Comparison Of Tolerance To Thermal Stress In Three Planthopper Species:Small Brown Planthopper,Brown Planthopper And White-backed Planthopper

Rice is one of the most grown and consumed crop in the world with primary nutrient values.However,it is being threatened by the deleterious effects of three planthopper species over the years.The crop suffers heavy impairments due to direct and indirect attacks.But in recent times,all the species are under serious temperature scrutiny as it is fluctuating increasingly.Fluctuating temperatures influenced the thermal tolerance limits and physiological mechanism of planthoppers and change their thermal adaptability.Therefore,this study was conducted to compare the difference in thermal tolerance of three species of planthoppers in terms of upper lethal and sublethal temperatures in the presence and absence of host plant material.Insects develop preventive strategies to withstand against extreme temperatures and three highly conserved heat shock protein genes were examined to validate the effects of these strategies at different low and high temperatures.1.Comparison of upper sublethal and lethal temperatures in three rice planthopper speciesTemperature is an important environmental factor for ectotherms,fitness and survival.The upper sub-lethal and lethal temperatures were compared using a temperature-controlled Weber-column in the absence and presence of host plant between adults of three closely related planthopper species,the small brown planthopper(Laodelphax striatellus,SBPH),the brown planthopper(Nilaparvata lugens,BPH)and the white-backed planthopper(Sogatella furcifera,WBPH),which are among the most serious rice pests across the world.The critical thermal maximums(CTmax)were higher in SB PH than in both BPH and WBPH,irrespective of the presence of host plant.The heat coma temperatures(HCT)were higher in BPH than in both SBPH and WBPH in the absence of host plant and higher in SBPH than in both BPH and WBPH in the presence of host plant.Regardless of species,CTmax and HCT were higher in the presence than in the absence of host plant.The upper lethal temperatures(ULT50)were higher in BPH than in both SBPH and WBPH.Between sexes,females showed higher thermal tolerance(HCT and ULT50)than males.The planthoppers also exhibited differences in behavioral thermotolerance in an increasing temperature regime,with fewer insects dropping-off or with dropping-off occurring at higher temperatures in SBPH than in BPH and WBPH.These results indicate that SBPH and BPH are more thermal tolerant than WBPH.In addition,the results highlight the importance of performing the measurement in an ecologically relevant setting(in the presence of host plant).These results are of significance in predicting population dynamics of the closely related planthopper species.2.Induced expression levels of Hsp90,Hsp70 and Hsp60 in three rice planthopper species in response to thermal stress treatmentsThe three planthopper species exhibit different thermal tolerance trends and heat shock proteins(Hsps)protect insects from thermal damage under temperatures stress conditions.To explore the induced expression levels of Hsps in planthopper species in response to thermal stress treatments,expression of three highly conserved genes of Hsp90,Hsp70 and Hsp60 were studied at different heat and cold shocks along with the investigation of different recovery period.In our results,heat stress(33 and 37?)up-regulated the expression of the three Hsps genes in all species,with even higher expression at 37?.Compared with the control group at 26?,Hsp90 produced the highest expression level in BPH at 37?(3.6-fold increase),followed by a 1.5-fold increase in SBPH at 33? when the insects experienced no recovery from heat shock.Conversely,cold shock at 5? and 15? did not change the expression profiles of Hsp90 gene in any species.During 3 h recovery,the expression level in SBPH up-regulated(2 fold at 5? and 2.6-fold increase at 33?)when compared with(0 h of recovery period)but decreased at 6 h recovery in all three species throughout the expression level measurements.Hsp70 was not significantly up-regulated under low and high temperatures except BPH(0.9-fold increase)at 37? when the insects experienced no recovery from heat shock,while recovery of 3 h induced(1-fold increase of expression)in WBPH when compared with control(0 h of recovery period).Significant Hsp60 expression was induced in WBPH at all temperatures with highest at 37?(2.2-fold increase)against control at 26? and(1.2-fold increase)during 3 h recovery when compared with(0 h of recovery period);Under cold shock at 15?,(0.9-fold increase)gene expression up-regulated in WBPH when compared with control at 26? but recovery period exhibited no effect on expression.These results validate the involvement of three Hsps genes in therrmal tolerance in the three species and their induction and expression levels are different under cold and heat shock stress,the effect of recovery time is fundamental on gene expression profiles.