Recessivity of pyrethroid resistance and limited interspecies hybridization across Hyalella clades supports rapid and independent origins of resistance * Haleigh C. Sever a , Jennifer R. Heim a , Victoria R. Lydy b , Courtney Y. Fung a , Kara E. Huff Hartz a , Marissa S. Giroux d , Nicolette Andrzejczyk d , Kaley M. Major c , Helen C. Poynton c , Michael J. Lydy a, * a Center for Fisheries, Aquaculture, and Aquatic Sciences, Department of Zoology, Southern Illinois University Carbondale, IL, USA b College of Natural and Applied Sciences: Department of Biology, Missouri State University, Springfield, MO, USA c School for the Environment, University of Massachusetts Boston, Boston, MA, USA d Department of Environmental Sciences, University of California, Riverside, CA, USA article info Article history: Received 29 April 2020 Received in revised form 18 June 2020 Accepted 18 June 2020 Available online 26 June 2020 Keywords: Resistance Pyrethroids Adaptation Recessive Hyalella azteca abstract Several populations of the amphipod, Hyalella azteca, have developed resistance to pyrethroid in- secticides due to non-target exposure, but the dominance of the resistance trait is unknown. The current study investigated the dominance level of point mutations in natural populations of insecticide-resistant H. azteca and determined whether H. azteca from different clades with and without resistant alleles can hybridize and produce viable offspring. A parent generation (P 0 ) of non-resistant homozygous wild type H. azteca was crossbred with pyrethroid-resistant homozygous mutant animals and the tolerance of the filial 1 (F 1 ) generation to the pyrethroid insecticide, permethrin, was measured. Then the genotypes of the F 1 generation was examined to assure heterozygosity. The resistant parents had permethrin LC 50 values that ranged from 52 to 82 times higher than the non-resistant animals and both crossbreeding experiments produced heterozygous hybrid offspring that had LC 50 values similar to the non-resistant H. azteca parent. Dominance levels calculated for each of the crosses showed values close to 0, con- firming that the L925I and L925V mutations were completely recessive. The lack of reproduction by hybrids of the C x D breeding confirmed that these clades are reproductively isolated and therefore introgression of adaptive alleles across these clades is unlikely. Potential evolutionary consequences of this selection include development of population bottlenecks, which may arise leading to fitness costs and reduced genetic diversity of H. azteca. © 2020 Elsevier Ltd. All rights reserved. 1. Introduction The widespread use of terrestrially applied pyrethroid in- secticides has led to unintentional ecological consequences in aquatic systems. Pyrethroid contamination in stream sediments is often the causal factor causing toxicity (Kuivila et al., 2012; Moran et al., 2017; Stehle et al., 2019) and an increasing number of studies have confirmed development of resistance in aquatic non-target species (Weston et al., 2013; Heim et al., 2018; Major et al., 2018). The development of pesticide resistance in non-target species is not a new phenomenon, being documented as early as the 1960s, when mosquito fish (Gambusia affinis) collected near cotton fields were reported to have higher tolerance to DDT than fish collected from areas with no known DDT exposure (Vinson et al., 1963). Addi- tionally, researchers connected resistance to the organophosphate insecticide chlorpyrifos in wood frogs (Lithobates sylvaticus) to proximity of agricultural fields in Pennsylvania, USA (Cothran et al., 2013). Moreover, Daphnia magna readily developed resistance to the carbamate insecticide carbaryl resulting in increased suscepti- bility to the bacterial endoparasite Pasteuria ramose (Jansen et al., 2011). More recently, the cryptic species complex, Hyalella azteca, has developed resistance to pyrethroid insecticides at several * This paper has been recommended for acceptance by Sarah Harmon. * Corresponding author. E-mail address: mlydy@siu.edu (M.J. Lydy). Contents lists available at ScienceDirect Environmental Pollution journal homepage: www.elsevier.com/locate/envpol https://doi.org/10.1016/j.envpol.2020.115074 0269-7491/© 2020 Elsevier Ltd. All rights reserved. Environmental Pollution 266 (2020) 115074