An insight into the antioxidant responses to exogenous oxidative stress agents and a glimpse into the role of catalase in the reproductive fitness of the Gulf Coast tick (Amblyomma maculatum)

Ticks are obligate blood-sucking ectoparasites that feed for prolonged periods of time, varying from few days to several weeks. Aside from dealing with the host’s hemostatic responses, tick must deal with Reactive Oxygen species (ROS) generated from the blood digestion. To fight the deleterious effects of ROS, tick utilizes a battery of antioxidant molecules in their repertoire, including catalase. The hypothesis underlying this study is that the elevated levels of ROS triggers the tick’s antioxidant machinery to alleviate oxidative stress. To test the hypothesis, the relative transcript level of selected antioxidant targets was determined by real time quantitative PCR in tick tissues after treatment with sub lethal doses of H₂O₂ and Paraquat. The selected tick’s antioxidant transcript levels were up-regulated from 2-18 folds in the tick tissues, signifying tick’s ability to activate the antioxidant machinery upon ROS exposure. An RNAi approach was used to silence tick catalase in order to assess the functional relevance of cat in tick hematophagy, hemostasis, and reproductive fitness. Catalase silencing was determined by the observed depletion in transcript level. However, it was found that an increase in reactive oxygen species level, it did not interfere with either tick hematophagy or phenotype. The transcript levels of various tick antioxidants in cat knocked-down tissues were differentially regulated, indicating the presence of alternate pathways of ROS control. Remarkably, application of a cocktail of cat dsRNA and cat inhibitor diminished the tick’s reproductive fitness. These findings support the presence of a robust antioxidant system and demonstrate the functional relevance of tick catalase in mitigating the reactive oxygen species generated during the prolonged tick feeding on the vertebrate host.