Distinct expression profiles of stress defense and DNA repair genes in Daphnia pulex exposed to cadmium, zinc, and quantum dots Song Tang a,b , Yonggan Wu c , Caitlin N. Ryan a , Shuangying Yu a , Guangqiu Qin d , Donn S. Edwards a , Gregory D. Mayer a,e,⇑ a The Institute of Environmental and Human Health, Texas Tech University, Lubbock, TX 79416, USA b School of Environment and Sustainability, University of Saskatchewan, Saskatoon, SK S7N 5B3, Canada c Department of Genetics, Stanford University, Stanford, CA 94305, USA d Institute of Toxicology, Guangxi Center for Disease Prevention and Control, Nanning, Guangxi 530028, China e Mount Desert Island Biological Laboratory, Salisbury Cove, ME 04672, USA highlights  CdSO 4 , ZnSO 4 , and Cd-based QDs are able to induce the mortality of Daphnia.  Both CdSO 4 and Cd-based QDs leads to Cd accumulation in vivo.  Both CdSO 4 and Cd-based QDs increased expression of metallothionein (MT-1).  GAPDH was the most reliable reference gene for RT-qPCR analysis in Daphnia.  Cd, Zn, and CdSe/ZnS QDs induced different patterns of gene expression. article info Article history: Received 14 February 2014 Received in revised form 31 May 2014 Accepted 4 June 2014 Handling Editor: Tamara S. Galloway Keywords: Nanoparticles Metallothionein Heat shock proteins Superoxide dismutase Reference genes Crustacean abstract The ever-increasing production and use of nanocrystaline semiconductors (Quantum dots; QDs) will inevitably result in increased appearance of these nanomaterials in the aquatic environment. However, the behavior and potential toxicity of heavy metal constituted nanoparticulates in aquatic invertebrates is largely unknown, especially with regard to molecular responses. The freshwater crustacean Daphnia pulex is a well-suited toxicological and ecological model to study molecular responses to environmental stressors. In this study, D. pulex were exposed for 48 h to sublethal doses of QDs (25% and 50% of LC50) with differing spectral properties (CdTe and CdSe/ZnS QDs) and Cd and Zn salts. Our data suggest that acute exposure to both CdSO 4 and Cd-based QDs leads to Cd uptake in vivo, which was biologically sup- ported by the observation of increased expression of metallothionein (MT-1). Furthermore, Cd, Zn, and CdSe/ZnS QDs induced different patterns of gene expression regarding stress defense and DNA repair, which furthers our knowledge regarding which response pathways are affected by nanoparticulate forms of metals versus ionic forms in aquatic crustaceans. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Nanocrystaline semiconductors (Quantum dots; QDs) are com- prised of a metalloid core, such as cadmium telluride (CdTe) or cadmium selenide (CdSe), and a metal/sulfide ‘cap’ or ‘shell’, most notably zinc sulphide (ZnS) (Hardman, 2006). These nanoparticles display unique size-related electrical and optical properties (Dua et al., 2010). The use of QDs is growing rapidly as new applications, especially in biomedical research, adopt this technology (Medintz et al., 2005). QDs have the ability to be released into the aquatic environment from a multitude of anthropogenic activities, includ- ing wastewater effluents from industrial, clinical, and research uses (Hardman, 2006). Many heavy metals that comprise QDs, such as cadmium (Cd), are known mutagens and carcinogens. Given the inherent metallic composition of QDs, there exists a potential for detrimental effects on aquatic biota, which warrants environmen- tal concern. The freshwater planktonic microcrustacean Daphnia is a cosmo- politan freshwater dweller in ponds and lakes (Shaw et al., 2007). Daphnia are keystone species in freshwater food chains and food webs, and an excellent bioindicator species for use in environmental http://dx.doi.org/10.1016/j.chemosphere.2014.06.011 0045-6535/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author at: The Institute of Environmental and Human Health, Texas Tech University, Box 41163, Lubbock, TX 79416, USA. Tel.: +1 806 885 4567; fax: +1 806 885 4577. E-mail address: greg.mayer@ttu.edu (G.D. Mayer). Chemosphere 120 (2015) 92–99 Contents lists available at ScienceDirect Chemosphere journal homepage: www.elsevier.com/locate/chemosphere