Comparative transcriptome analysis reveals the biological mechanism of selective cadmium enrichment in Tegillarca granosa

The bloody clam Tegillarca granosa, is a commercial benthic bivalve in China that can selectively accumulate cadmium (Cd) among the metals in the marine environment, leading to high Cd accumulation. To explore the molecular mechanism of selective Cd enrichment in T. granosa, transcriptomic analyses of gills under heavy metal stress were performed. In this study, a copper (Cu)-exposed group was used as the positive control, a natural seawater group was used as the negative control, and the experimental groups were exposed to Cd or Cd & Cu. Twelve gill transcriptome libraries of T. granosa under heavy metals stress were constructed and comparatively analyzed. A total of 704,103,872 clean reads were obtained, 68.01% of which were successfully mapped to the reference genome of T. granosa. Additionally, 1045, 1201 and 931 differentially expressed genes (DEGs) were detected in the Cd vs. C, Cd & Cu vs. C and Cu vs. C comparisons, respectively. KEGG enrichment analysis showed that the “endocytosis”, “olfactory conduction” and “osteoclast differentiation” pathways were significantly enriched. Furthermore, seven genes involved in the cell entry, transportation, storage, and detoxification of heavy metals were screened and selected to validate the transcriptome data by qRT-PCR. Four selective Cd enrichment-related functional genes, CNGA3, NCXs (NCX1 and NCX2) and PIP5K, were screened; the processes of Cd cell entry, intracellular transport and storage were described, and hypotheses about these processes were put forth. In conclusion, the effective selective enrichment and regulatory network of Cd in T. granosa were revealed, providing a new theoretical basis for cultivating new varieties with low Cd accumulation in T. granosa.