| The cotton aphid(Aphis gossypii Glover),also known as the melon aphid,belongs to Hemiptera,Aphididae and is a worldwide polyphagous pest with a variety of host-specialized biotypes,typical including Cucurbitaceae-and Malvaceae-specialized biotype(CU and MA biotype).The fitness decreases significantly when the host-specialized biotypes are introduced to non-native hosts.The mechanisms remain unknown at present.Studies have shown that the feeding by phytophagous insects stimulate or interfere with plant defenses,thereby altering the fitness of later insects on the plant.How the host-specialized A.gossypii performs on the non-native hosts that have been infested by other conspecific biotypes still need to be explored.In this study,using CU and MA biotype as materials,we investigated the performance of the host-specialized A.gossypii on native and non-native hosts that have been pre-infested,and explored the causes from the perspective of plant physiological metabolism and defense.1.Performance of CU and MA biotype on pre-infested cucumber leavesMA biotype could not carry out phloem ingestion,developed into abnormal morphs(yellow dwarfs)and reduced fecundity on healthy cucumber leaves,as well as on cucumber leaves that were pre-infested with CU biotype for 2 d.However,MA biotype achieved phloem ingestion,and developed into large dark-green morphs and improved fecundity significantly on cucumber leaves that were pre-infested with CU biotype for 14 d.Pre-infestation with CU biotype for 14 d also had advantages for CU biotype,manifesting as increased fecundity and shortened probing time.The performance of MA biotype that were cultured for 3 generations on cucumber leaves that were pre-infested by CU biotype for 14 d did not differ from that of CU biotype on healthy cucumber leaves.These results indicated that MA biotype can’t adapt to healthy cucumber,but can adapt to pre-infested cucumber with CU biotype for 14 d.The living experience on pre-infested cucumber enabled the MA biotype to adapt to healthy cucumber and achieved alteration of host specialization without changing genetic background.2.Metabolomic analysis of the pre-infested cucumber leavesA total of 13 categories 658 metabolites were detected in cucumber leaves that were pre-infested with CU biotype using widely targeted metabolome analysis,which belonged to nutrients such as lipids,amino acids and derivatives,saccharides and alcohols,nucleotides and derivatives,and anti-insect secondary metabolites such as flavonoids,phenolic acids,terpenoids,organic acids and alkaloids.There were 177 differential metabolites between 14-d pre-infested and healthy cucumber leaves,with 100% sugars and 91.30% lipids up-regulated,63.64% of organic acids and 55.56% of flavonoids down-regulated.There were 85 differential metabolites between 2-d pre-infested and healthy cucumber leaves,with 100% lipids and 85.71% nucleotides,and 86.67% phenolic acids,83.33% alkaloids,80.00% organic acids,62.50% flavonoids up-regulated.There were 178 differential metabolites between 14-d pre-infested and 2-d pre-infested cucumber leaves,with 66.67% sugars,56.52% lipids and 56.25% amino acids up-regulated,60.60% flavonoids and 54.50% organic acids down-regulated.The differential metabolites were mainly enriched in the pathways of tryptophan metabolism,flavonoid biosynthesis,phenylpropanoid biosynthesis,fructose and mannose metabolism.These results showed that pre-infestation of CU biotype for 2 d induced the increase of anti-insect secondary metabolites in cucumber leaves,while pre-infestation 14 d induced the increase of nutrient content and the decrease of anti-insect secondary metabolites in cucumber leaves.3.Functional assessment of phytohormones on pre-infestation effectsSalicylic acid(SA)content was decreased and jasmonic acid(JA)content was increased significantly in cucumber leaves that were pre-infested with CU biotype for 14 d,compared with healthy and 2-d pre-infested cucumber leaves,and the highest SA content in cucumber leaves pre-infested for 2 d.The expression of SA-related genes PR1,PAL,PAD4 and JA-related genes OPR3,LOX2,AOS were consistent with the trend of SA and JA content.Exogenous application of MeJA increased the JA content and decreased the SA content in cucumber leaves,which enhanced the adaptability of MA biotype on healthy cucumber leaves.Exogenous application of SA increased SA content and decreased JA content in cucumber leaves,which reduced the adaptability of MA biotype on 14-d pre-infested cucumber leaves meaning the pre-infestation effect of cucumber leaves was lost,and reduced the adaptability of CU biotype on healthy cucumber leaves.SA and JA are important signal substances that initiate plant defense responses.These results show that pre-infestation of CU biotype for 14 d inhibited the accumulation of SA and increased the content of JA in cucumber leaves,and broke the defense response of cucumber to become more susceptible.The external application of SA increased the resistance of cucumber to A.gossypii,indicating that the SA pathway is the defense pathway of cucumber against A.gossypii.In conclusion,MA biotype can’t adapt to healthy and 2-d pre-infested cucumber leaves,but can adapt to pre-infested cucumber with CU biotype for 14 d.The reason is that the pre-infestation of CU biotype interferes with the hormonal signals and physiological metabolism of cucumber and disrupts the defense system,thus facilitating the colonization of MA biotype on cucumber,suggesting that MA biotype can’t adapt to healthy cucumber because it can’t regulate the defense response of cucumber,and if the defense system of cucumber is pre-disrupted,MA biotype can be adapted to cucumber.The living experience on pre-infested cucumber enabled the MA biotype to adapt to healthy cucumber,indicating A.gossypii can achieve alteration of host specialization without changing genetic background.The results gives an important direction to explore the mechanism of host-specialized biotype formation in A.gossypii,and has guiding significance for the prevention and control of A.gossypii. |
