Historical exposure to chemicals reduces tolerance to novel chemical stress in Daphnia (waterflea)

Abstract

Until the last few decades, anthropogenic chemicals used in most production processes have not been comprehensively assessed for their risk and impact on wildlife and humans. They are transported globally and usually end up in the environment as unintentional pollutants, causing long-term adverse effects. Modern toxicology practices typically use acute toxicity tests of unrealistic concentrations of chemicals to determine their safe use, missing pathological effects arising from long-term exposures to environmentally relevant concentrations. Here, we study the transgenerational effect of environmentally relevant concentrations of five chemicals on the priority list of international regulatory frameworks on the keystone species Daphnia magna. We expose Daphnia genotypes resurrected from the sedimentary archive of a lake with a known history of chemical pollution to the five chemicals to understand how historical exposure to chemicals influences adaptive responses to novel chemical stress. We measure within- and transgenerational plasticity in fitness-linked life history traits following exposure of "experienced" and "naive" genotypes to novel chemical stress. As the revived Daphnia originate from the same genetic pool sampled at different times in the past, we are able to quantify the long-term evolutionary impact of chemical pollution by studying genome-wide diversity and identifying functional pathways affected by historical chemical stress. Our results suggest that historical exposure to chemical stress causes reduced genome-wide diversity, leading to lower cross-generational tolerance to novel chemical stress. Lower tolerance is underpinned by reduced gene diversity at detoxification, catabolism and endocrine genes in experienced genotypes. We show that these genes sit within pathways that are conserved and potential chemical targets in other species, including humans.

FIGURE 1

Experimental design. Four genotypes of Daphnia magna, previously resurrected from dormant embryos, were used for transgenerational exposures to five chemicals: PFOS (70 ng L⁻¹), diclofenac (2 mg L⁻¹), trimethoprim (2 mg L⁻¹), atrazine (0.2 mg L⁻¹) and arsenic (1,000 µg L⁻¹). The four genotypes were resurrected from different times in the past, and were either “naïve” (black ‐ LRII36_1 [<1950] and blue, LRV12_3 [1960–1970]) or experienced (green, LRV8.5_3 [1980–1990] and red, LRV0_1 [>1999)] to chemicals. The clonal lines from the four genotypes were maintained in common garden conditions for at least two generations before the experiment to control for maternal effect. Five clonal replicates of each genotype, randomly selected from the control environment, were exposed to the five chemicals for three generations (G). Coloured squares represent genotypes at each generation (G). Some genotypes went extinct in G2 and G3

FIGURE 2

Phenotypic trajectory analysis. PTA on the four genotypes of Daphnia magna used in transgenerational exposure to five chemical classes (PFOS [70 ng L⁻¹], diclofenac [2 mg L⁻¹], trimethoprim [2 mg L⁻¹], atrazine [0.2 mg L⁻¹] and arsenic [1,000 µg L⁻¹]), resulting from multivariate response of five fitness‐linked life history traits. Open circles represent the control (nonexposed clonal replicates) and full circles represent the exposed clonal replicates. Genotype centroids are connected by reaction norms (solid lines), showing phenotypic change in direction and length. Differences among genotypes in terms of magnitude (M) and direction (θ) of plastic response are all significant. The statistics supporting the PTA are given in Table S1. Genotypes are colour‐coded as in Figure 1

FIGURE 3

Genomic diversity. Alpha diversity measured at (a) genome‐wide and (b) chromosome‐level in the four genotypes used in this study; (c) number of significantly divergent genes between each pair of genotypes. The genotypes are colour coded as in Figure 1: LRII36_1 (<1950; black), LRV12_3 (1960–1970; blue), LRV8.5_3 (1975–1985; green) and LRV0_1 (> 1999; red).