Crab Callinectes Sapidus Rathbun. J Crustac Biol. 5(2):207–215. doi:10.2307/1547867.
Ewer J. 2005. How the ecdysozoan changed its coat. PLoS Biol. 3(10):1696–1699. doi:10.1371/journal.pbio.0030349.
Gijswijt MJ, Deul DH, de Jong BJ. 1979. Inhibition of chitin synthesis by benzoyl-phenylurea insecticides, III. Similarity in action in Pieris brassicae (L.) with Polyoxin D. Pestic Biochem Physiol. 12(1):87–94. doi:10.1016/0048-3575(79)90098-1.
Heming BS. 2018. Insect development and evolution. Ithaca: Cornell University Press.
Mohammed, A. M. A.; DIab, M. R.; Abdelsattar, M.; Khalil, S. M. S. Characterization and RNAi-Mediated Knockdown of Chitin Synthase A in the Potato Tuber Moth, Phthorimaea Operculella. Sci. Rep. 2017, 7 (1), 1–12. https://doi.org/10.1038/s41598-017-09858-y.
Lee RM. 1961. The variation of blood volume with age in the desert locust (Schistocerca gregaria Forsk.). J Insect Physiol. 6(1):36–51. doi:10.1016/0022-1910(61)90090-7.
OECD (2003), Proposal for an Enhanced Test Guideline. Daphnia magna Reproduction Test. Draft OECD Guidel. Test. Chem. Enhanc. Tech. Guid. Doc. 211 21.
OECD (2004), Test No. 202: Daphnia sp. Acute Immobilisation Test, OECD Guidelines for the Testing of Chemicals, Section 2, OECD Publishing, Paris, https://doi.org/10.1787/9789264069947-en.
OECD (2012), Test No. 211: Daphnia magna Reproduction Test, OECD Guidelines for the Testing of Chemicals, Section 2, OECD Publishing, Paris, https://doi.org/10.1787/9789264185203-en.
Tellam RL, Vuocolo T, Johnson SE, Jarmey J, Pearson RD. 2000. Insect chitin synthase. cDNA sequence, gene organization and expression. Eur J Biochem. 267(19):6025–6043. doi:10.1046/j.1432-1327.2000.01679.x.
Wang, Z.; Yang, H.; Zhou, C.; Yang, W. J.; Jin, D. C.; Long, G. Y. Molecular Cloning, Expression, and Functional Analysis of the Chitin Synthase 1 Gene and Its Two Alternative Splicing Variants in the White-Backed Planthopper, Sogatella Furcifera (Hemiptera: Delphacidae). Sci. Rep. 2019, 9 (1), 1–14. https://doi.org/10.1038/s41598-018-37488-5.