Cadmium (Cd) is present in the environment in low concentrations. Elevated soil Cd has been found to have significant impact on reproduction and gene expression in earthworms, an ecotoxicological sentinel species. This research explored the impact of varying concentrations of Cd on gene expression and reproduction in E. fetida.
Artificial soil was spiked with three different Cd solutions; Low (30 mg/kg) medium (90 mg/kg) and high (270 mg/kg) concentrations. Four replicates from each concentration and control were maintained. During the test period, 10 worms were introduced into each beaker, the worms were fed oatmeal and water loss was replenished every 2 days. At days 28 and 56 the number of cocoons per beaker were counted and worms selected for RNA extraction. Before RNA extraction the worms were depurated. The extracted RNA was sequenced using an Illumina HiSeq 2500. Trinity and Trinotate and Transdecoder freeware programs were used to assemble and functionally annotate a de novo transcriptome from the extracted RNA. EdgeR was used to identify differentially expressed transcripts between the four conditions.
34 genes were identified as significantly differentially expressed (p= 0.001) between the four conditions with minimum 15 fold change in expression level. Of particular interest to reproduction this included Chondroitin proteoglycan 2 (CPG2) and Follistatin (FST). CPG2 has a role in polar body extrusion during cytokinesis in embryo development and was not expressed under the highest Cd concentration. FST is a specific inhibitor of the biosynthesis and secretion of pituitary follicle stimulation hormone (FSH). FST was highly expressed under high Cd concentrations and not expressed at all in low Cd concentration. Cocoon production of E. fetida at 28 days was negatively correlated with increasing FST levels. FST could be a potential biomarker of the reproductive impact of Cd exposure.