| Western flower thrips(WFT),Frankliniella occidentalis(Pergande)is a major agricultural insect pest,and Beauveria bassiana is one of the most important biological factors controlling WFT.The infected insects with pathogens(or parasites)can change their body temperature by moving to a higher or lower temperature environment,which is called behavioral thermoregulation.Behavioral thermoregulation has significant effects on insect-pathogen interaction systems.Here,we tested the behavioral thermoregulation of infected thrips with Beauveria bassiana and its effects on survival,reproduction,sex ratio and related gene expression.The effects of temperature on the growth kinetics and virulence of Beauveria bassiana in WFT were also investigated.This study provided a theoretical basis for improving the control efficiency of B.bassiana against WFT.The main findings are as follows: 1 The cold-seeking behavior of infected WFT with B.bassianaIn our experiments,healthy WFT preferred a temperature environment of 24°C,while those infected with B.bassiana tended to a lower temperature of 12°C,showing cold-seeking behavior.The low temperature of 12°C changed the strategy of survival and reproduction of infected WFT.Compared with those kept at 24°C,the infected thrips had significantly higher survival rate and less total number of eggs at 12°C.At 24°C,compared with healthy ones,the infected thrips laid more eggs at earlier stage and less eggs at later stage.At 12°C,the infected thrips laid more eggs at later stage than healthy ones.The lower temperature 12°C changed the sex ratio of the infected WFT.For the contemporary thrips,the sex ratio(♂:♀)of the infected thrips decreased with time at both temperature conditions,and it is always higher at 12°C than it is at at 24°C.For the offspring,the proportion of female at 12°C(37.0%)was significantly higher than that at 24°C(28.5%).The survival rate from egg to adulthood at 12°C(84%)was significantly higher than that(80%)at 24°C.The lower temperature 12°C delays the growth of fungi in the infected thrips.The copy number of fungi in infected thrips at 12°C was significantly lower than that at 24°C since day2.2 Transcriptome analysis of infected western flower thrips by B.bassiana under cold seeking behaviorTranscriptome sequencing technology was used to compare the differential gene expression in healthy and infected thrips at 12°C and 24°C.In total,77,155 unigenes were obtained and 34,515 genes were annotated.A total number of 521 differentially-expressed genes were obtained by compared infected thrips at 12°C with at 24°C.225 genes were common differential expressed genes in healthy and infected thrips,named FTO-DEG,the remaining 296 genes are unique genes that responsed to infection at 12°C in thrips,named STO-DEG.74 genes of FTO-DEG were annotated by GO annotation and were enriched in 29 pathways.97 genes of STO-DEG were annotated by GO annotation and were enriched in 48 pathways.The results showed that the lower temperature of 12°C caused a series of immune-related responses and stress-related responses in the infected thrips,which increased the ability of the infected thrips to fight the infection of Beauveria bassiana.3 Identification and q RT-PCR analysis of related genes in cold-seeking thrips infected with fungiLower temperature of 12 °C induced up-expression of immune-related genes in infected thrips.This paper examined the expression of signal transduction genes(Toll),genes associated with T-cell resistance(Ant),signal regulation and amplification genes(Ser),and immunity-inducing effector genes(Per)at 12°C and 24 °C.The results showed that,in addition that Ant gene began to decline on the 3rd day,the expression levels of Toll,Ser and Per genes all increased with the increased time.Moreover,the expression of immune-related genes at 12°C was higher than that at 24°C.The lower temperature of 12°C changed the response pattern of stress-related genes.This paper examined the expression of three genes encoding Trehalose(Tre),Proline(Pro),and heat shock protein(Hsp)during cold-stress.Whether thrips were infected or not,the expression of Tre,Pro,and Hsp genes in thrips increased firstly and then decreased or increased again at 12°C;at 24°C,the expression levels of Tre,Pro,and Hsp genes slowly increased with the increased time,in addition that the relative expression level of Tre gene in infected thrips on the 2nd day was not significantly different from that on the 3rd day,the expression levels of other genes were significantly increased on the 3rd day.4 Growth kinetics and virulence of two Beauveria bassiana isolates in Frankliniella occidentalis under different temperaturesThe bioassay results of B.bassiana against WFT showed that the virulence of isolate SCWJ-2 was significantly higher than that of GZGY-1-3 under 30°C,but there was no significant difference between them under 25°C and 20°C.In the vitro experiments,the colony diameter of isolate SCWJ-2 was significantly larger than those of isolate GZGY-1-3 under all test temperatures.The results of q RT-PCR showed that the gene copy number of SCWJ-2 in vivo was higher than that of GZGY-1-3 under 30°C,but there was no significant difference between them under 25°C and 20°C.The number of fungal gene copies in vivo is in accordance with the result of bioassay,but not with results of the plate colony diameters in vitro culture.B.bassiana isolate SCWJ-2 infected F.occidentalis more effectively than isolate GZGY-1-3 under high temperature. |
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