Establishing transcriptomic platforms for two common marine molluscan biomonitoring species

Abstract

Since industrialisation, continuous use and release of man-made chemicals in marine environments have posed health risks to both marine organisms, and humans who consume contaminated seafood. Although analytical tools allow us to quantify many chemical contaminants in environmental samples at low concentrations (μg/L or ng/L), they cannot capture all chemicals and the concentration data cannot be directly used to predict health effects to marine organisms or humans. To rectify such shortcomings, biomarker approaches are useful to indicate toxic effects of chemical contaminants to marine biomonitors. With the advance of next generation sequencing (NGS) technologies, it is now possible to construct transcriptomes of any species in a short time at an affordable cost. This study aimed to create transcriptomes for two common biomonitor species, namely the intertidal whelk Reishia clavigera and the green-lipped mussel Perna viridis, and apply such genomic information to reveal the toxic mechanisms of organotin compounds (OTs) as examples.

Among OTs, tributyltin (TBT) and triphenyltin (TPT) can cause the development of imposex in female gastropods and inhibit growth in bivalves. Yet, their toxic mechanisms remain largely unknown. This study, therefore, investigated their toxic mechanisms in the two biomonitor species using NGS technology with an aid of bioinformatic analysis.

Before the exposure experiments, a global de novo transcriptome of R. clavigera was constructed from five target tissues (i.e., digestive gland, gonad, head ganglia, penis and remaining body tissues) of the male and female whelks using Illumina RNA sequencing. The resultant transcriptome comprised 280,652 assembled transcripts and 228,124 assembled unigenes. This transcriptome database offers molecular resources for diagnostics of environmental health and studying toxic mechanisms of chemical contaminants.

Female whelks were chronically exposed to TBT or TPT for 120 days and analysed for RNA expression over time. The results suggested that OTs can up-regulate the steroidogenic unigene steroid 17-alpha-hydroxylase/17,20-lyase (CYP17A1) and thereby increase biosynthesis of androstenedione and free testosterone, which may, in turn, promote imposex development. Concurrently, OTs-mediated up-regulation of nuclear receptor (NR) peroxisome proliferator-activated receptor gamma (PPARγ) may trigger the regulation of its downstream biological processes that are associated with imposex development. Moreover, OTs-mediated regulation of phosphorylation and inhibition of proteases possibly modulate the NR activities and hence indirectly govern imposex development.

Likewise, P. viridis were chronically exposed to TPT for 28 days and analysed for RNA expression. The results indicated a wide range of adverse effects of TPT to the mussel. TPT regulates the steroid hormone system by activating steroidogenic unigene CYP17A1, NR unigene retinoic X receptor alpha (RXRα) and estrogen receptor (ER) in their gills, and modulating 17 beta hydroxysteroid dehydrogenase type 12 (HSD17B12) and vitamin D3 receptor (VDR) in their hepatopancreas. TPT can cause impairment of lipid metabolism, function of detoxification and cellular apoptosis, and also suppress transcription of shell protein that may lead to abnormal shell formation.

This study clearly demonstrated the effectiveness and usefulness of NGS in ecotoxicological studies, providing more insights on the toxic mechanisms of target contaminants and identifying diagnostic biomarkers for effective biomonitoring of marine pollution.