Changes in expression of stress genes in copepods feeding upon a non-brevetoxin-producing strain of the dinoflagellate Karenia brevis

Abstract

Effects of the toxic Gulf of Mexico dinoflagellate Karenia brevis on its copepod grazers are equivocal, in that this dinoflagellate is ingested by various copepods, but may be toxic to, and/or nutritionally inadequate for grazers. Recent studies have shown that when the Mediterranean copepod Calanus helgolandicus fed on K. brevis, egg production rates and egg viability declined steadily over time, reaching almost zero levels after 5 days of feeding (Turner et al., 2012). The K. brevis strain used in that study was chemically analyzed and shown not to contain any brevetoxins, but may have contained other harmful metabolites. Here we present gene expression analyses by reverse transcription-quantitative PCR (RT-qPCR) in C. helgolandicus after different exposure times (3, 5 and 8 days) to the same non-brevetoxin-containing K. brevis strain examined previously. The analyzed genes have a primary role in generic stress responses, defense systems (e.g. aldehyde, free fatty acid and free radical detoxification) and apoptosis regulation in other organisms. After 3 days of feeding on K. brevis, gene expression levels in copepods did not show significant changes. However, after 5 days of exposure, expression of stress-response genes was significantly reduced. After prolonged exposure for 8 days, expression of genes for 5 of 6 aldehyde dehydrogenases was strongly reduced, but expression of other genes was not coherently reduced, or increased. Microscopic observations revealed that after only a single day of feeding on K. brevis, freshly-spawned eggs were morphologically similar to those produced by copepods feeding on control diets, but after 3 days of feeding on K. brevis, eggs exhibited altered membrane cell morphology and apoptotic features such as granulation and degeneration of the cytoplasm matrix, compared to the unaltered eggs produced by copepods feeding on control diets. The changes in gene expression levels in copepods feeding on K. brevis were similar to those noted previously for copepods feeding upon diatoms that contained oxylipins, which interfered with copepod reproductive success. Changes in gene expression in copepods feeding upon toxic algal diets may offer important early warning signals of copepod stress responses to these diets.