Impacts Of Antibiotic And Gut Bacteria On The Fitness Of The Diamondback Moth,Plutella Xylostella(L.)

The diamondback moth(DBM),Plutella xylostella(L.)is the most important pest of cruciferous crops in wide areas of the world.Advances in molecular techniques have improved our knowledge of the gut microbial composition of P.xylostella,but we remain largely ignorant of many of the processes which maintain the integrity of the association,particularly those which mediate the transport of nutrients and the regulation of the microbial biomass.The purpose of this study is to figure out the effect of gut bacteria on the fitness of P.xylostella,and to illustrate the function of gut bacteria in the moth.The effect of gut bacteria on the fitness of P.xylostella was investigated in the laboratory.Firstly,by supplying antibiotics of different concentrations into the insects’diet and raising the moth with different generations,the relationship between antibiotic,gut bacteria and insect was analyzed,the toxic effect of antibiotics on the insect was valued and verified.On the other hand,by replying the gut bacteria to the host which had eliminated gut bacteria,a positive effect of gut bacteria on the fitness of P.xylostella was observed.Main results are as follows:1.Impact of antibiotic treatment on the fitness of P.xylostellaWe firstly raise the P.xylostella which is under antibiotic treatment for four generations,and then evaluate the influence of antibiotic treatment on the fitness of DBM.It is turned out that the antibiotic-treatment not only deprived gut bacterium from the moth,but also lead to hosts’ fitness disadvantages.Meanwhile,with increasing generations,significant differences among treatment groups also increased.Depriving gut bacteria by antibiotic treatment result in significantly retarded development,arrested growth,reduced weights,declined pupation rates,descended fecundity,shorted female and male life-span of P.xylostella,and the significant differences of each parameter were trend to increase when generation increase.While it might have no great influence on hatching rate,female-male rate or eclosion rate.We cultured the P.xylostella containing little gut bacteria on the artificial diet with three different-concentrations antibiotics,and then evaluated the toxic effect of antibiotic treatment on the fitness of P.xylostella.Results showed that toxic effect of antibiotic treatment enforced with the increase of generation,especially the highest antibiotic dose dramatically decreased the pupation rate and total amount of egg of P.xylostella.Our study indicated the toxic effect of antibiotic,and the effects on host were in a concentration-dependent manner.Moreover,the growth,development and fecundity of P.xylostellai were greatly influenced when treatment time prolonged.In order to explore the influence of capsule-transmitted gut bacterium on the fitness of P.xylostella,we performed the experiment by egg surface sterilization.However,it demonstrated that the fitness of P.xylostella was not influenced.2.Impact of gut bacteria on the fitness of P.xylostellaWe cultured the P.xylostella with little gut bacteria on the leave for one generation,finding that there were numbers of bacteria in this insect gut.Then we transfered the moths to artificial diet in the second generation,finding that there were little bacteria in the gut just as before.These facts indicate that most gut bacteria of the moth come from food.After eliminating the gut bacteria by antibiotic treatment,one or more bacteria were replied to the larvae.Experiment showed that pupation rate was significantly increased after gut bacteria were replied.In conclusion,this study demonstrated that antibiotic treatment resulted in significantly retarded growth,development and reproduction of P.xylostella in a time-dependent and concentration-dependent manner.Furthermore,the negative effects on the host fitness was partly caused by the toxic effect of antibiotic and partly caused by the deficiency of gut bacteria.Further study verified the positive effect of gut bacteria by replying them to the sterilized insect,illustrating that the gut bacteria might play a promote role in the fitness of P.xylostella.