Effects Of Virus And Magnetic Field On The Development,Reproduction And Behaviour Of Rice Planthopper

Insects,as being the largest animal group on earth,are interacting with their surrounding environment(including biotic and abiotic factors),which is precisely the main study topics of Insect Ecology.The interactions are also one of the major forces in insect evolution.Based on Insect Ecology,the adaptive strategies,as well as the corresponding mechanisms of insects in response to their environment can be explored by studying the effects of biotic and abiotic factors on insects.Furthermore,this study can be expected to offer new ideas in developing methods in biological and physical control.Rice planthopper of brown planthopper,Nilaparvata lugens,white-backed planthopper,Sogotalla furcifera and small brown planthopper,Laodelphax striatellus are migratory insect pests,which are frequently occurring to cause heavily damage on rice production in Asia.Moreover,the rice planthoppers can result in damage on cereal crops via transmitting plant virus,especially for L.striatellus as being the vector of rice stripe virus(RSV).Here,by studying the effects of RSV infection on the physiology and behaviour of L.striatellus,we can expect to reveal the mechanisms of mutual adaptation strategy of insects and virus during their interaction progress,and further discover some new strategies in controlling plant viruses and their insect vectors.To date,the mostly general opinion on the migratory characteristics of N.lugens,S.furcifera and L.striatellus is that they are likely to migrate with the airflow.Recently,the long-distance migratory monarch butterfly,Danaus plexippus showed the ability to orientate with geomagnetic field,this provides the basis for the study on magnetic orientation and magnetobiology of rice planthoppers.For a long time,the earth's magnetic field(i.e.,geomagnetic field,GMF),as being an easily neglected but ubiquitous abiotic factor,has attracted more and more attention to the research of the magnetic bio-effects on organisms triggered by the change of the intensity,as well as its proved knowledge of providing orientation information.Although the study of bio-effects triggered by near-zero magnetic field(NZMF)which is regarded as a good approximation to simulate GMF decay of the earth and the zero-magnetic environment in space is now in an primary stage,it is of great significance to discover the ways of insects in respond to the decay of GMF and the magnetoreception mechanism of insects.In this research,we conducted the study of interactions between the biotic RSV and its vector L.Striatellus,and the magnetic bioeffects triggered by the abiotic NZMF on the three main species of rice planthoppers,N.lugens,S.furcifera and L.striatellus.The main results of this study are as follows:1.Effects of RSV infection on the development and reproduction of its vector,small brown planthopper,L.striatellusIn this study,the RSV-L.striatellus-YLS(yeast-like symbionts)system was investigated.The results indicated that the RSV infection resulted in down-regulated JHAMT and up-regulated CYP307A1 in the juvenile hormone and ecdysteroid pathways of the 5th instar nymph,and it is consistent with the accelerated nymphal development.In general,the shortened nymphal development can strengthen the horizontal transmission of RSV and promote the population expansion of L.striatellus.Moreover,the RSV infection also increased body weight of the newly emerged adults,this may be caused by the increase in abundance of YLS in the adults with RSV infection,and it is beneficial for the fitness of L.striatellus and RSV.Except for the beneficial effects of RSV infection on L.striatellus,the RSV-infected L.striatellus exhibited a significant decrease in fecundity that could be triggered by the down-regulation of Vitellogenin(Vg).Based on the principle of optimal fitness during the evolving procedure,the beneficial and deleterious effects induced by the RSV infection are likely to be explained by the strategy of "Trade-off' during the co-evolution of the RSV-L.striatellus-YLS system.2.Effects of RSV infection on the feeding behaviour and flight capacity of its vectors,small brown planthopper,L.striatellusIn this study,we systematically identified eight feeding waveforms of L.striatellus referenced to the EPG study of other piercing-sucking insects(e.g.,rice planthoppers and aphids),including the NX waveform which is suggested to represent the scalpellum over-penetrated the cells during the piercing-sucking process.The result indicated that the total number of P waveform of RSV-infected females was significantly lower than that of the females without RSV infection,indicating higher probability of successful probing for the RSV-infected L.striatellus.Moreover,the RSV infection significantly decreased the duration of NP waveform and significantly increased the duration of N4b waveform for the female adults.The effects of RSV infection on the duration of NP and N4b waveforms for the males were opposite to the females,showing the existence of sexual dimorphism in the duration of NP and N4b waveforms of L.striatellus feeding in response to RSV infection.The RSV-triggered changes in feeding behaviour could represent the outcome of co-evolutionary