| High adaptability to environmental adversity of an exotic insect pest is an important mechanism for its successful invasion. Sycamore lace bug, Corythucha ciliata, is a species that is native to temperate regions in North America, and now is invading subtropical urban areas, seriously damaging sycamore trees(Platanus spp.) in China. During the spread of the bug in subtropical urban areas in China, it is often subjected to high temperature stress. We found that this adverse condition did not limit its development, reproduction and establishment in the newly-invaded regions. Therefore, we predict that this species has a high thermal tolerance which may contribute to its adaptation to high temperatures in these regions. Using the approaches to population ecology, chemical ecology and molecular ecology, this thesis examined its tolerance to high temperatures of C. ciliata and its physiological mechanisms. This paper focused on:(1) survival and reproduction of C. ciliata under constant temperatures and several heat-shock conditions;(2) changs in heat-resistant substances, antioxidant metabolism and HSPs after exposure to high temperatures. The aims of this study were to reveal the tolerance to high temperatures of C. ciliata and its corresponding physiological mechanisms after its invasion in subtropical China. The results would enhance our understanding of the mechanisms that the species rapidly invades different temperature zones, eventually offering experimental evidence for the early warning and control of C. ciliata. The main results are summarized below:1. Development and fecundity of C. ciliata reared on leaves of London plane tree (P.×acerifolia) were investigated at seven constant temperatures (16,19,22,26,30,33and36℃) and at80%relative humidity and a14L:10D photoperiod. The developmental time significantly decreased with increasing temperature. The developmental time from egg hatching to adult emergence was respectively47.6,35.0,24.1,20.0and17.1days at the temperatures of19,22,26,30and33℃. C. ciliata could not finish the full development at16and36℃. The threshold temperature (C) estimated for egg-to-adult development was11.17℃, and thermal constant (K)370.57degree-days. Longevity of females was found to be the shortest (17.7days) at33℃and the longest (58.9days) at16℃, and that of males the shortest (19.7days) at33℃and the longest (59.7days) at16℃. Fecundity was the highest at30℃, being286.8eggs per female over an oviposition period of8.9days. Female lifetime fecundity was reduced at other temperatures, being the lowest (87.7eggs per female) at19℃. The population trend index (I) of C. ciliata was the highest (130.1) at30℃and the lowest (24.9) at19℃. Therefore, the optimal developmental temperature for C. ciliata was determined to be30℃.2. Experiments were conducted to examine the effects of high temperature (35,37,39,41,43and45℃), period of exposure (0.5,1,2,4,6and8h) and developmental stage (egg, nymph and adult) on the survival of C. ciliata in the laboratory. The effects of heat hardening of nymphs and adults of the insect on their tolerance at lethal temperature (exposure to33,35,37,39and41℃for1h prior to transfer to43℃for2h) were also studied. The results showed that survival of eggs, nymphs and adults (all above80%) was not significantly affected by temperatures ranging from35to41℃, but decreased rapidly with increasing temperature and duration of exposure above41℃. The lethal temperature to cause a50%mortality of all developmental stages decreased with the increase in the duration of exposure from0.5to8h. Taken together, eggs had the highest thermotolerance, followed by adults and nymphs in a decreasing order. Thermotolerance of male adults was slightly greater than that of females. Survival of nymphs, female and male adults at43℃for2h significantly increased after heat hardening in non-lethal high temperature for1h, and the optimal hardening temperature was37℃. It is concluded that C. ciliata has great thermotolerance, and heat hardening could improve its tolerance.3. Reproduction and fitness of F1generation in C. ciliata were assessed after the adults were exposed to the temperatures of26(control),35,37,39,41,43, or45℃for2h, and then were transferred to26℃in the laboratory. The results showed that heat-shock temperatures ranging from35to41℃did not significantly affect survival, longevity and fecundity of adults, but heat shock at43and45℃significantly reduced these parameters. Exposing parent females to heat-shock treatments from35to41℃did not affect the hatching rate of their eggs or the proportion of female F1progeny but slightly and significantly reduced survival of the nymphs. C. ciliata, therefore, could tolerate high temperatures less than41℃, and the temperatures below this had little influence on the reproduction and offsprings in C. ciliata.4. In order to understand the physiological mechanisms of survival of C. ciliata in response to high temperature stress, several physiological and biochemical parameters were measured for the adults in laboratory populations after a2h exposure to33,35,37,39,41and43℃, and the field populations from8:00to14:00with a2 h interval on July-29m,2011in Shanghai. The results showed that the body absolute water content significantly reduced with the increasing temperature while triglycerides, total protein, mannitol, sorbitol, malondialdehyde (MDA), lactate dehydrogenase (LDH), superoxide dismutase (SOD), glutathione (GSH), catalase (CAT) and glutathione (GR) significantly increased. Similar treadency was also observed in field population. The physiological mechanisms, therefore, for adults being capable to maintain a higher survival rate below41℃, were closely related to a series of physiological and biochemical adjustments in the bodies of the bugs. This metabolic defense mechanism may inprove the tolerance to high tempersture stress in C. ciliata, when it is hit by the natural heat.5. In order to reveal the physiological mechanisms of adaptation to high temperature at the molecular level in C. ciliata, a full-length cDNA encoding HSP70from C. ciliata was cloned and characterized by RT-PCR and RACE technique. The complete cDNA (2256bp) contained a1917bp open reading frame encoding639amino acid residues with the predicted molecular weight of70.55kD, with the formula of C3085H4943N871O988S16and the isoelectric point of5.45, and carried an important and intact HSP70signature sequence. The nucleotide sequence of the cDNA was very similar to that of the HSP70cDNA sequence of some other insect species (>80%). In order to explore the effects of HSP70on tolerance to high temperatures, a standard system was designed for real-time fluorescence quantitative RT-PCR, with which the relative expression of HSP70gene in different populations in response to high temperatures was detected, i.e., the laboratory population of adults after exposure to33,35,37,39,41and43℃for2h, and the field population at8:00,10:00,12:00and14:00on July-29th,2011in Shanghai. The results showed that in the laboratory populations, the relative expression levels of HSP70gene significantly increased as the temperatures increased when they were heat shocked at37-41℃for2h. The expression amount of HSP70after exposure to41℃was2710times higher than that after33℃; but when heat shocked at43℃, the expression amount of HSP70declined rapidly. In the field populations, the relative expression levels of HSP70gene also significantly increased as the temperature increased during the day. At12:00and14:00pm, the expression amount of HSP70increased almost226and771times higher than that at8:00am. Therefore, the HSP70might play an important role in avoiding or reducing thermal injury of in C. ciliata by high temperatures. Based on a comprehensive analysis of the above results, we believe that C. ciliata can tolerate high temperatures below41℃in subtropical urban areas in China, in which the high temperature extremes may not be a factor to limit the survival, reproduction in C. ciliata. This capability is related with a variety of physiological and metabolic processes, e.g., acceleration of water loss, synthesis of polyhydroxy, synthesis of heat shock proteins, increased activity of antioxidant protection system metabolism. The ecological plasticity may contribute to its expansion into lower latitude areas in China where Platanus trees are widely planted. Our findings will be useful for predicting dynamics of C. ciliata and understanding the mechanisms underlying the invasion of this important urban invader among different temperatrure zones. |
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