| Parasitoid wasps are important natural enemies of agricultural pests.During co-evolution process,hosts have evolved to form multiple defensive strategies against parasitoid wasps,which mainly include humoral and cellular immunity.The behavioral defense is also another type of effective strategy to defend parasitoids,however,it is not well studied.Defensive behaviors are always costly and energy-intensive.In high-risk environments,defensive behaviors favor organism survival.However,in low-risk environments,defensive behaviors are not only wasting energy,but also impairing other critical activities.As such,it is important for animals to adjust appropriate response decisions when dealing with different levels of threats.In this study,Leptopilina boulardi and Drosophila melanogaster were used as a study system to investigate how the hosts recognize the parasitoids,assess their thret level and the mechanisms of the behavioral defenses.(1)The young L.boulardi exposure decreases host’s oviposition rate.Female L.boulardi parasitoids could induce about 50% reduction in the host’s egg laying,while parasitoids of either non natural enemy or male do not.A long-term exposure experiment showed that the oviposition rate of host returned to normal after a certain period of exposure time.When host’s egg laying returned to normal,the hosts were re-exposed to young or old parasitoids,and we found hosts lay fewer eggs again when exposed to young parasitoids but still presented normal when exposed to old parasitoids.The results indicate that the decreased host oviposition rate is a kind of defensive behavior,and is related to parasitoid ages.(2)Hosts recognize the age of parasitoids through visual perception of the wasp search behavior.When the blind hosts were exposed to L.boulardi,they typically decreased oviposition rate.However,olfactory-deficient mutants did not.These results suggest that the hosts perceive parasitoids through visual system.When LC4 neurons were blocked,the exposed flies didn’t show a reduction in oviposition rate,indicating that this blockage eliminates the threats of young female parasitoids.The parasitism efficiency of L.boulardi gradually decreased with age.After reducing the parasitic rate of young parasitoids by knocking down Orco gene and ablating antennae,the exposed hosts didn’t decrease oviposition rate,demonstrating that hosts don’t develop defensive behaviors against parasitoids with low-threats.We then found that the search frequency and search speed of the old,Orco-knocked and antennae-ablated parasitoids were significantly decreased compared with the young parasitoids.These results suggests that hosts use vision to perceive the search behavior of parasitoids,determine the danger levels of parasitoids,and develop different defensive strategies.(3)Exposure to L.boulardi inhibits host ovulation.After dissecting the ovaries of hosts exposed to young and old parasitoids,we found that the ovaries of hosts exposed to young parasitoids were significantly larger than controls,and contain more mature eggs.The ovaries of hosts exposed to old parasitoids showed similar size to controls.These results suggest that young parasitoids might impair the ovulation process of hosts.Next,we checked the status of key characters of ovulation,including follicular rupture,Mmp2 activity,and oviduct muscle relaxation.We found that the exposure to young parasitoids significantly reduce host follicular rupture and Mmp2 enzyme activity,and affect oviduct muscle relaxation.(4)Octopamine mediates host oviposition changes under parasitoid stress.The q PCR analysis of octopamine(OA)synthesis genes(Tdc2 and Tβh)in host brain and ventral nerve cord(VNC)reveals that the RNA levels of OA synthesis genes decreased in host VNCs when encounting young parasitoid exposed.Analysis of OA levels by immunohistochemical staining showed that OA levels were reduced in both VNC and reproductive system after exposure to young parasitoids.After activating OA neurons and injecting OA mimic,the decreased oviposition were significantly rescued.These results suggest that host Drosophila reduce the OA content in the reproductive system by down-regulating OA synthesis in the VNC,leading to decreased oviposition under the stress of young parasitoids.In summary,this study reveals that the hosts have evolved the ability to assess the risk of parasitoid wasps,and made adaptive defense behaviors of oviposition for different risk levels of parasitoid wasps.We also investigated the basis for parasitoid risk assessment and the molecular mechanisms underlying adaptive defensive behaviors of oviposition.These findings provide insights and references for insect behavioral defense and the risk assessment of insects to natural enemies. |
