Thermal Adaptation In The Potato Early Blight Pathogen Alternaria Alternata

Potato early blight caused by Alternaria alternata is a ubiquitous global fungal disease,which causes serious damage to potato production.In this study,the thermal adaptation of mycelial growth rate and pathogenicity were studied through a combination of microsatellite markers,common garden experiment and biological statistics method to analysis the selection of temperature on mycelial growth rate and pathogenicity of 206 A.alternata isolates from China.Acclimatization experiment and detection of relative expression of heat shock protein genes were used to test the adaptability on high and low temperatures and the influence of heat shock proteins in temperature adaptation.Therefore,we can evaluate pathogen evolution potential and guide the sustainable management of potato early blight.The main results of the study are as follows:1.The mycelium growth rate of A.alternata increased with the temperature,reaching a maximum rate at 25℃,above this level,growth rate declined.The optimum mycelium growth temperature of A.alternata ranged from 24℃ to 29℃ although significant variation(～5℃)existed in different isolates.The negative correlation between maximum mycelial growth rate and temperature range of A.alternata suggesting the growth rate had a fitness cost to temperature,isolates with higher growth rate usually had narrow growth temperature range.The mycelium growth rate increased with the temperature under global warming,reaching 6.3%increased after 3℃,then declined,indicating A.alternata maybe bring more serious damage to agricultural production.Population differentiation(QST)based on the mycelial growth rate of A.alternata was significantly greater than population genetic differentiation(GST)based on SSR markers loci at 25℃,while GST was significantly greater than QST at 19℃,22℃,28℃ and 31℃.Optimum growth temperatures of each population were around 26℃,the temperature which could significantly distinguish isolates between populations,so the genetic differentiation was higher.While under other temperature,growth rate inhibited by hot or chill temperature,and mycelial growth rate had fitness cost on growth temperature range,hot or chill temperature choosing lower growth rate isolates with wider temperature range,therefore the population differentiation was smaller,temperature played a stabilizing selection on the evolution of mycelial growth rate.Correlation between geographic distance of sample locations and gene flow based on the SSR neutral marker loci and quantitative character also suggesting selection against mutants harmonized the genetic difference of geographically distant populations accumulated by random drift.2.There were significantly differences on the latent period,the area of lesion and the growth rate of lesion between different population and experimental temperature.As the temperature increased from 19℃ to 31℃,the latent period of A.alternata decreased,while the area of the lesion and the growth rate of lesion increased.In seven A.alternata populations,phenotypic plasticity of growth rate of lesion were significantly greater than the heritability,suggesting that environmental factor played a more important role in the growth rate of lesion.Population differentiation(QST)based on the growth rate of lesion of A.alternata was significantly greater than population genetic differentiation(GST)based on SSR markers loci,suggesting that differences in pathogenicity among A.alternata populations were caused by diversifying selection,different selection pressures driving pathogens adapted to the local environments.In the appropriate temperature(22℃～31℃)for A.alternata,mycelial growth rate had fitness cost on growth rate of lesion,meaning that isolates with lower mycelium growth rate and wider mycelium growth temperature range usually had higher pathogenicity.3.A.alternata was grow better under the hot temperature,when the temperature was higher than the optimum temperature,growth rate reduced slowly with temperature,but fell sharply when surpassing a threshold temperature,while when the temperature was lower than the optimum temperature,the growth rate first sharply declined with temperature,but decreased slowly when reaching a threshold temperature.The adaptability of A.alternata to the high temperature was better than low temperature,hot adaptation of acclimatized isolates increased gradually from 32℃ to 35℃,and chill adaptation of acclimatized isolates reduced gradually from 18 ℃～14 ℃.4.The expression of A.alternata heat shock protein genes under hot and chill acclimatization were not up-regulation or down-regulation consistency,this may be related to different adaptation to temperature of isolates in the long-term evolution,but the expression of heat shock protein genes in many isolates with low growth were higher,suggesting that heat shock protein may be played an important role in the process of A.alternata adapted to the environment temperature.Two isolates up-regulated both on hsp60 and hsp90 were found highly adaptation to acclimatization temperature,indicating that hsp60 and hsp90 may be involved in the adaptation of A.alternata to temperature.The results showed the mycelial growth rate under a stabilizing selection by temperature and the pathogenicity of A.alternata adapted to the local environments,the mycelial growth rate of A.alternata not only had a fitness cost to temperature range,but also had a fitness cost to the growth rate of lesion under the optimum temperature(22℃～31℃),however,A.alternata had strong adaptation to high temperature,themycelial growth rate and pathogenicity were higher in hot temperature,so the prevention and control of A.alternata is an essential part in potato disease sustainable management.