| Summary: | Paralytic shellfish toxins (PST) could be accumulated in bivalves and cause safety problems. To protect public health, bivalves are examined for PST contamination before entering the market, usually by high-performance liquid chromatography (HPLC) or LC-tandem mass spectrometry (LC-MS/MS) in the lab, which needs PST standards not all available and is time-consuming for large sample sizes. To detect PST toxicity in bivalves rapidly and sensitively, a biomarker gene is highly demanded, but the related study is very limited. In this study, we fed a commercially important bivalve, <i>Patinopecten yessoensis</i>, with the PST-producing dinoflagellate <i>Alexandrium catenella</i>. After 1, 3, and 5 days of exposure, both PST concentrations and toxicity levels in the digestive gland continuously increased. Transcriptome analysis revealed that the differentially expressed genes were significantly enriched in oxidation-reduction process, which included the cytochrome P450 genes (<i>CYP</i>s), type I iodothyronine deiodinase (<i>IOD1</i>s), peroxidasin (<i>PXDN</i>), and acyl-Coenzyme A oxidase 1 (<i>ACOX1</i>) at day 1 and a superoxide dismutase (<i>SOD</i>) at day 5, highlighting the crucial roles of these genes in response to oxidative stress induced by PST. Among the 33 continuously upregulated genes, five showed a significant correlation between gene expression and PST concentration, with the highest correlation present in <i>PyC1QL4-1</i>, the gene encoding Complement C1Q-like protein 4, C1QL4. In addition, the correlation between <i>PyC1QL4-1</i> expression and PST toxicity was also the highest. Further analysis in another aquaculture scallop (<i>Chlamys farreri</i>) indicated that the expression of <i>CfC1QL4-1</i>, the homolog of <i>PyC1QL4-1</i>, also exhibited significant correlations with both PST toxicity and concentration. Our results reveal the gene expression responses of scallop digestive glands to PST-producing algae and indicate that the <i>C1QL4-1</i> gene might be a potential biomarker for PST monitoring in scallops, which may provide a convenient way for the early warning and sensitive detection of PST contamination in the bivalves.
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