Rapid Evolution Of Salt Tolerance In Zooplankton

The global freshwater salinization caused by various anthropogenic activities and climate change creates salinity levels not experienced of most freshwater organisms,which threaten the freshwater community structure and biodiversity.However,little is known about the ability of freshwater organisms to adapt to increased salinity,so it is significant to understand the evolutionary response of organisms to salinity changes.In this study,the ability and the adaptation of zooplankton to changing salinity has been explored.The main contents are as follows:The existence of intraspecific variation would provide raw materials for the evolution of organisms.In this study,two common small-sized cladocerans--Moina micrura and Ceriodaphnia cornuta were used to test the intraspecific variation in salinity tolerance.The acute experiments(control;5 ‰)and life table experiments(control;1.5 ‰)were conducted by using the 13 populations of M.micrura and 22 populations of C.cornuta,which were hatched from Dianshan Lake or collected in the Huangpu River.The results showed that there were significant intraspecific variations of salt tolerance in both two species.Several populations showed higher salt tolerance.The tolerance indexes of C.cornuta from Dianshan Lake were quite different,and the highest value was 5.9 times of the lowest.While in the remaining populations,the indexes of the high salt tolerance populations were 2-3 times that of the low salt tolerance populations.The significant differences of life history traits under different treatments showed that the two species had phenotypic plasticity to elevated salinity,and the significant interaction between population and different salinity treatment indicated the genetic basis for the plasticity difference.The differential responses to elevated salinity make it possible for the natural populations to adapt to the long-term exposure to increases of salinity.Experimental evolution was used to further explore whether the presence and removal of salinity could lead to the rapid evolution and recovery of zooplankton.The experiment was carried out for three months with C.cornuta.After two months of the selection experiment at the control and 0.75 ‰ salinity conditions,the C.cornuta populations could evolve salt tolerance rapidly.The populations from the 0.75 ‰selection line showed the higher tolerance than the populations from the control line.The traits including size at maturity,age at maturity and intrinsic rate of population increase showed the significant evolutionary responses.And except for size at maturity,significant phenotypic plasticity was observed in all traits.The results of the relaxation experiments showed the degradation of the C.cornuta populations,which had evolved salt tolerance before.The resistance of the 0.75 ‰ selection line was significantly lower than the control line,and the evolved traits couldn't be restored to their initial state.This may be due to the loss of the most sensitive genotypes,resulting in a decline in genetic diversity.Meanwhile,the salt-tolerant genotypes may have low fitness under freshwater conditions.The hypothesis requires further data on genotype dynamics.In order to better understand the response of zooplankton to changing salinity in the wild,the differences in the community structure of resting eggs and the hatching of dominant species under different salt concentrations(0 ‰,5 ‰,10 ‰,15 ‰)in the freshwater site and brackish water site were analyzed in this study.The results showed that there was significant difference in the community structure of resting eggs between two sites,which was caused by water salinity.Copepods(Sinocalanus tenellus)dominated in the resting egg bank in brackish site,while cladocerans dominated in the freshwater site.The results of hatching experiment showed that 5 ‰ was the optimal hatching condition for the three kinds of resting eggs(S.tenellus,Brachionus,C.cornuta).The resting egg bank may contribute to the recovery of zooplankton communities after experiencing the disturbance of salinity.More studies were needed for examining how zooplankton respond when they experiencing larger increases over long period.Above all,it has been found that zooplankton may rapidly evolve salt tolerance to cope with freshwater salinization through phenotypic plasticity mediated life history and morphological traits.Given the importance of zooplankton in freshwater food webs,this kind of evolutionary tolerance may allow them to mitigate the effects of freshwater salinization on freshwater ecosystems.This study provides a scientific basis for biodiversity conservation and ecological restoration under the background of freshwater salinization.Though it is not possible to draw correct conclusions about the specific mechanism of salt tolerance in zooplankton,and more studies are needed to support it.What's more,it is necessary to do further research on the evolutionary response of natural populations to long-term changes in salinity,the molecular mechanism of salt tolerance evolution,multi-species and multi-factor interactions in the future.