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

Until the last few decades, anthropogenic chemicals used in most
production processes didn’t have a comprehensive assessment of 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 practises 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
originates 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.