Transcriptome Analysis and Identification of Cadmium-Induced Oxidative Stress Response Genes in Different Meretrix meretrix Developmental Stages

Simple Summary Cadmium (Cd) is a major aquatic pollutant that can cause toxic effects on aquatic animals, including the economically important clam Meretrix meretrix. A transcriptomic analysis was performed on M. meretrix fertilized eggs, D-shaped larvae, pediveligers, and postlarvae following exposure to Cd2+-containing seawater. A number of genes related to the oxidative stress response were found to be differentially expressed in the fertilized egg and the different larval forms when exposed to Cd2+. These included the CCO, Ndh, HPX, A2M, STF, and pro-C3 genes, and changes in their expression levels in response to Cd2+ exposure were confirmed by qRT-PCR assays. In addition, qRT-PCR also revealed marked changes in the expression of two other oxidative stress-related genes, Nrf2 and MT, in the fertilized egg and different larval forms of M. meretrix in response to Cd2+ exposure. The results suggested that changes in oxidative stress-related genes might be crucial for M. meretrix to cope with the adverse effects of Cd during its development. Abstract Cadmium (Cd) is one of the major pollutants in the aquatic environment, and it can easily accumulate in aquatic animals and result in toxic effects by changing the metabolism of the body, causing a serious impact on the immune system, reproductive system, and the development of offspring. The clam Meretrix meretrix is one of the commercially important species that is cultivated in large-scale aquaculture in China. To elucidate the underlying molecular mechanisms of Cd2+ in the developmental processes, fertilized eggs and larvae of M. meretrix at different developmental stages were exposed to Cd2+ (27.2 mg L−1 in natural seawater) or just natural seawater without Cd2+ (control), and high-throughput transcriptome sequencing and immunohistochemistry techniques were used to analyze the toxic effects of Cd on larvae at different early developmental stages. The results revealed 31,914 genes were differentially expressed in the different stages of M. meretrix development upon treatment with Cd2+. Ten of these genes were differentially expressed in all stages of development examined, but they comprised only six unigenes (CCO, Ndh, HPX, A2M, STF, and pro-C3), all of which were related to the oxidative stress response. Under Cd exposure, the expression levels of CCO and Ndh were significantly upregulated in D-shaped and pediveliger larvae, while pro-C3 expression was significantly upregulated in the fertilized egg, D-shaped larva, and pediveliger. Moreover, HPX, A2M, and STF expression levels in the fertilized egg and pediveliger larvae were also significantly upregulated. In contrast, CCO, Ndh, HPX, A2M, STF, and pro-C3 expression levels in the postlarva were all downregulated under Cd exposure. Besides the genes with changes in expression identified by the transcriptome, the expression of two other oxidative stress-related genes (MT and Nfr2) was also found to change significantly in the different developmental stages of M. meretrix upon Cd exposure, confirming their roles in combating oxidative stress. Overall, the findings of this study indicated that Cd would interfere with cellular respiration, ion transport, and immune response through inducing oxidative stress, and changes in the expression of oxidative stress-related genes might be an important step for M. meretrix to deal with the adverse effects of Cd at different stages of its development.