Study On The Rapid Evolution Of Heat Tolerance In Small And Medium-sized Zooplankton

Understand the strategies of species adapting to warming is particularly important as global warming brings new challenges to the survival of species.Invertebrates can respond to environmental warming through plasticity or evolutionary responses in physiology,morphology,life history and behavior.Aquatic organisms are gradually becoming miniaturized under climate warming,and the study of thermal adaptation to small aquatic invertebrates is of great significance for predicting the response of organisms to the future warming environment.The method of resurrection ecology can conveniently and effectively observe the adaptation trajectory of species to environmental changes.And experimental evolution is a powerful tool to help study whether organisms can adapt to environmental changes.In this study,two small and medium-sized zooplanktons—Ceriodaphnia cornuta(C.cornuta)and Moina micrura(M.micrura)are the research objects.The research conducted experiments on critical thermal maximum(CT_max),knockdown time test,life table statistics,and predation avoidance behaviors(alertness)of resurrected historical populations and geographic populations,and obtained the heat tolerance,morphology and life history traits of the two kind of water flea,so as to evaluate the adaptation trajectory and adaptability of the water flea to the thermal environment.At the same time,in addition to exploring the rapid evolution potential of natural populations to warming,we simulated natural temperature fluctuations in the laboratory and conducted a two-month thermal selection evolution experiment to explore the rapid adaptability and evolutionary potential of C.cornuta and M.micrura.The main results are as follows:1.Historical population records of heat tolerance of small and medium zooplanktonIn this study,we successfully resurrected the historical populations of two zooplankton species in Dianshan Lake,Shanghai for 60 years,and reproduced the historical evolutionary track of heat tolerance of the coexisting medium and small zooplankton(C.cornuta and M.micrura).The CT_max experiment detected in water flea from different historical periods and life table experiment was carried out at 25? and 29? to record the heat tolerance and life history traits of water flea.The results showed that there were significant differences in the heat tolerance of the two water flea species,but no heat tolerance evolution occurred in the historical population,and there was no significant difference in the life history of the historical population.However,we have observed significant differences in the fitness and age at maturity of the two species of water flea at different temperatures.Among them,under the condition of increasing4?,the water flea appeared premature,indicating that the two species have phenotypic plasticity to 4? heating.In addition,in the historical populations of Dianshan Lake,the heat tolerance of water flea is related to the highest temperature in summer.2.Geographic population variation of heat tolerance of small and medium zooplanktonIn this study,we collected small and medium-sized zooplanktons in the waters of the Huangpu River in Shanghai—C.cornuta and M.micrura in six different geographical populations.CT_max and alertness of each group were detected,and the life table experiment at25? and 29? were performed on M.micrura to compare the geographical variations in heat tolerance,life history,and alertness of the two kinds of water flea.From the results of CT_maxand life table experiments,there is a significant geographic variation in heat tolerance among the water flea populations in Huangpu River,and the difference in life history traits is significantly affected by the interaction of the acclimation temperature and the geographic location of the population.However,there was no significant difference in alertness between the two geographic populations of water flea.At the same time,compared with other locations,the HP5 and HP8 populations of M.micrura show faster growth rate,earlier maturity age and smaller size at maturity in the life history,but its heat tolerance is lower.This may be a trade-off between the heat tolerance and life history of this species.In addition,we found that the fitness of the geographic population of M.micrura in the Huangpu River geographic population is negatively correlated with the maximum temperature in June,that is,the higher the maximum temperature in June,the lower the intrinsic growth rate of the water flea.3.Selection of experimental populations for heat tolerance of small and medium zooplanktonUsing the resurrected historical clones,two new populations of zooplankton species were formed,and thermal stress selective evolution experiments were conducted at three sets of temperatures(constant 25?,constant 29?,and fluctuating 26-32?)to explore thermal evolution potential of the two kinds of water flea.evolution studies in laboratory have also shown that both C.cornuta and M.micrura have great potential for thermal evolution,among which phenotypic plasticity may plays a key role.Under the fluctuating temperature regime,the heat tolerance of the water flea is different from that of the constant condition.For the constant and fluctuating systems,the CT_maxof the two types of water flea is higher under the constant condition,but the knockdown time is longer under the fluctuating condition and alertness is better.It is found that there is a trade-off between critical high temperature and heat shock time,which may be a choice produced by water flea at constant and fluctuating temperatures.This suggests that while considering the increase in global average temperature,we must also consider the impact of fluctuating extreme temperatures.Otherwise,there may be deviations in predicting the ability and direction of biological heat tolerance and adapting to the thermal environment.In this study,we found that water flea may be mediated by temperature plasticity to adapt to balloon climate warming and extreme temperatures,and small and medium-sized zooplankton have greater potential for heat tolerance plasticity.This study provides important evidence for the response strategies of small and medium sized freshwater zooplankton to global warming,and helps formulate future biological protection strategies under global warming.It is not yet possible to make accurate conclusions about the specific mechanism of water flea adapting to the temperature environment,and more research is needed to verify it.In the future,it is necessary to conduct further research from the perspective of the combination of molecular and phenotypic factors and the interaction of multiple species from individual to community levels.