interactions between the RSV and its vector,L.striatellus,since fitness of RSV relies heavily on its vectors' fitness and ability to transmit itself to new host plants.On the other hand,the flight speed is significantly decreased and the flight distance is markedly decreased(P>0.05)for the 2-day-old newly emerged macropterous male of L.striatellus with RSV infection,compared with those without RSV infection.The results suggested a potential deleterious effect triggered by RSV infection on the flight capacity of male adults of L.striatellus.3.Effects of near zero magnetic field(NZMF)on the development and reproduction of L.striatellus,N.lugens and S.furciferaIn this study,we compared the effects of near zero magnetic field(NZMF)and geomagnetic field(GMF)on the development and reproduction of small brown planthopper,L.striatellus,brown planthopper,N.lugens and white-backed planthopper,S.furcifera.Compared with GMF,the results indicated as following:(1)L.striatellus:NZMF had no significant effects on egg duration,but it significantly extended the total nymphal duration of L.striatellus.Moreover,NZMF significantly decreased body weight of the newly emerged adult,and significantly decreased the fecundity of female L.striatellus.Furthermore,NZMF also significantly decreased the relative Vg transcript level,which is likely to be the reason of the loss in female fecundity of L.striatellus.(2)N.lugens:NZMF significantly extended egg duration of N.lugens and had no significant effects on the nymphal duration of females,but it significantly extended the nymphal development of male N.lugens.Moreover,NZMF significantly decreased body weight of the newly emerged adults,and significantly decreased the fecundity of female N.lugens.Furthermore,NZMF significantly decreased the relative Vg transcript level,which is likely to be the reason of the loss in female fecundity of N.Lugens.(3)S.furcifera:NZMF significantly prolonged egg duration and nymphal duration of S.furcifera.Moreover,NZMF significantly decreased body weight of the newly emerged females and the female fecundity of S.furcifera.And NZMF is likely to affect the circadian rhythm of Vg transcription based on the data from the temporal expression of Vg in S.furcifera.On the other hand,S.furcifera was selected to explore the molecular mechanism of the NZMF effects on the development.The temporal dynamic expression of some key genes in the 5th instar nymph of S.furcifera,i.e.,CRY1 and CRY2 as putative magnetosensors,JHAMT,FAMeT and JHEH in the juvenile hormone pathway,and CYP307A1 in the ecdysone pathway were investigated.The results indicated that the expression of CRY1 and CRY2 was significantly up-regulated under the NZMF at the first sampling time point,suggesting their developmental stage-specific patterns and potential upstream location in magnetic response.And the gene expression patterns of JHAMT,JHEH and CYP307A1 were consistent with the NZMF-triggered delay in nymphal development of S.furcifera.In this study,we first proposed the potential mechanisms of the magnetic bioeffects triggered by the abiotic NZMF,that is,the Cryptochromes-mediated hormone signaling pathway in the context of magnetobiology.The results revealed the potential key roles of hormone regulation for insects in adapting to the abiotic factors.4.Effects of NZMF on phototaxis and flight capacity of rice planthoppers,taking S.furcifera for exampleCompared to GMF,NZMF significantly increased the proportion of brachypterous S.furcifera.The proportion of the males and females flying to light under the NZMF was significantly higher than that under the GMF within the first 2 days of adult stage.By examining the temporal gene expression patterns of Cryptochromes(Cry1 and Cry2)which exhibit multifunction of photoreceptors and magnetoreception,etc.,we found significantly down-regulated transcript expression of Cry1 and Cry2 at the first sampling time point.Thus we suggested that the change of phototaxis of S.furcifera may be caused by the influence of NZMF on the photoreceptor function of Cryptochromes.Moreover,Compared to GMF,NZMF significantly decreased the flight speed while increased the flight time and flight distance of S.furcifera.The phenotype changes may be mainly induced by the transcription changes of AKH and AKHR in the adipokinetic hormone pathway which is related to the lipid energy metabolism.The transcription change of OAR in the octopamine pathway which is related to the flight activity may also contribute to these changes.Here,these findings also revealed the potential existence of the Cryptochromes-mediated hormone signalling transduction model.In fact,the base of rice planthopper is located at low latitudes,where the intensity of GMF is rather lower than the average value but the temperature is high enough to compensate for the adverse effects on nymph development and body weight of rice planthoppers.This also suggests the adaptation strategy of rice planthopper in respond to the abiotic NZMF.In conclusion,this thesis explores the adaptive strategies of insects to biotic and abiotic environmental factors as well as their corresponding mechanisms.And hormone regulation always plays important roles in the adaptation of insects to environment,no matter biotic and abiotic environmental factors.