Induction of hemocyte apoptosis by Ovomermis sinensis: Implications for host immune suppression.

The entomopathogenic nematode, Ovomermis sinensis, is a parasite of some common lepidopteran pests. O. sinensis is able to overcome the immune system of its hosts and eventually kill the hosts when it emerges. We provide insight into how the mermithid nematode overcomes the immune response of the host Helicoverpa armigera. Our results indicate that O. sinensis actively inhibits host immune responses as evidenced by hemocyte nodulation, spreading behavior, lysozyme activity and melanization. However, O. sinensis did not inhibit host immune responses through immune gene activation. Moreover, the research on the immune depressive strategies of O. sinensis revealed that the parasite did not inhibit host effector molecules, but did reduce the number of hemocytes. Flow cytometry analysis revealed that the host hemocytes were apoptotic within 24 h, and no hemocytes were present after 72 h. In addition, our in vivo and in vitro studies showed that the survival rate of O. sinensis was increased when hemocyte proliferation was inhibited. Our findings suggest that O. sinensis inhibited host immune responses by inducing apoptosis of host hemocytes.

Cloning and expression analysis of the DEAD-box/RNA helicase Oslaf-1 in Ovomermis sinensis.

Ovomermis sinensis is a potentially-valuable nematode for controlling insect pests. The parasitic stage of the nematode absorbs nutrients in its host's hemolymph to maintain its growth development and then kills the host when it emerges. At present, little known about its reproductive development, particularly the responsible molecular mechanism. More detailed research on the genes of reproductive development will not only help us understand the mechanisms underlying sex differentiation in the nematode, but would also be valuable for successfully cultivating them in vitro and using them for biocontrol. In this study, we used the homology cloning method to clone the full-length cDNA of a DEAD-box family gene (Oslaf-1) from O. sinensis. Then, using qRT-PCR technology to detect the expression pattern of the Oslaf-1 gene at different development stages and tissues, the gene was found to be highly expressed in the post-parasitic stage (P < 0.01) and ovarian (P < 0.05) of O. sinensis. Western blot analysis showed the same result that the gene is associated with gonadal development and function, but is not gonad-specific. In situ hybridization further demonstrated that the gene is widely expressed in early embryos and is mainly distributed in the gonadal area. However, the signal was mainly concentrated in the reproductive primordia in pre-parasitic juveniles. RNA interference (RNAi) studies revealed that the sex ratio of O. sinensis soaked in dsRNA of Oslaf-1 was not statistically different than the gfp dsRNA treated groups. Our results suggest that Oslaf-1 may play a vital role in the reproductive systems of the nematode. In addition, we speculate that the Oslaf-1 gene plays an important role during embryonic development and that it occurs and develops in the gonads of pre-parasitic juveniles of O. sinensis.