With the rapid pace of industrialization and frequent occurrence of marine oil spill accidents, persistent organic pollutants (POPs) are globally detectable in terrestrial and marine environment (Heskett et al. 2012; Wurl and Obbard et al. 2004). Polycyclic aromatic hydrocarbons (PAHs) are one of the most widespread POPs in marine ecosystem, which represent a potential health hazard to aquatic organisms (Ankley et al. 2003; Kim et al. 2013). Due to the carcinogenicity, teratogenicity and mutagenicity of PAHs, sixteen chemicals of them have been classified as the prior controlled contaminants by US Environmental Protection Agency (US EPA) (Office of the Federal Registration, 1982). During the last decades, numerous studies have observed that PAHs pollution could induce adverse effects on reproduction of marine organisms, including sex reversal, delaying gonadal development and spawn probability (Chikae et al. 2004; Rochman et al. 2014; Vignet et al. 2016). For example, reduction in the successful rate of reproduction as well as survival rate of outputs were observed in maternal oyster Crassostrea gigas exposed to a kind of model PAHs, benzo[a]pyrene (B[a]P), during ovarian development (Choy et al. 2007). Consequently, the phenomenon has attracted great concerns due to the strong evidences that PAHs possess high potential to be reproductive toxicants towards marine organisms.
Currently, plenty of studies suggested an association between PAHs exposure and a range of reproductive toxicity effects in marine organisms, such as endocrine disruption (Collier et al. 2013; Yamamoto et al. 2017), DNA damage (Everaarts and Sarkar 1996; Meier et al. 2020), and histological alterations of gonads (Chukwuka et al. 2019; Louiz et al. 2009). However, information on the underlying toxic mechanisms of PAHs to marine invertebrates, such as mollusks (e.g., bivalves) is relatively scarce (Cuevas et al. 2015; Jing-jing et al. 2009; Sarker et al. 2018; Simão et al. 2020). Traditional studies in bivalves, investigating the toxic mechanism of POPs on reproduction, are based on adverse effects as well as the transcriptional changes of well-known functional genes (Boulais et al. 2018; Yang et al. 2020a; Yang et al. 2020b). Unfortunately, many data gaps still remain in bivalves (Liu et al. 2020). Hence, there is an urgent need for a more efficient method that could comprehensively elucidate the potential molecular mechanism of PAHs exposure on reproduction hazards in mollusks (Honda and Suzuki 2020).
Compared with traditional toxicological data, omics approaches can provide high-throughput data of numerous biomolecules and reflect the integrated global responses within organisms (Chen et al. 2019a; Zhang et al. 2010). Over the past decade, transcriptomic analysis has been well developed and widely introduced in reproductive toxicology studies in many aquatic species, such as fishes (Berg et al. 2016; Colli-Dula et al. 2018; Schiller et al. 2013), crustaceans (Liu et al. 2019; Yu et al. 2018), and bivalves (An et al. 2014; Bachère et al. 2017). Recently, Suman et al. (2020) determined the acute and chronic toxicity of polystyrene microplastics on brine shrimp, and also exhibited the detailed transcriptomic data to enhance current acknowledgement of their molecular toxicity. Overall, the integration of transcriptomic profiling in toxicology and toxicological data can provide a deeper understanding of the mechanism behind toxic processes.
Studies have demonstrated that stage-dependent induction of reproductive toxicity under POPs exposure, especially for early-life stage animals, were the most sensitive groups to given dose (Collier et al. 2013). Additionally, life-cycle POPs exposure has been proved to cause more severe impairment to reproductive functions in fish, such as disturbing steroidogenesis, blocking oocyte development and delaying spawning (Chen et al. 2019b; Horri et al. 2018; Timme-Laragy et al. 2006). However, the majority of previous studies merely conducted as short-term exposure experiments in bivalves, and focused on the specific reproductive stages (mainly at mature stage) during gonadal development (Alonso et al., 2019; Smolarz et al., 2017). The limited research has restricted our comprehensive knowledge on the reproduction-toxic effects of PAHs in bivalves.
Scallop Chlamys farreri is a typical bivalve for coastal monitoring programs, which is frequently used for toxicological studies (Zhang et al. 2011). In the present study, female C. farreri were dosed with different concentrations of B[a]P (0, 0.04, 0.4 and 4 µg/L) during ovarian development stages (proliferative, growth, mature, and spawn stage), respectively. Where 4 µg/L B[a]P exposure group and control group were conducted for ovarian transcriptome to screen reproductive-toxicology genes for following analysis. Overall, this study first highlighted the multi-stage molecular toxic mechanism by combining transcriptomic analysis and reproductive toxic indicators in bivalves. This would not only promote our understanding of the molecular toxicity of B[a]P in bivalves, but also provide vital data for future relevant ecotoxicological studies.