Physiological And Transcriptional Responses To Thermal Stress In Littorina Brevicula Along Chinese Coastline

Global temperature has been escalating and the frequency of marine heat wave has been shown to increase as a result of the on going climate change scenario,which poses severe challenges to the growth,development,reproduction and survival of the organisms.Previous studies have shown certain species inhabiting in the intertidal zone have already lived in the edge of their upper thermal tolerance limit,therefore,research on the thermal tolerance and response of these organisms to increasing temperature is both pressing and of great ecological value.Periwinkle Littorina brevicula is one of the common species on the upper zone of a rocky shore.It is frequently affected by high temperature.Thermal tolerance and the mechanisms in response to high temperature of L.brevicula from different populations are believed to vary due to the changing thermal regimes.Therefore,better understanding of the thermal tolerance and responsive molecular mechanism of this species is a crucial step to evaluate the impact of extreme thermal events(increasing in frequency and strength)on intertidal ecosystems under the global warming background.In this study,integrated methods of physiology and molecular biology were adopted to ascertain the physiological and molecular response of L.brevicula to the thermal stress.By comparatively analyzing the survival,cardiac performance and transcriptional responses of L.brevicula from different populations,several results were concluded as follows:1.In order to find out the thermal tolerance of summer populations and compare the difference of temperature tolerance between populations,we studied survival patterns of L.brevicula in summer at four temperatures(47 ?,49 ?,51 ? and 53 ?).The analysis of the Kaplan-Meier survival estimates,median lethal temperature(LT50) and Cox proportional hazards results showed that L.brevicula has high thermal tolerance.By comparing the Kaplan-Meier survival estimates,median lethal temperature and Cox proportional hazards of different populations(Dalian,Qinhuangdao,Qingdao,Lianyungang,Binhaigang,Lvsi and Dongtou),a significant difference in thermal tolerance among populations was observed.At the same time,it also showed that thermal tolerance of the populations in the northern distribution areas is lower than that in the central and southern,as the populations in the southern distribution areas lower than that in the central.In addition,thermal tolerance ability did not completely agree with latitudinal gradients and presented a complex mosaic of thermal environments.2.In order to examine the thermal tolerance of summer populations and understand the tolerance differences and patterns,we determined the thermal performance curves (TPCs)of individual L.brevicula in summer populations.We found this species had different thermal tolerance patterns in response to low-or high-thermal stress as a result of the relationship between the heart rates and temperatures.Results of the TPCs,Arrhenius break temperature(ABT)and Flat line temperature(FLT)data in eight populations(Dalian,Qinhuangdao,Dongying,Qingdao,Lianyungang,Binhaigang,Lvsi and Dongtou)indicated significant differences in thermal tolerance among populations.Thermal tolerance of populations in central distribution areas was significantly higher than the marginal populations.It is consistent with the results of the LT50.No significant thermal tolerance variation was observed with latitudinal gradients.In addition,the Dongying and Qinhuangdao populations were weaker to resisit the thermal stress than others,and we believed that theywere less affected by tidal action.3.In order to examine the thermal tolerance and the pattern of winter populations,we compare the thermal tolerance differences between the populations,as well as between the seasons(summer vs.winter).We studied the TPCs of L.brevicula in winter populations,compared with the ABT and FLT among five populations (Lianyungang,Zhonganpeng,Lvsi,Dongtou and Xiamen),it showed significant differences in thermal tolerance among the populations.Thermal tolerance of the populations in central distribution areas were significantly higher than the southern marginal populations,it was the same both in winter and summer.Besides,the heart rate of Zhonganpeng population was always higher than the others,presumably due to the "preparative defense" strategy under low-thermal stress.4.In order to investigate the transcriptional response to thermal stress,we detected the transcriptome of L.brevicula in five populations(Lianyungang,Zhonganpeng,Lvsi,Dongtou and Xiamen)under different temperatures.Through comparing differential transcriptomic expressions at 3 temperatures(25 ? control,37 ?,45 °C),we found that organisms synthesis abundant heat shock protein family(Hsps)in thermal stress process,Hsps involved in molecular stabilization of protein and protein degradation,and heat shock protein 70 family(Hsp70s)performed major functions in process.Cytochrome P450 family(CYP450)and Glutathione s-transferase(GSTs)were also found,they involved in the oxidative stress caused by the thermal process.Meanwhile,the expression patterns of the transcripts varied at low or high temperatures.In addition,by comparing the transcriptome differential expression among these populations,there were differences in the expression patterns and abilities of transcriptomes in response to thermal stress.