Molecular characterization and expression of vitellogenin (Vg) genes from the cyclopoid copepod, Paracyclopina nana exposed to heavy metals

Induction of vitellogenin (Vg) has been used as a biomarker of exposure to heavy metals and endocrine-disruption chemicals (EDCs) in aquatic organisms. Here, we identified the full-length Vg1 and Vg2 sequence from the brackish water copepod, Paracyclopina nana. Vg1 gene contained 5718 bp of the open reading frame (ORF) that encoded the putative protein of 1905 amino acids residues, while Vg2 gene consisted of 5442 bp of ORF, encoding the putative protein of 1813 amino acids residues. P. nana Vgs showed highly conserved domains in the N-terminal region. The phylogenetic analysis revealed that P. nana Vgs are distinct from other arthropods, such as insects and decapods, as it formed a clade with other copepods, Tigriopus japonicus and salmon louse (Lepeophtheirus salmonis). The expression of Vg transcripts was detectable after the copepodid stages 4-5. Female copepods expressed over 83 times and 223 times more Vg1 and Vg2 transcripts, respectively, than males. When copepods were exposed to heavy metals (0.1 mg/L Cd, 0.4 mg/L Cu, and 2 mg/L AsIII) for 24, 48, 72, and 96 h, P. nana Vg transcripts were highly induced in a time-dependent manner. Interestingly, Vg2 gene was more susceptible than Vg1 to trace heavy metal exposure. This finding indicates that P. nana Vgs provide a potential indicator for assessing the toxic effect of heavy metals. In addition, we suggest P. nana as a potential model species for risk-assessment to environmental pollutants in brackish water.