Abstract
Insects have developed an efficient defence system against microorganisms, which involves both humoral and cellular mechanisms. Recent studies on insect defence system are aimed at utilizing it in the battle against mosquito-borne diseases. However, mounting immune responses of insects has proved to impose fitness costs. The current study was conducted to test the costs of enhancing the immune responses of Aedes caspius by oral administration of thymoquinone, the active ingredient of the black seed oil, Nigella sativa L. (Ranunculaceae). The obtained results showed an effective humoral activity against Micrococcus luteus (NCTC 2665, Sigma-Aldrich, UK) and Bacillus cereus when mosquitoes were injected with lipopolysaccharide (LPS), Bacillus thuringiensis or B. cereus. However, this activity showed no effect against B. thuringiensis. Furthermore, oral administration of thymoquinone enhanced the humoral activity against B. cereus but not against B. thuringiensis. On the other hand, thymoquinone-enhanced immunity imposed reproductive costs in terms of higher percentages of follicular apoptosis and resorption. This observation has been confirmed by the semi-quantitative reverse transcriptase polymerase chain reaction (RT-PCR) analysis, which showed a relatively higher expression of selected caspases genes, namely: CASP 18, CASP 19 and CASP 20, but not CASP 21, transcripts in immune enhanced mosquitoes compared to non-enhanced ones. Based on these results, this study suggests that enhancing the humoral activity by thymoquinone proved to be costly in terms of triggering follicular apoptosis and resorption. Thus, as part of the scenario of immunity-reproduction conflict, it was concluded that the impact of immune stimulation and/or enhancement on the vector reproduction constitutes a limiting factor to the utilization of thymoquinone in the immuno-control strategy against mosquito-borne diseases.