The transcriptome-wide effects of exposure to a pyrethroid pesticide on the Critically Endangered delta smelt Hypomesus transpacificus

Research into the effects of contaminants on fishes is often conducted on wellstudied model test species, whose responses may be different than those of endangered species. We developed an oligonucleotide microarray consisting of 12 595 genes to examine the effects of the pesticide permethrin on an endangered fish in California, USA, the delta smelt Hypomesus transpacificus. The microarray was used in combination with quantitative PCR (qPCR) assessments. We exposed larval delta smelt to permethrin for 96 h at concentrations of 0.69, 1.37, 2.56, 4.84, 12.88 and 24.94 μg l−1, and compared the responses to a control group. The 96 h 50% lethal concentration (LC50) was 4.07 μg l−1, which is lower than those reported for model test species and similar to other endangered species. With increasing exposure concentration, we detected the differential expression of 3342 microarray features with elevated expression of genes involved in protein degradation and apoptosis, and decreased expression of immune function genes. Functional analysis indicated that genes involved in protein degradation, immune function, an unfolded protein response, meta bolism and cell signaling cascades were affected by exposure to permethrin. Many of the gene responses in the 0.69 μg l−1 treatment group differed in their directional change in expression from those at higher exposure concentrations, suggesting a potential mechanistic threshold of sub-lethal toxicity at concentrations below the lowest observed effect concentration (LOEC) of 2.56 μg l−1. These results are consistent with non-monotonic response patterns to contaminants, and demonstrate the effects on a sensitive fish species of a widely used pyrethroid pesticide at concentrations below those that affect model test species.