Are Toxicological Responses in Laboratory (Inbred) Zebrafish Representative of Those in Outbred (Wild) Populations? − A Case Study with an Endocrine Disrupting Chemical

Abstract

Laboratory animals tend to be more inbred and less genetically diverse than wild populations, and thus may differ in their susceptibility to chemical stressors. We tested this hypothesis by comparing the responses of related inbred (theoretical inbreeding F(IT) = n + 0.25) and outbred (F(IT) = n) zebrafish (Danio rerio) WIK/Wild family lines to an endocrine disrupting chemical, clotrimazole. Exposure of inbred and outbred zebrafish to 2.9 μg clotrimazole/L had no effect on survival, growth, or gonadal development. Exposure of both lines to 43.7 μg clotrimazole/L led to male-biased sex ratios compared with controls (87% versus 55% and 92% vs 64%, for inbred and outbred males, respectively), advanced germ cell development, and reduced plasma 11-ketotestosterone concentrations in males. However, outbred males (but not inbred males) developed testis that were more than twice the weight of controls, which corresponded with a proliferation of Leydig cells and maintenance of the expression (rather than down-regulation occurring in inbreds) of gonadal aromatase (cyp19a1a) and insulin-like growth factor (igf1). Our results illustrate that the effects of an endocrine disrupting chemical (clotrimazole) on some end points (here testis development) can differ between inbred and outbred zebrafish. This highlights the need for reporting pedigree/genetic information and consistency in the responses of laboratory animals (e.g., by using model compounds as positive controls).

Climate change and pollution speed declines in zebrafish populations

“…We found that elevating the temperature from 28°C to 33°C increased the somatic growth rate, but this growth increase was not the key determinant of sexual differentiation in our study (i.e., it did not favor the development of females). Instead, male-skewed sex ratios in the elevated temperature and/or clotrimazole exposure treatments appeared to be mediated by the inhibition of P450(CYP)19 aromatase, as has been found in other studies (39,40,42). Gene expression of cyp19a1a encoding aromatase was up-regulated in the few females remaining in the inbred, elevated temperature, and/or high-level clotrimazole exposure treatments, and we hypothesize that this up-regulation was the result of a compensatory response to P450(CYP)19 aromatase inhibition in these individuals.…”

“…✗This "low dose × low temperature" treatment was not included in our analyses because we knew, from our previous work (40,83), that the lowdose clotrimazole (2 μg/L) alone would not affect population-relevant end points (survival, growth, gonadal development or fecundity), and we sought to maximize replication for the other treatment regimes.…”

“…Each exposure tank was separated by a sealed glass partition into two lanes, each with four screened flow-through compartments containing either inbred or outbred fish families. This arrangement avoided any possible exchange of pheromones secreted into the water between inbred and outbred fish (40). Each compartment was stocked with four fish taken at random from any given inbred or outbred family, and assignment of tanks to each treatment was also randomized.…”

“…The zebrafish is a freshwater species native to the Indian subcontinent and has been studied widely as a model animal in biomedical and environmental science (36)(37)(38). Importantly, the sex-determining mechanism in zebrafish is known to be modified by environmental factors, and independent exposures to elevated water temperatures (21,39), and to chemicals that inhibit aromatase, have been shown to induce male development in this species (39,40). The enzyme aromatase [cytochrome P450(CYP)19] mediates the conversion of male hormones (androgens) to female hormones (estrogens), and thus plays a pivotal role in sex assignment in zebrafish and other vertebrates (41).…”

The Parthenogenetic Cosmopolitan Chironomid, Paratanytarsus grimmii, as a New Standard Test Species for Ecotoxicology: Culturing Methodology and Sensitivity to Aqueous Pollutants

“…However, laboratory Chironomus spp. cultures often suffer from genetic impoverishment (Nowak et al 2007b), reducing their similarity with the wild populations they are supposed to represent (Brown et al 2011). Genetic impoverishment also leads to inbreeding depression, which can compromise toxicological test results (Nowak et al 2007a).…”